Convert CONFIG_SYS_CONSOLE_IS_IN_ENV to Kconfig

[platform/kernel/u-boot.git] / README

# SPDX-License-Identifier: GPL-2.0+
#
# (C) Copyright 2000 - 2013
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.

Summary:
========

This directory contains the source code for U-Boot, a boot loader for
Embedded boards based on PowerPC, ARM, MIPS and several other
processors, which can be installed in a boot ROM and used to
initialize and test the hardware or to download and run application
code.

The development of U-Boot is closely related to Linux: some parts of
the source code originate in the Linux source tree, we have some
header files in common, and special provision has been made to
support booting of Linux images.

Some attention has been paid to make this software easily
configurable and extendable. For instance, all monitor commands are
implemented with the same call interface, so that it's very easy to
add new commands. Also, instead of permanently adding rarely used
code (for instance hardware test utilities) to the monitor, you can
load and run it dynamically.


Status:
=======

In general, all boards for which a configuration option exists in the
Makefile have been tested to some extent and can be considered
"working". In fact, many of them are used in production systems.

In case of problems see the CHANGELOG file to find out who contributed
the specific port. In addition, there are various MAINTAINERS files
scattered throughout the U-Boot source identifying the people or
companies responsible for various boards and subsystems.

Note: As of August, 2010, there is no longer a CHANGELOG file in the
actual U-Boot source tree; however, it can be created dynamically
from the Git log using:

        make CHANGELOG


Where to get help:
==================

In case you have questions about, problems with or contributions for
U-Boot, you should send a message to the U-Boot mailing list at
<u-boot@lists.denx.de>. There is also an archive of previous traffic
on the mailing list - please search the archive before asking FAQ's.
Please see https://lists.denx.de/pipermail/u-boot and
https://marc.info/?l=u-boot

Where to get source code:
=========================

The U-Boot source code is maintained in the Git repository at
https://source.denx.de/u-boot/u-boot.git ; you can browse it online at
https://source.denx.de/u-boot/u-boot

The "Tags" links on this page allow you to download tarballs of
any version you might be interested in. Official releases are also
available from the DENX file server through HTTPS or FTP.
https://ftp.denx.de/pub/u-boot/
ftp://ftp.denx.de/pub/u-boot/


Where we come from:
===================

- start from 8xxrom sources
- create PPCBoot project (https://sourceforge.net/projects/ppcboot)
- clean up code
- make it easier to add custom boards
- make it possible to add other [PowerPC] CPUs
- extend functions, especially:
  * Provide extended interface to Linux boot loader
  * S-Record download
  * network boot
  * ATA disk / SCSI ... boot
- create ARMBoot project (https://sourceforge.net/projects/armboot)
- add other CPU families (starting with ARM)
- create U-Boot project (https://sourceforge.net/projects/u-boot)
- current project page: see https://www.denx.de/wiki/U-Boot


Names and Spelling:
===================

The "official" name of this project is "Das U-Boot". The spelling
"U-Boot" shall be used in all written text (documentation, comments
in source files etc.). Example:

        This is the README file for the U-Boot project.

File names etc. shall be based on the string "u-boot". Examples:

        include/asm-ppc/u-boot.h

        #include <asm/u-boot.h>

Variable names, preprocessor constants etc. shall be either based on
the string "u_boot" or on "U_BOOT". Example:

        U_BOOT_VERSION          u_boot_logo
        IH_OS_U_BOOT            u_boot_hush_start


Versioning:
===========

Starting with the release in October 2008, the names of the releases
were changed from numerical release numbers without deeper meaning
into a time stamp based numbering. Regular releases are identified by
names consisting of the calendar year and month of the release date.
Additional fields (if present) indicate release candidates or bug fix
releases in "stable" maintenance trees.

Examples:
        U-Boot v2009.11     - Release November 2009
        U-Boot v2009.11.1   - Release 1 in version November 2009 stable tree
        U-Boot v2010.09-rc1 - Release candidate 1 for September 2010 release


Directory Hierarchy:
====================

/arch                   Architecture-specific files
  /arc                  Files generic to ARC architecture
  /arm                  Files generic to ARM architecture
  /m68k                 Files generic to m68k architecture
  /microblaze           Files generic to microblaze architecture
  /mips                 Files generic to MIPS architecture
  /nds32                Files generic to NDS32 architecture
  /nios2                Files generic to Altera NIOS2 architecture
  /powerpc              Files generic to PowerPC architecture
  /riscv                Files generic to RISC-V architecture
  /sandbox              Files generic to HW-independent "sandbox"
  /sh                   Files generic to SH architecture
  /x86                  Files generic to x86 architecture
  /xtensa               Files generic to Xtensa architecture
/api                    Machine/arch-independent API for external apps
/board                  Board-dependent files
/boot                   Support for images and booting
/cmd                    U-Boot commands functions
/common                 Misc architecture-independent functions
/configs                Board default configuration files
/disk                   Code for disk drive partition handling
/doc                    Documentation (a mix of ReST and READMEs)
/drivers                Device drivers
/dts                    Makefile for building internal U-Boot fdt.
/env                    Environment support
/examples               Example code for standalone applications, etc.
/fs                     Filesystem code (cramfs, ext2, jffs2, etc.)
/include                Header Files
/lib                    Library routines generic to all architectures
/Licenses               Various license files
/net                    Networking code
/post                   Power On Self Test
/scripts                Various build scripts and Makefiles
/test                   Various unit test files
/tools                  Tools to build and sign FIT images, etc.

Software Configuration:
=======================

Configuration is usually done using C preprocessor defines; the
rationale behind that is to avoid dead code whenever possible.

There are two classes of configuration variables:

* Configuration _OPTIONS_:
  These are selectable by the user and have names beginning with
  "CONFIG_".

* Configuration _SETTINGS_:
  These depend on the hardware etc. and should not be meddled with if
  you don't know what you're doing; they have names beginning with
  "CONFIG_SYS_".

Previously, all configuration was done by hand, which involved creating
symbolic links and editing configuration files manually. More recently,
U-Boot has added the Kbuild infrastructure used by the Linux kernel,
allowing you to use the "make menuconfig" command to configure your
build.


Selection of Processor Architecture and Board Type:
---------------------------------------------------

For all supported boards there are ready-to-use default
configurations available; just type "make <board_name>_defconfig".

Example: For a TQM823L module type:

        cd u-boot
        make TQM823L_defconfig

Note: If you're looking for the default configuration file for a board
you're sure used to be there but is now missing, check the file
doc/README.scrapyard for a list of no longer supported boards.

Sandbox Environment:
--------------------

U-Boot can be built natively to run on a Linux host using the 'sandbox'
board. This allows feature development which is not board- or architecture-
specific to be undertaken on a native platform. The sandbox is also used to
run some of U-Boot's tests.

See doc/arch/sandbox.rst for more details.


Board Initialisation Flow:
--------------------------

This is the intended start-up flow for boards. This should apply for both
SPL and U-Boot proper (i.e. they both follow the same rules).

Note: "SPL" stands for "Secondary Program Loader," which is explained in
more detail later in this file.

At present, SPL mostly uses a separate code path, but the function names
and roles of each function are the same. Some boards or architectures
may not conform to this.  At least most ARM boards which use
CONFIG_SPL_FRAMEWORK conform to this.

Execution typically starts with an architecture-specific (and possibly
CPU-specific) start.S file, such as:

        - arch/arm/cpu/armv7/start.S
        - arch/powerpc/cpu/mpc83xx/start.S
        - arch/mips/cpu/start.S

and so on. From there, three functions are called; the purpose and
limitations of each of these functions are described below.

lowlevel_init():
        - purpose: essential init to permit execution to reach board_init_f()
        - no global_data or BSS
        - there is no stack (ARMv7 may have one but it will soon be removed)
        - must not set up SDRAM or use console
        - must only do the bare minimum to allow execution to continue to
                board_init_f()
        - this is almost never needed
        - return normally from this function

board_init_f():
        - purpose: set up the machine ready for running board_init_r():
                i.e. SDRAM and serial UART
        - global_data is available
        - stack is in SRAM
        - BSS is not available, so you cannot use global/static variables,
                only stack variables and global_data

        Non-SPL-specific notes:
        - dram_init() is called to set up DRAM. If already done in SPL this
                can do nothing

        SPL-specific notes:
        - you can override the entire board_init_f() function with your own
                version as needed.
        - preloader_console_init() can be called here in extremis
        - should set up SDRAM, and anything needed to make the UART work
        - there is no need to clear BSS, it will be done by crt0.S
        - for specific scenarios on certain architectures an early BSS *can*
          be made available (via CONFIG_SPL_EARLY_BSS by moving the clearing
          of BSS prior to entering board_init_f()) but doing so is discouraged.
          Instead it is strongly recommended to architect any code changes
          or additions such to not depend on the availability of BSS during
          board_init_f() as indicated in other sections of this README to
          maintain compatibility and consistency across the entire code base.
        - must return normally from this function (don't call board_init_r()
                directly)

Here the BSS is cleared. For SPL, if CONFIG_SPL_STACK_R is defined, then at
this point the stack and global_data are relocated to below
CONFIG_SPL_STACK_R_ADDR. For non-SPL, U-Boot is relocated to run at the top of
memory.

board_init_r():
        - purpose: main execution, common code
        - global_data is available
        - SDRAM is available
        - BSS is available, all static/global variables can be used
        - execution eventually continues to main_loop()

        Non-SPL-specific notes:
        - U-Boot is relocated to the top of memory and is now running from
                there.

        SPL-specific notes:
        - stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined and
                CONFIG_SPL_STACK_R_ADDR points into SDRAM
        - preloader_console_init() can be called here - typically this is
                done by selecting CONFIG_SPL_BOARD_INIT and then supplying a
                spl_board_init() function containing this call
        - loads U-Boot or (in falcon mode) Linux


Configuration Options:
----------------------

Configuration depends on the combination of board and CPU type; all
such information is kept in a configuration file
"include/configs/<board_name>.h".

Example: For a TQM823L module, all configuration settings are in
"include/configs/TQM823L.h".


Many of the options are named exactly as the corresponding Linux
kernel configuration options. The intention is to make it easier to
build a config tool - later.

- ARM Platform Bus Type(CCI):
                CoreLink Cache Coherent Interconnect (CCI) is ARM BUS which
                provides full cache coherency between two clusters of multi-core
                CPUs and I/O coherency for devices and I/O masters

                CONFIG_SYS_FSL_HAS_CCI400

                Defined For SoC that has cache coherent interconnect
                CCN-400

                CONFIG_SYS_FSL_HAS_CCN504

                Defined for SoC that has cache coherent interconnect CCN-504

The following options need to be configured:

- CPU Type:     Define exactly one, e.g. CONFIG_MPC85XX.

- Board Type:   Define exactly one, e.g. CONFIG_MPC8540ADS.

- 85xx CPU Options:
                CONFIG_SYS_PPC64

                Specifies that the core is a 64-bit PowerPC implementation (implements
                the "64" category of the Power ISA). This is necessary for ePAPR
                compliance, among other possible reasons.

                CONFIG_SYS_FSL_TBCLK_DIV

                Defines the core time base clock divider ratio compared to the
                system clock.  On most PQ3 devices this is 8, on newer QorIQ
                devices it can be 16 or 32.  The ratio varies from SoC to Soc.

                CONFIG_SYS_FSL_PCIE_COMPAT

                Defines the string to utilize when trying to match PCIe device
                tree nodes for the given platform.

                CONFIG_SYS_FSL_ERRATUM_A004510

                Enables a workaround for erratum A004510.  If set,
                then CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV and
                CONFIG_SYS_FSL_CORENET_SNOOPVEC_COREONLY must be set.

                CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV
                CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV2 (optional)

                Defines one or two SoC revisions (low 8 bits of SVR)
                for which the A004510 workaround should be applied.

                The rest of SVR is either not relevant to the decision
                of whether the erratum is present (e.g. p2040 versus
                p2041) or is implied by the build target, which controls
                whether CONFIG_SYS_FSL_ERRATUM_A004510 is set.

                See Freescale App Note 4493 for more information about
                this erratum.

                CONFIG_A003399_NOR_WORKAROUND
                Enables a workaround for IFC erratum A003399. It is only
                required during NOR boot.

                CONFIG_A008044_WORKAROUND
                Enables a workaround for T1040/T1042 erratum A008044. It is only
                required during NAND boot and valid for Rev 1.0 SoC revision

                CONFIG_SYS_FSL_CORENET_SNOOPVEC_COREONLY

                This is the value to write into CCSR offset 0x18600
                according to the A004510 workaround.

                CONFIG_SYS_FSL_DSP_DDR_ADDR
                This value denotes start offset of DDR memory which is
                connected exclusively to the DSP cores.

                CONFIG_SYS_FSL_DSP_M2_RAM_ADDR
                This value denotes start offset of M2 memory
                which is directly connected to the DSP core.

                CONFIG_SYS_FSL_DSP_M3_RAM_ADDR
                This value denotes start offset of M3 memory which is directly
                connected to the DSP core.

                CONFIG_SYS_FSL_DSP_CCSRBAR_DEFAULT
                This value denotes start offset of DSP CCSR space.

                CONFIG_SYS_FSL_SINGLE_SOURCE_CLK
                Single Source Clock is clocking mode present in some of FSL SoC's.
                In this mode, a single differential clock is used to supply
                clocks to the sysclock, ddrclock and usbclock.

                CONFIG_SYS_CPC_REINIT_F
                This CONFIG is defined when the CPC is configured as SRAM at the
                time of U-Boot entry and is required to be re-initialized.

                CONFIG_DEEP_SLEEP
                Indicates this SoC supports deep sleep feature. If deep sleep is
                supported, core will start to execute uboot when wakes up.

- Generic CPU options:
                CONFIG_SYS_BIG_ENDIAN, CONFIG_SYS_LITTLE_ENDIAN

                Defines the endianess of the CPU. Implementation of those
                values is arch specific.

                CONFIG_SYS_FSL_DDR
                Freescale DDR driver in use. This type of DDR controller is
                found in mpc83xx, mpc85xx as well as some ARM core SoCs.

                CONFIG_SYS_FSL_DDR_ADDR
                Freescale DDR memory-mapped register base.

                CONFIG_SYS_FSL_DDR_EMU
                Specify emulator support for DDR. Some DDR features such as
                deskew training are not available.

                CONFIG_SYS_FSL_DDRC_GEN1
                Freescale DDR1 controller.

                CONFIG_SYS_FSL_DDRC_GEN2
                Freescale DDR2 controller.

                CONFIG_SYS_FSL_DDRC_GEN3
                Freescale DDR3 controller.

                CONFIG_SYS_FSL_DDRC_GEN4
                Freescale DDR4 controller.

                CONFIG_SYS_FSL_DDRC_ARM_GEN3
                Freescale DDR3 controller for ARM-based SoCs.

                CONFIG_SYS_FSL_DDR1
                Board config to use DDR1. It can be enabled for SoCs with
                Freescale DDR1 or DDR2 controllers, depending on the board
                implemetation.

                CONFIG_SYS_FSL_DDR2
                Board config to use DDR2. It can be enabled for SoCs with
                Freescale DDR2 or DDR3 controllers, depending on the board
                implementation.

                CONFIG_SYS_FSL_DDR3
                Board config to use DDR3. It can be enabled for SoCs with
                Freescale DDR3 or DDR3L controllers.

                CONFIG_SYS_FSL_DDR3L
                Board config to use DDR3L. It can be enabled for SoCs with
                DDR3L controllers.

                CONFIG_SYS_FSL_IFC_BE
                Defines the IFC controller register space as Big Endian

                CONFIG_SYS_FSL_IFC_LE
                Defines the IFC controller register space as Little Endian

                CONFIG_SYS_FSL_IFC_CLK_DIV
                Defines divider of platform clock(clock input to IFC controller).

                CONFIG_SYS_FSL_LBC_CLK_DIV
                Defines divider of platform clock(clock input to eLBC controller).

                CONFIG_SYS_FSL_DDR_BE
                Defines the DDR controller register space as Big Endian

                CONFIG_SYS_FSL_DDR_LE
                Defines the DDR controller register space as Little Endian

                CONFIG_SYS_FSL_DDR_SDRAM_BASE_PHY
                Physical address from the view of DDR controllers. It is the
                same as CONFIG_SYS_DDR_SDRAM_BASE for  all Power SoCs. But
                it could be different for ARM SoCs.

                CONFIG_SYS_FSL_DDR_INTLV_256B
                DDR controller interleaving on 256-byte. This is a special
                interleaving mode, handled by Dickens for Freescale layerscape
                SoCs with ARM core.

                CONFIG_SYS_FSL_DDR_MAIN_NUM_CTRLS
                Number of controllers used as main memory.

                CONFIG_SYS_FSL_OTHER_DDR_NUM_CTRLS
                Number of controllers used for other than main memory.

                CONFIG_SYS_FSL_HAS_DP_DDR
                Defines the SoC has DP-DDR used for DPAA.

                CONFIG_SYS_FSL_SEC_BE
                Defines the SEC controller register space as Big Endian

                CONFIG_SYS_FSL_SEC_LE
                Defines the SEC controller register space as Little Endian

- MIPS CPU options:
                CONFIG_SYS_INIT_SP_OFFSET

                Offset relative to CONFIG_SYS_SDRAM_BASE for initial stack
                pointer. This is needed for the temporary stack before
                relocation.

                CONFIG_XWAY_SWAP_BYTES

                Enable compilation of tools/xway-swap-bytes needed for Lantiq
                XWAY SoCs for booting from NOR flash. The U-Boot image needs to
                be swapped if a flash programmer is used.

- ARM options:
                CONFIG_SYS_EXCEPTION_VECTORS_HIGH

                Select high exception vectors of the ARM core, e.g., do not
                clear the V bit of the c1 register of CP15.

                COUNTER_FREQUENCY
                Generic timer clock source frequency.

                COUNTER_FREQUENCY_REAL
                Generic timer clock source frequency if the real clock is
                different from COUNTER_FREQUENCY, and can only be determined
                at run time.

- Tegra SoC options:
                CONFIG_TEGRA_SUPPORT_NON_SECURE

                Support executing U-Boot in non-secure (NS) mode. Certain
                impossible actions will be skipped if the CPU is in NS mode,
                such as ARM architectural timer initialization.

- Linux Kernel Interface:
                CONFIG_MEMSIZE_IN_BYTES         [relevant for MIPS only]

                When transferring memsize parameter to Linux, some versions
                expect it to be in bytes, others in MB.
                Define CONFIG_MEMSIZE_IN_BYTES to make it in bytes.

                CONFIG_OF_LIBFDT

                New kernel versions are expecting firmware settings to be
                passed using flattened device trees (based on open firmware
                concepts).

                CONFIG_OF_LIBFDT
                 * New libfdt-based support
                 * Adds the "fdt" command
                 * The bootm command automatically updates the fdt

                OF_TBCLK - The timebase frequency.

                boards with QUICC Engines require OF_QE to set UCC MAC
                addresses

                CONFIG_OF_BOARD_SETUP

                Board code has addition modification that it wants to make
                to the flat device tree before handing it off to the kernel

                CONFIG_OF_SYSTEM_SETUP

                Other code has addition modification that it wants to make
                to the flat device tree before handing it off to the kernel.
                This causes ft_system_setup() to be called before booting
                the kernel.

                CONFIG_OF_IDE_FIXUP

                U-Boot can detect if an IDE device is present or not.
                If not, and this new config option is activated, U-Boot
                removes the ATA node from the DTS before booting Linux,
                so the Linux IDE driver does not probe the device and
                crash. This is needed for buggy hardware (uc101) where
                no pull down resistor is connected to the signal IDE5V_DD7.

- vxWorks boot parameters:

                bootvx constructs a valid bootline using the following
                environments variables: bootdev, bootfile, ipaddr, netmask,
                serverip, gatewayip, hostname, othbootargs.
                It loads the vxWorks image pointed bootfile.

                Note: If a "bootargs" environment is defined, it will override
                the defaults discussed just above.

- Cache Configuration:
                CONFIG_SYS_L2CACHE_OFF- Do not enable L2 cache in U-Boot

- Cache Configuration for ARM:
                CONFIG_SYS_L2_PL310 - Enable support for ARM PL310 L2 cache
                                      controller
                CONFIG_SYS_PL310_BASE - Physical base address of PL310
                                        controller register space

- Serial Ports:
                CONFIG_PL011_SERIAL

                Define this if you want support for Amba PrimeCell PL011 UARTs.

                CONFIG_PL011_CLOCK

                If you have Amba PrimeCell PL011 UARTs, set this variable to
                the clock speed of the UARTs.

                CONFIG_PL01x_PORTS

                If you have Amba PrimeCell PL010 or PL011 UARTs on your board,
                define this to a list of base addresses for each (supported)
                port. See e.g. include/configs/versatile.h

                CONFIG_SERIAL_HW_FLOW_CONTROL

                Define this variable to enable hw flow control in serial driver.
                Current user of this option is drivers/serial/nsl16550.c driver

- Autoboot Command:
                CONFIG_BOOTCOMMAND
                Only needed when CONFIG_BOOTDELAY is enabled;
                define a command string that is automatically executed
                when no character is read on the console interface
                within "Boot Delay" after reset.

                CONFIG_RAMBOOT and CONFIG_NFSBOOT
                The value of these goes into the environment as
                "ramboot" and "nfsboot" respectively, and can be used
                as a convenience, when switching between booting from
                RAM and NFS.

- Serial Download Echo Mode:
                CONFIG_LOADS_ECHO
                If defined to 1, all characters received during a
                serial download (using the "loads" command) are
                echoed back. This might be needed by some terminal
                emulations (like "cu"), but may as well just take
                time on others. This setting #define's the initial
                value of the "loads_echo" environment variable.

- Removal of commands
                If no commands are needed to boot, you can disable
                CONFIG_CMDLINE to remove them. In this case, the command line
                will not be available, and when U-Boot wants to execute the
                boot command (on start-up) it will call board_run_command()
                instead. This can reduce image size significantly for very
                simple boot procedures.

- Regular expression support:
                CONFIG_REGEX
                If this variable is defined, U-Boot is linked against
                the SLRE (Super Light Regular Expression) library,
                which adds regex support to some commands, as for
                example "env grep" and "setexpr".

- Device tree:
                CONFIG_OF_CONTROL
                If this variable is defined, U-Boot will use a device tree
                to configure its devices, instead of relying on statically
                compiled #defines in the board file. This option is
                experimental and only available on a few boards. The device
                tree is available in the global data as gd->fdt_blob.

                U-Boot needs to get its device tree from somewhere. This can
                be done using one of the three options below:

                CONFIG_OF_SEPARATE
                If this variable is defined, U-Boot will build a device tree
                binary. It will be called u-boot.dtb. Architecture-specific
                code will locate it at run-time. Generally this works by:

                        cat u-boot.bin u-boot.dtb >image.bin

                and in fact, U-Boot does this for you, creating a file called
                u-boot-dtb.bin which is useful in the common case. You can
                still use the individual files if you need something more
                exotic.

                CONFIG_OF_BOARD
                If this variable is defined, U-Boot will use the device tree
                provided by the board at runtime instead of embedding one with
                the image. Only boards defining board_fdt_blob_setup() support
                this option (see include/fdtdec.h file).

- Watchdog:
                CONFIG_WATCHDOG
                If this variable is defined, it enables watchdog
                support for the SoC. There must be support in the SoC
                specific code for a watchdog. For the 8xx
                CPUs, the SIU Watchdog feature is enabled in the SYPCR
                register.  When supported for a specific SoC is
                available, then no further board specific code should
                be needed to use it.

                CONFIG_HW_WATCHDOG
                When using a watchdog circuitry external to the used
                SoC, then define this variable and provide board
                specific code for the "hw_watchdog_reset" function.

                CONFIG_SYS_WATCHDOG_FREQ
                Some platforms automatically call WATCHDOG_RESET()
                from the timer interrupt handler every
                CONFIG_SYS_WATCHDOG_FREQ interrupts. If not set by the
                board configuration file, a default of CONFIG_SYS_HZ/2
                (i.e. 500) is used. Setting CONFIG_SYS_WATCHDOG_FREQ
                to 0 disables calling WATCHDOG_RESET() from the timer
                interrupt.

- Real-Time Clock:

                When CONFIG_CMD_DATE is selected, the type of the RTC
                has to be selected, too. Define exactly one of the
                following options:

                CONFIG_RTC_PCF8563      - use Philips PCF8563 RTC
                CONFIG_RTC_MC13XXX      - use MC13783 or MC13892 RTC
                CONFIG_RTC_MC146818     - use MC146818 RTC
                CONFIG_RTC_DS1307       - use Maxim, Inc. DS1307 RTC
                CONFIG_RTC_DS1337       - use Maxim, Inc. DS1337 RTC
                CONFIG_RTC_DS1338       - use Maxim, Inc. DS1338 RTC
                CONFIG_RTC_DS1339       - use Maxim, Inc. DS1339 RTC
                CONFIG_RTC_DS164x       - use Dallas DS164x RTC
                CONFIG_RTC_ISL1208      - use Intersil ISL1208 RTC
                CONFIG_RTC_MAX6900      - use Maxim, Inc. MAX6900 RTC
                CONFIG_RTC_DS1337_NOOSC - Turn off the OSC output for DS1337
                CONFIG_SYS_RV3029_TCR   - enable trickle charger on
                                          RV3029 RTC.

                Note that if the RTC uses I2C, then the I2C interface
                must also be configured. See I2C Support, below.

- GPIO Support:
                CONFIG_PCA953X          - use NXP's PCA953X series I2C GPIO

                The CONFIG_SYS_I2C_PCA953X_WIDTH option specifies a list of
                chip-ngpio pairs that tell the PCA953X driver the number of
                pins supported by a particular chip.

                Note that if the GPIO device uses I2C, then the I2C interface
                must also be configured. See I2C Support, below.

- I/O tracing:
                When CONFIG_IO_TRACE is selected, U-Boot intercepts all I/O
                accesses and can checksum them or write a list of them out
                to memory. See the 'iotrace' command for details. This is
                useful for testing device drivers since it can confirm that
                the driver behaves the same way before and after a code
                change. Currently this is supported on sandbox and arm. To
                add support for your architecture, add '#include <iotrace.h>'
                to the bottom of arch/<arch>/include/asm/io.h and test.

                Example output from the 'iotrace stats' command is below.
                Note that if the trace buffer is exhausted, the checksum will
                still continue to operate.

                        iotrace is enabled
                        Start:  10000000        (buffer start address)
                        Size:   00010000        (buffer size)
                        Offset: 00000120        (current buffer offset)
                        Output: 10000120        (start + offset)
                        Count:  00000018        (number of trace records)
                        CRC32:  9526fb66        (CRC32 of all trace records)

- Timestamp Support:

                When CONFIG_TIMESTAMP is selected, the timestamp
                (date and time) of an image is printed by image
                commands like bootm or iminfo. This option is
                automatically enabled when you select CONFIG_CMD_DATE .

- Partition Labels (disklabels) Supported:
                Zero or more of the following:
                CONFIG_MAC_PARTITION   Apple's MacOS partition table.
                CONFIG_ISO_PARTITION   ISO partition table, used on CDROM etc.
                CONFIG_EFI_PARTITION   GPT partition table, common when EFI is the
                                       bootloader.  Note 2TB partition limit; see
                                       disk/part_efi.c
                CONFIG_SCSI) you must configure support for at
                least one non-MTD partition type as well.

- IDE Reset method:
                CONFIG_IDE_RESET_ROUTINE - this is defined in several
                board configurations files but used nowhere!

                CONFIG_IDE_RESET - is this is defined, IDE Reset will
                be performed by calling the function
                        ide_set_reset(int reset)
                which has to be defined in a board specific file

- ATAPI Support:
                CONFIG_ATAPI

                Set this to enable ATAPI support.

- LBA48 Support
                CONFIG_LBA48

                Set this to enable support for disks larger than 137GB
                Also look at CONFIG_SYS_64BIT_LBA.
                Whithout these , LBA48 support uses 32bit variables and will 'only'
                support disks up to 2.1TB.

                CONFIG_SYS_64BIT_LBA:
                        When enabled, makes the IDE subsystem use 64bit sector addresses.
                        Default is 32bit.

- SCSI Support:
                CONFIG_SYS_SCSI_MAX_LUN [8], CONFIG_SYS_SCSI_MAX_SCSI_ID [7] and
                CONFIG_SYS_SCSI_MAX_DEVICE [CONFIG_SYS_SCSI_MAX_SCSI_ID *
                CONFIG_SYS_SCSI_MAX_LUN] can be adjusted to define the
                maximum numbers of LUNs, SCSI ID's and target
                devices.

                The environment variable 'scsidevs' is set to the number of
                SCSI devices found during the last scan.

- NETWORK Support (PCI):
                CONFIG_E1000_SPI
                Utility code for direct access to the SPI bus on Intel 8257x.
                This does not do anything useful unless you set at least one
                of CONFIG_CMD_E1000 or CONFIG_E1000_SPI_GENERIC.

                CONFIG_NATSEMI
                Support for National dp83815 chips.

                CONFIG_NS8382X
                Support for National dp8382[01] gigabit chips.

- NETWORK Support (other):
                CONFIG_CALXEDA_XGMAC
                Support for the Calxeda XGMAC device

                CONFIG_LAN91C96
                Support for SMSC's LAN91C96 chips.

                        CONFIG_LAN91C96_USE_32_BIT
                        Define this to enable 32 bit addressing

                CONFIG_SMC91111
                Support for SMSC's LAN91C111 chip

                        CONFIG_SMC91111_BASE
                        Define this to hold the physical address
                        of the device (I/O space)

                        CONFIG_SMC_USE_32_BIT
                        Define this if data bus is 32 bits

                        CONFIG_SMC_USE_IOFUNCS
                        Define this to use i/o functions instead of macros
                        (some hardware wont work with macros)

                        CONFIG_SYS_DAVINCI_EMAC_PHY_COUNT
                        Define this if you have more then 3 PHYs.

                CONFIG_FTGMAC100
                Support for Faraday's FTGMAC100 Gigabit SoC Ethernet

                        CONFIG_FTGMAC100_EGIGA
                        Define this to use GE link update with gigabit PHY.
                        Define this if FTGMAC100 is connected to gigabit PHY.
                        If your system has 10/100 PHY only, it might not occur
                        wrong behavior. Because PHY usually return timeout or
                        useless data when polling gigabit status and gigabit
                        control registers. This behavior won't affect the
                        correctnessof 10/100 link speed update.

                CONFIG_SH_ETHER
                Support for Renesas on-chip Ethernet controller

                        CONFIG_SH_ETHER_USE_PORT
                        Define the number of ports to be used

                        CONFIG_SH_ETHER_PHY_ADDR
                        Define the ETH PHY's address

                        CONFIG_SH_ETHER_CACHE_WRITEBACK
                        If this option is set, the driver enables cache flush.

- TPM Support:
                CONFIG_TPM
                Support TPM devices.

                CONFIG_TPM_TIS_INFINEON
                Support for Infineon i2c bus TPM devices. Only one device
                per system is supported at this time.

                        CONFIG_TPM_TIS_I2C_BURST_LIMITATION
                        Define the burst count bytes upper limit

                CONFIG_TPM_ST33ZP24
                Support for STMicroelectronics TPM devices. Requires DM_TPM support.

                        CONFIG_TPM_ST33ZP24_I2C
                        Support for STMicroelectronics ST33ZP24 I2C devices.
                        Requires TPM_ST33ZP24 and I2C.

                        CONFIG_TPM_ST33ZP24_SPI
                        Support for STMicroelectronics ST33ZP24 SPI devices.
                        Requires TPM_ST33ZP24 and SPI.

                CONFIG_TPM_ATMEL_TWI
                Support for Atmel TWI TPM device. Requires I2C support.

                CONFIG_TPM_TIS_LPC
                Support for generic parallel port TPM devices. Only one device
                per system is supported at this time.

                        CONFIG_TPM_TIS_BASE_ADDRESS
                        Base address where the generic TPM device is mapped
                        to. Contemporary x86 systems usually map it at
                        0xfed40000.

                CONFIG_TPM
                Define this to enable the TPM support library which provides
                functional interfaces to some TPM commands.
                Requires support for a TPM device.

                CONFIG_TPM_AUTH_SESSIONS
                Define this to enable authorized functions in the TPM library.
                Requires CONFIG_TPM and CONFIG_SHA1.

- USB Support:
                At the moment only the UHCI host controller is
                supported (PIP405, MIP405); define
                CONFIG_USB_UHCI to enable it.
                define CONFIG_USB_KEYBOARD to enable the USB Keyboard
                and define CONFIG_USB_STORAGE to enable the USB
                storage devices.
                Note:
                Supported are USB Keyboards and USB Floppy drives
                (TEAC FD-05PUB).

                CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the
                txfilltuning field in the EHCI controller on reset.

                CONFIG_USB_DWC2_REG_ADDR the physical CPU address of the DWC2
                HW module registers.

- USB Device:
                Define the below if you wish to use the USB console.
                Once firmware is rebuilt from a serial console issue the
                command "setenv stdin usbtty; setenv stdout usbtty" and
                attach your USB cable. The Unix command "dmesg" should print
                it has found a new device. The environment variable usbtty
                can be set to gserial or cdc_acm to enable your device to
                appear to a USB host as a Linux gserial device or a
                Common Device Class Abstract Control Model serial device.
                If you select usbtty = gserial you should be able to enumerate
                a Linux host by
                # modprobe usbserial vendor=0xVendorID product=0xProductID
                else if using cdc_acm, simply setting the environment
                variable usbtty to be cdc_acm should suffice. The following
                might be defined in YourBoardName.h

                        CONFIG_USB_DEVICE
                        Define this to build a UDC device

                        CONFIG_USB_TTY
                        Define this to have a tty type of device available to
                        talk to the UDC device

                        CONFIG_USBD_HS
                        Define this to enable the high speed support for usb
                        device and usbtty. If this feature is enabled, a routine
                        int is_usbd_high_speed(void)
                        also needs to be defined by the driver to dynamically poll
                        whether the enumeration has succeded at high speed or full
                        speed.

                If you have a USB-IF assigned VendorID then you may wish to
                define your own vendor specific values either in BoardName.h
                or directly in usbd_vendor_info.h. If you don't define
                CONFIG_USBD_MANUFACTURER, CONFIG_USBD_PRODUCT_NAME,
                CONFIG_USBD_VENDORID and CONFIG_USBD_PRODUCTID, then U-Boot
                should pretend to be a Linux device to it's target host.

                        CONFIG_USBD_MANUFACTURER
                        Define this string as the name of your company for
                        - CONFIG_USBD_MANUFACTURER "my company"

                        CONFIG_USBD_PRODUCT_NAME
                        Define this string as the name of your product
                        - CONFIG_USBD_PRODUCT_NAME "acme usb device"

                        CONFIG_USBD_VENDORID
                        Define this as your assigned Vendor ID from the USB
                        Implementors Forum. This *must* be a genuine Vendor ID
                        to avoid polluting the USB namespace.
                        - CONFIG_USBD_VENDORID 0xFFFF

                        CONFIG_USBD_PRODUCTID
                        Define this as the unique Product ID
                        for your device
                        - CONFIG_USBD_PRODUCTID 0xFFFF

- ULPI Layer Support:
                The ULPI (UTMI Low Pin (count) Interface) PHYs are supported via
                the generic ULPI layer. The generic layer accesses the ULPI PHY
                via the platform viewport, so you need both the genric layer and
                the viewport enabled. Currently only Chipidea/ARC based
                viewport is supported.
                To enable the ULPI layer support, define CONFIG_USB_ULPI and
                CONFIG_USB_ULPI_VIEWPORT in your board configuration file.
                If your ULPI phy needs a different reference clock than the
                standard 24 MHz then you have to define CONFIG_ULPI_REF_CLK to
                the appropriate value in Hz.

- MMC Support:
                The MMC controller on the Intel PXA is supported. To
                enable this define CONFIG_MMC. The MMC can be
                accessed from the boot prompt by mapping the device
                to physical memory similar to flash. Command line is
                enabled with CONFIG_CMD_MMC. The MMC driver also works with
                the FAT fs. This is enabled with CONFIG_CMD_FAT.

                CONFIG_SH_MMCIF
                Support for Renesas on-chip MMCIF controller

                        CONFIG_SH_MMCIF_ADDR
                        Define the base address of MMCIF registers

                        CONFIG_SH_MMCIF_CLK
                        Define the clock frequency for MMCIF

- USB Device Firmware Update (DFU) class support:
                CONFIG_DFU_OVER_USB
                This enables the USB portion of the DFU USB class

                CONFIG_DFU_NAND
                This enables support for exposing NAND devices via DFU.

                CONFIG_DFU_RAM
                This enables support for exposing RAM via DFU.
                Note: DFU spec refer to non-volatile memory usage, but
                allow usages beyond the scope of spec - here RAM usage,
                one that would help mostly the developer.

                CONFIG_SYS_DFU_DATA_BUF_SIZE
                Dfu transfer uses a buffer before writing data to the
                raw storage device. Make the size (in bytes) of this buffer
                configurable. The size of this buffer is also configurable
                through the "dfu_bufsiz" environment variable.

                CONFIG_SYS_DFU_MAX_FILE_SIZE
                When updating files rather than the raw storage device,
                we use a static buffer to copy the file into and then write
                the buffer once we've been given the whole file.  Define
                this to the maximum filesize (in bytes) for the buffer.
                Default is 4 MiB if undefined.

                DFU_DEFAULT_POLL_TIMEOUT
                Poll timeout [ms], is the timeout a device can send to the
                host. The host must wait for this timeout before sending
                a subsequent DFU_GET_STATUS request to the device.

                DFU_MANIFEST_POLL_TIMEOUT
                Poll timeout [ms], which the device sends to the host when
                entering dfuMANIFEST state. Host waits this timeout, before
                sending again an USB request to the device.

- Journaling Flash filesystem support:
                CONFIG_JFFS2_NAND
                Define these for a default partition on a NAND device

                CONFIG_SYS_JFFS2_FIRST_SECTOR,
                CONFIG_SYS_JFFS2_FIRST_BANK, CONFIG_SYS_JFFS2_NUM_BANKS
                Define these for a default partition on a NOR device

- Keyboard Support:
                See Kconfig help for available keyboard drivers.

                CONFIG_KEYBOARD

                Define this to enable a custom keyboard support.
                This simply calls drv_keyboard_init() which must be
                defined in your board-specific files. This option is deprecated
                and is only used by novena. For new boards, use driver model
                instead.

- Video support:
                CONFIG_FSL_DIU_FB
                Enable the Freescale DIU video driver.  Reference boards for
                SOCs that have a DIU should define this macro to enable DIU
                support, and should also define these other macros:

                        CONFIG_SYS_DIU_ADDR
                        CONFIG_VIDEO
                        CONFIG_CFB_CONSOLE
                        CONFIG_VIDEO_SW_CURSOR
                        CONFIG_VGA_AS_SINGLE_DEVICE
                        CONFIG_VIDEO_LOGO
                        CONFIG_VIDEO_BMP_LOGO

                The DIU driver will look for the 'video-mode' environment
                variable, and if defined, enable the DIU as a console during
                boot.  See the documentation file doc/README.video for a
                description of this variable.

- LCD Support:  CONFIG_LCD

                Define this to enable LCD support (for output to LCD
                display); also select one of the supported displays
                by defining one of these:

                CONFIG_ATMEL_LCD:

                        HITACHI TX09D70VM1CCA, 3.5", 240x320.

                CONFIG_NEC_NL6448AC33:

                        NEC NL6448AC33-18. Active, color, single scan.

                CONFIG_NEC_NL6448BC20

                        NEC NL6448BC20-08. 6.5", 640x480.
                        Active, color, single scan.

                CONFIG_NEC_NL6448BC33_54

                        NEC NL6448BC33-54. 10.4", 640x480.
                        Active, color, single scan.

                CONFIG_SHARP_16x9

                        Sharp 320x240. Active, color, single scan.
                        It isn't 16x9, and I am not sure what it is.

                CONFIG_SHARP_LQ64D341

                        Sharp LQ64D341 display, 640x480.
                        Active, color, single scan.

                CONFIG_HLD1045

                        HLD1045 display, 640x480.
                        Active, color, single scan.

                CONFIG_OPTREX_BW

                        Optrex   CBL50840-2 NF-FW 99 22 M5
                        or
                        Hitachi  LMG6912RPFC-00T
                        or
                        Hitachi  SP14Q002

                        320x240. Black & white.

                CONFIG_LCD_ALIGNMENT

                Normally the LCD is page-aligned (typically 4KB). If this is
                defined then the LCD will be aligned to this value instead.
                For ARM it is sometimes useful to use MMU_SECTION_SIZE
                here, since it is cheaper to change data cache settings on
                a per-section basis.


                CONFIG_LCD_ROTATION

                Sometimes, for example if the display is mounted in portrait
                mode or even if it's mounted landscape but rotated by 180degree,
                we need to rotate our content of the display relative to the
                framebuffer, so that user can read the messages which are
                printed out.
                Once CONFIG_LCD_ROTATION is defined, the lcd_console will be
                initialized with a given rotation from "vl_rot" out of
                "vidinfo_t" which is provided by the board specific code.
                The value for vl_rot is coded as following (matching to
                fbcon=rotate:<n> linux-kernel commandline):
                0 = no rotation respectively 0 degree
                1 = 90 degree rotation
                2 = 180 degree rotation
                3 = 270 degree rotation

                If CONFIG_LCD_ROTATION is not defined, the console will be
                initialized with 0degree rotation.

                CONFIG_LCD_BMP_RLE8

                Support drawing of RLE8-compressed bitmaps on the LCD.

                CONFIG_I2C_EDID

                Enables an 'i2c edid' command which can read EDID
                information over I2C from an attached LCD display.

- MII/PHY support:
                CONFIG_PHY_CLOCK_FREQ (ppc4xx)

                The clock frequency of the MII bus

                CONFIG_PHY_RESET_DELAY

                Some PHY like Intel LXT971A need extra delay after
                reset before any MII register access is possible.
                For such PHY, set this option to the usec delay
                required. (minimum 300usec for LXT971A)

                CONFIG_PHY_CMD_DELAY (ppc4xx)

                Some PHY like Intel LXT971A need extra delay after
                command issued before MII status register can be read

- IP address:
                CONFIG_IPADDR

                Define a default value for the IP address to use for
                the default Ethernet interface, in case this is not
                determined through e.g. bootp.
                (Environment variable "ipaddr")

- Server IP address:
                CONFIG_SERVERIP

                Defines a default value for the IP address of a TFTP
                server to contact when using the "tftboot" command.
                (Environment variable "serverip")

                CONFIG_KEEP_SERVERADDR

                Keeps the server's MAC address, in the env 'serveraddr'
                for passing to bootargs (like Linux's netconsole option)

- Gateway IP address:
                CONFIG_GATEWAYIP

                Defines a default value for the IP address of the
                default router where packets to other networks are
                sent to.
                (Environment variable "gatewayip")

- Subnet mask:
                CONFIG_NETMASK

                Defines a default value for the subnet mask (or
                routing prefix) which is used to determine if an IP
                address belongs to the local subnet or needs to be
                forwarded through a router.
                (Environment variable "netmask")

- BOOTP Recovery Mode:
                CONFIG_BOOTP_RANDOM_DELAY

                If you have many targets in a network that try to
                boot using BOOTP, you may want to avoid that all
                systems send out BOOTP requests at precisely the same
                moment (which would happen for instance at recovery
                from a power failure, when all systems will try to
                boot, thus flooding the BOOTP server. Defining
                CONFIG_BOOTP_RANDOM_DELAY causes a random delay to be
                inserted before sending out BOOTP requests. The
                following delays are inserted then:

                1st BOOTP request:      delay 0 ... 1 sec
                2nd BOOTP request:      delay 0 ... 2 sec
                3rd BOOTP request:      delay 0 ... 4 sec
                4th and following
                BOOTP requests:         delay 0 ... 8 sec

                CONFIG_BOOTP_ID_CACHE_SIZE

                BOOTP packets are uniquely identified using a 32-bit ID. The
                server will copy the ID from client requests to responses and
                U-Boot will use this to determine if it is the destination of
                an incoming response. Some servers will check that addresses
                aren't in use before handing them out (usually using an ARP
                ping) and therefore take up to a few hundred milliseconds to
                respond. Network congestion may also influence the time it
                takes for a response to make it back to the client. If that
                time is too long, U-Boot will retransmit requests. In order
                to allow earlier responses to still be accepted after these
                retransmissions, U-Boot's BOOTP client keeps a small cache of
                IDs. The CONFIG_BOOTP_ID_CACHE_SIZE controls the size of this
                cache. The default is to keep IDs for up to four outstanding
                requests. Increasing this will allow U-Boot to accept offers
                from a BOOTP client in networks with unusually high latency.

- DHCP Advanced Options:
                You can fine tune the DHCP functionality by defining
                CONFIG_BOOTP_* symbols:

                CONFIG_BOOTP_NISDOMAIN
                CONFIG_BOOTP_BOOTFILESIZE
                CONFIG_BOOTP_NTPSERVER
                CONFIG_BOOTP_TIMEOFFSET
                CONFIG_BOOTP_VENDOREX
                CONFIG_BOOTP_MAY_FAIL

                CONFIG_BOOTP_SERVERIP - TFTP server will be the serverip
                environment variable, not the BOOTP server.

                CONFIG_BOOTP_MAY_FAIL - If the DHCP server is not found
                after the configured retry count, the call will fail
                instead of starting over.  This can be used to fail over
                to Link-local IP address configuration if the DHCP server
                is not available.

                CONFIG_BOOTP_DHCP_REQUEST_DELAY

                A 32bit value in microseconds for a delay between
                receiving a "DHCP Offer" and sending the "DHCP Request".
                This fixes a problem with certain DHCP servers that don't
                respond 100% of the time to a "DHCP request". E.g. On an
                AT91RM9200 processor running at 180MHz, this delay needed
                to be *at least* 15,000 usec before a Windows Server 2003
                DHCP server would reply 100% of the time. I recommend at
                least 50,000 usec to be safe. The alternative is to hope
                that one of the retries will be successful but note that
                the DHCP timeout and retry process takes a longer than
                this delay.

 - Link-local IP address negotiation:
                Negotiate with other link-local clients on the local network
                for an address that doesn't require explicit configuration.
                This is especially useful if a DHCP server cannot be guaranteed
                to exist in all environments that the device must operate.

                See doc/README.link-local for more information.

 - MAC address from environment variables

                FDT_SEQ_MACADDR_FROM_ENV

                Fix-up device tree with MAC addresses fetched sequentially from
                environment variables. This config work on assumption that
                non-usable ethernet node of device-tree are either not present
                or their status has been marked as "disabled".

 - CDP Options:
                CONFIG_CDP_DEVICE_ID

                The device id used in CDP trigger frames.

                CONFIG_CDP_DEVICE_ID_PREFIX

                A two character string which is prefixed to the MAC address
                of the device.

                CONFIG_CDP_PORT_ID

                A printf format string which contains the ascii name of
                the port. Normally is set to "eth%d" which sets
                eth0 for the first Ethernet, eth1 for the second etc.

                CONFIG_CDP_CAPABILITIES

                A 32bit integer which indicates the device capabilities;
                0x00000010 for a normal host which does not forwards.

                CONFIG_CDP_VERSION

                An ascii string containing the version of the software.

                CONFIG_CDP_PLATFORM

                An ascii string containing the name of the platform.

                CONFIG_CDP_TRIGGER

                A 32bit integer sent on the trigger.

                CONFIG_CDP_POWER_CONSUMPTION

                A 16bit integer containing the power consumption of the
                device in .1 of milliwatts.

                CONFIG_CDP_APPLIANCE_VLAN_TYPE

                A byte containing the id of the VLAN.

- Status LED:   CONFIG_LED_STATUS

                Several configurations allow to display the current
                status using a LED. For instance, the LED will blink
                fast while running U-Boot code, stop blinking as
                soon as a reply to a BOOTP request was received, and
                start blinking slow once the Linux kernel is running
                (supported by a status LED driver in the Linux
                kernel). Defining CONFIG_LED_STATUS enables this
                feature in U-Boot.

                Additional options:

                CONFIG_LED_STATUS_GPIO
                The status LED can be connected to a GPIO pin.
                In such cases, the gpio_led driver can be used as a
                status LED backend implementation. Define CONFIG_LED_STATUS_GPIO
                to include the gpio_led driver in the U-Boot binary.

                CONFIG_GPIO_LED_INVERTED_TABLE
                Some GPIO connected LEDs may have inverted polarity in which
                case the GPIO high value corresponds to LED off state and
                GPIO low value corresponds to LED on state.
                In such cases CONFIG_GPIO_LED_INVERTED_TABLE may be defined
                with a list of GPIO LEDs that have inverted polarity.

- I2C Support:
                CONFIG_SYS_NUM_I2C_BUSES
                Hold the number of i2c buses you want to use.

                CONFIG_SYS_I2C_DIRECT_BUS
                define this, if you don't use i2c muxes on your hardware.
                if CONFIG_SYS_I2C_MAX_HOPS is not defined or == 0 you can
                omit this define.

                CONFIG_SYS_I2C_MAX_HOPS
                define how many muxes are maximal consecutively connected
                on one i2c bus. If you not use i2c muxes, omit this
                define.

                CONFIG_SYS_I2C_BUSES
                hold a list of buses you want to use, only used if
                CONFIG_SYS_I2C_DIRECT_BUS is not defined, for example
                a board with CONFIG_SYS_I2C_MAX_HOPS = 1 and
                CONFIG_SYS_NUM_I2C_BUSES = 9:

                 CONFIG_SYS_I2C_BUSES   {{0, {I2C_NULL_HOP}}, \
                                        {0, {{I2C_MUX_PCA9547, 0x70, 1}}}, \
                                        {0, {{I2C_MUX_PCA9547, 0x70, 2}}}, \
                                        {0, {{I2C_MUX_PCA9547, 0x70, 3}}}, \
                                        {0, {{I2C_MUX_PCA9547, 0x70, 4}}}, \
                                        {0, {{I2C_MUX_PCA9547, 0x70, 5}}}, \
                                        {1, {I2C_NULL_HOP}}, \
                                        {1, {{I2C_MUX_PCA9544, 0x72, 1}}}, \
                                        {1, {{I2C_MUX_PCA9544, 0x72, 2}}}, \
                                        }

                which defines
                        bus 0 on adapter 0 without a mux
                        bus 1 on adapter 0 with a PCA9547 on address 0x70 port 1
                        bus 2 on adapter 0 with a PCA9547 on address 0x70 port 2
                        bus 3 on adapter 0 with a PCA9547 on address 0x70 port 3
                        bus 4 on adapter 0 with a PCA9547 on address 0x70 port 4
                        bus 5 on adapter 0 with a PCA9547 on address 0x70 port 5
                        bus 6 on adapter 1 without a mux
                        bus 7 on adapter 1 with a PCA9544 on address 0x72 port 1
                        bus 8 on adapter 1 with a PCA9544 on address 0x72 port 2

                If you do not have i2c muxes on your board, omit this define.

- Legacy I2C Support:
                If you use the software i2c interface (CONFIG_SYS_I2C_SOFT)
                then the following macros need to be defined (examples are
                from include/configs/lwmon.h):

                I2C_INIT

                (Optional). Any commands necessary to enable the I2C
                controller or configure ports.

                eg: #define I2C_INIT (immr->im_cpm.cp_pbdir |=  PB_SCL)

                I2C_ACTIVE

                The code necessary to make the I2C data line active
                (driven).  If the data line is open collector, this
                define can be null.

                eg: #define I2C_ACTIVE (immr->im_cpm.cp_pbdir |=  PB_SDA)

                I2C_TRISTATE

                The code necessary to make the I2C data line tri-stated
                (inactive).  If the data line is open collector, this
                define can be null.

                eg: #define I2C_TRISTATE (immr->im_cpm.cp_pbdir &= ~PB_SDA)

                I2C_READ

                Code that returns true if the I2C data line is high,
                false if it is low.

                eg: #define I2C_READ ((immr->im_cpm.cp_pbdat & PB_SDA) != 0)

                I2C_SDA(bit)

                If <bit> is true, sets the I2C data line high. If it
                is false, it clears it (low).

                eg: #define I2C_SDA(bit) \
                        if(bit) immr->im_cpm.cp_pbdat |=  PB_SDA; \
                        else    immr->im_cpm.cp_pbdat &= ~PB_SDA

                I2C_SCL(bit)

                If <bit> is true, sets the I2C clock line high. If it
                is false, it clears it (low).

                eg: #define I2C_SCL(bit) \
                        if(bit) immr->im_cpm.cp_pbdat |=  PB_SCL; \
                        else    immr->im_cpm.cp_pbdat &= ~PB_SCL

                I2C_DELAY

                This delay is invoked four times per clock cycle so this
                controls the rate of data transfer.  The data rate thus
                is 1 / (I2C_DELAY * 4). Often defined to be something
                like:

                #define I2C_DELAY  udelay(2)

                CONFIG_SOFT_I2C_GPIO_SCL / CONFIG_SOFT_I2C_GPIO_SDA

                If your arch supports the generic GPIO framework (asm/gpio.h),
                then you may alternatively define the two GPIOs that are to be
                used as SCL / SDA.  Any of the previous I2C_xxx macros will
                have GPIO-based defaults assigned to them as appropriate.

                You should define these to the GPIO value as given directly to
                the generic GPIO functions.

                CONFIG_SYS_I2C_INIT_BOARD

                When a board is reset during an i2c bus transfer
                chips might think that the current transfer is still
                in progress. On some boards it is possible to access
                the i2c SCLK line directly, either by using the
                processor pin as a GPIO or by having a second pin
                connected to the bus. If this option is defined a
                custom i2c_init_board() routine in boards/xxx/board.c
                is run early in the boot sequence.

                CONFIG_I2C_MULTI_BUS

                This option allows the use of multiple I2C buses, each of which
                must have a controller.  At any point in time, only one bus is
                active.  To switch to a different bus, use the 'i2c dev' command.
                Note that bus numbering is zero-based.

                CONFIG_SYS_I2C_NOPROBES

                This option specifies a list of I2C devices that will be skipped
                when the 'i2c probe' command is issued.  If CONFIG_I2C_MULTI_BUS
                is set, specify a list of bus-device pairs.  Otherwise, specify
                a 1D array of device addresses

                e.g.
                        #undef  CONFIG_I2C_MULTI_BUS
                        #define CONFIG_SYS_I2C_NOPROBES {0x50,0x68}

                will skip addresses 0x50 and 0x68 on a board with one I2C bus

                        #define CONFIG_I2C_MULTI_BUS
                        #define CONFIG_SYS_I2C_NOPROBES {{0,0x50},{0,0x68},{1,0x54}}

                will skip addresses 0x50 and 0x68 on bus 0 and address 0x54 on bus 1

                CONFIG_SYS_SPD_BUS_NUM

                If defined, then this indicates the I2C bus number for DDR SPD.
                If not defined, then U-Boot assumes that SPD is on I2C bus 0.

                CONFIG_SYS_RTC_BUS_NUM

                If defined, then this indicates the I2C bus number for the RTC.
                If not defined, then U-Boot assumes that RTC is on I2C bus 0.

                CONFIG_SOFT_I2C_READ_REPEATED_START

                defining this will force the i2c_read() function in
                the soft_i2c driver to perform an I2C repeated start
                between writing the address pointer and reading the
                data.  If this define is omitted the default behaviour
                of doing a stop-start sequence will be used.  Most I2C
                devices can use either method, but some require one or
                the other.

- SPI Support:  CONFIG_SPI

                Enables SPI driver (so far only tested with
                SPI EEPROM, also an instance works with Crystal A/D and
                D/As on the SACSng board)

                CONFIG_SOFT_SPI

                Enables a software (bit-bang) SPI driver rather than
                using hardware support. This is a general purpose
                driver that only requires three general I/O port pins
                (two outputs, one input) to function. If this is
                defined, the board configuration must define several
                SPI configuration items (port pins to use, etc). For
                an example, see include/configs/sacsng.h.

                CONFIG_SYS_SPI_MXC_WAIT
                Timeout for waiting until spi transfer completed.
                default: (CONFIG_SYS_HZ/100)     /* 10 ms */

- FPGA Support: CONFIG_FPGA

                Enables FPGA subsystem.

                CONFIG_FPGA_<vendor>

                Enables support for specific chip vendors.
                (ALTERA, XILINX)

                CONFIG_FPGA_<family>

                Enables support for FPGA family.
                (SPARTAN2, SPARTAN3, VIRTEX2, CYCLONE2, ACEX1K, ACEX)

                CONFIG_FPGA_COUNT

                Specify the number of FPGA devices to support.

                CONFIG_SYS_FPGA_PROG_FEEDBACK

                Enable printing of hash marks during FPGA configuration.

                CONFIG_SYS_FPGA_CHECK_BUSY

                Enable checks on FPGA configuration interface busy
                status by the configuration function. This option
                will require a board or device specific function to
                be written.

                CONFIG_FPGA_DELAY

                If defined, a function that provides delays in the FPGA
                configuration driver.

                CONFIG_SYS_FPGA_CHECK_CTRLC
                Allow Control-C to interrupt FPGA configuration

                CONFIG_SYS_FPGA_CHECK_ERROR

                Check for configuration errors during FPGA bitfile
                loading. For example, abort during Virtex II
                configuration if the INIT_B line goes low (which
                indicated a CRC error).

                CONFIG_SYS_FPGA_WAIT_INIT

                Maximum time to wait for the INIT_B line to de-assert
                after PROB_B has been de-asserted during a Virtex II
                FPGA configuration sequence. The default time is 500
                ms.

                CONFIG_SYS_FPGA_WAIT_BUSY

                Maximum time to wait for BUSY to de-assert during
                Virtex II FPGA configuration. The default is 5 ms.

                CONFIG_SYS_FPGA_WAIT_CONFIG

                Time to wait after FPGA configuration. The default is
                200 ms.

- Configuration Management:

                CONFIG_IDENT_STRING

                If defined, this string will be added to the U-Boot
                version information (U_BOOT_VERSION)

- Vendor Parameter Protection:

                U-Boot considers the values of the environment
                variables "serial#" (Board Serial Number) and
                "ethaddr" (Ethernet Address) to be parameters that
                are set once by the board vendor / manufacturer, and
                protects these variables from casual modification by
                the user. Once set, these variables are read-only,
                and write or delete attempts are rejected. You can
                change this behaviour:

                If CONFIG_ENV_OVERWRITE is #defined in your config
                file, the write protection for vendor parameters is
                completely disabled. Anybody can change or delete
                these parameters.

                Alternatively, if you define _both_ an ethaddr in the
                default env _and_ CONFIG_OVERWRITE_ETHADDR_ONCE, a default
                Ethernet address is installed in the environment,
                which can be changed exactly ONCE by the user. [The
                serial# is unaffected by this, i. e. it remains
                read-only.]

                The same can be accomplished in a more flexible way
                for any variable by configuring the type of access
                to allow for those variables in the ".flags" variable
                or define CONFIG_ENV_FLAGS_LIST_STATIC.

- Protected RAM:
                CONFIG_PRAM

                Define this variable to enable the reservation of
                "protected RAM", i. e. RAM which is not overwritten
                by U-Boot. Define CONFIG_PRAM to hold the number of
                kB you want to reserve for pRAM. You can overwrite
                this default value by defining an environment
                variable "pram" to the number of kB you want to
                reserve. Note that the board info structure will
                still show the full amount of RAM. If pRAM is
                reserved, a new environment variable "mem" will
                automatically be defined to hold the amount of
                remaining RAM in a form that can be passed as boot
                argument to Linux, for instance like that:

                        setenv bootargs ... mem=\${mem}
                        saveenv

                This way you can tell Linux not to use this memory,
                either, which results in a memory region that will
                not be affected by reboots.

                *WARNING* If your board configuration uses automatic
                detection of the RAM size, you must make sure that
                this memory test is non-destructive. So far, the
                following board configurations are known to be
                "pRAM-clean":

                        IVMS8, IVML24, SPD8xx,
                        HERMES, IP860, RPXlite, LWMON,
                        FLAGADM

- Access to physical memory region (> 4GB)
                Some basic support is provided for operations on memory not
                normally accessible to U-Boot - e.g. some architectures
                support access to more than 4GB of memory on 32-bit
                machines using physical address extension or similar.
                Define CONFIG_PHYSMEM to access this basic support, which
                currently only supports clearing the memory.

- Error Recovery:
                CONFIG_NET_RETRY_COUNT

                This variable defines the number of retries for
                network operations like ARP, RARP, TFTP, or BOOTP
                before giving up the operation. If not defined, a
                default value of 5 is used.

                CONFIG_ARP_TIMEOUT

                Timeout waiting for an ARP reply in milliseconds.

                CONFIG_NFS_TIMEOUT

                Timeout in milliseconds used in NFS protocol.
                If you encounter "ERROR: Cannot umount" in nfs command,
                try longer timeout such as
                #define CONFIG_NFS_TIMEOUT 10000UL

        Note:

                In the current implementation, the local variables
                space and global environment variables space are
                separated. Local variables are those you define by
                simply typing `name=value'. To access a local
                variable later on, you have write `$name' or
                `${name}'; to execute the contents of a variable
                directly type `$name' at the command prompt.

                Global environment variables are those you use
                setenv/printenv to work with. To run a command stored
                in such a variable, you need to use the run command,
                and you must not use the '$' sign to access them.

                To store commands and special characters in a
                variable, please use double quotation marks
                surrounding the whole text of the variable, instead
                of the backslashes before semicolons and special
                symbols.

- Command Line Editing and History:
                CONFIG_CMDLINE_PS_SUPPORT

                Enable support for changing the command prompt string
                at run-time. Only static string is supported so far.
                The string is obtained from environment variables PS1
                and PS2.

- Default Environment:
                CONFIG_EXTRA_ENV_SETTINGS

                Define this to contain any number of null terminated
                strings (variable = value pairs) that will be part of
                the default environment compiled into the boot image.

                For example, place something like this in your
                board's config file:

                #define CONFIG_EXTRA_ENV_SETTINGS \
                        "myvar1=value1\0" \
                        "myvar2=value2\0"

                Warning: This method is based on knowledge about the
                internal format how the environment is stored by the
                U-Boot code. This is NOT an official, exported
                interface! Although it is unlikely that this format
                will change soon, there is no guarantee either.
                You better know what you are doing here.

                Note: overly (ab)use of the default environment is
                discouraged. Make sure to check other ways to preset
                the environment like the "source" command or the
                boot command first.

                CONFIG_DELAY_ENVIRONMENT

                Normally the environment is loaded when the board is
                initialised so that it is available to U-Boot. This inhibits
                that so that the environment is not available until
                explicitly loaded later by U-Boot code. With CONFIG_OF_CONTROL
                this is instead controlled by the value of
                /config/load-environment.

- TFTP Fixed UDP Port:
                CONFIG_TFTP_PORT

                If this is defined, the environment variable tftpsrcp
                is used to supply the TFTP UDP source port value.
                If tftpsrcp isn't defined, the normal pseudo-random port
                number generator is used.

                Also, the environment variable tftpdstp is used to supply
                the TFTP UDP destination port value.  If tftpdstp isn't
                defined, the normal port 69 is used.

                The purpose for tftpsrcp is to allow a TFTP server to
                blindly start the TFTP transfer using the pre-configured
                target IP address and UDP port. This has the effect of
                "punching through" the (Windows XP) firewall, allowing
                the remainder of the TFTP transfer to proceed normally.
                A better solution is to properly configure the firewall,
                but sometimes that is not allowed.

                CONFIG_STANDALONE_LOAD_ADDR

                This option defines a board specific value for the
                address where standalone program gets loaded, thus
                overwriting the architecture dependent default
                settings.

- Frame Buffer Address:
                CONFIG_FB_ADDR

                Define CONFIG_FB_ADDR if you want to use specific
                address for frame buffer.  This is typically the case
                when using a graphics controller has separate video
                memory.  U-Boot will then place the frame buffer at
                the given address instead of dynamically reserving it
                in system RAM by calling lcd_setmem(), which grabs
                the memory for the frame buffer depending on the
                configured panel size.

                Please see board_init_f function.

- Automatic software updates via TFTP server
                CONFIG_UPDATE_TFTP
                CONFIG_UPDATE_TFTP_CNT_MAX
                CONFIG_UPDATE_TFTP_MSEC_MAX

                These options enable and control the auto-update feature;
                for a more detailed description refer to doc/README.update.

- MTD Support (mtdparts command, UBI support)
                CONFIG_MTD_UBI_WL_THRESHOLD
                This parameter defines the maximum difference between the highest
                erase counter value and the lowest erase counter value of eraseblocks
                of UBI devices. When this threshold is exceeded, UBI starts performing
                wear leveling by means of moving data from eraseblock with low erase
                counter to eraseblocks with high erase counter.

                The default value should be OK for SLC NAND flashes, NOR flashes and
                other flashes which have eraseblock life-cycle 100000 or more.
                However, in case of MLC NAND flashes which typically have eraseblock
                life-cycle less than 10000, the threshold should be lessened (e.g.,
                to 128 or 256, although it does not have to be power of 2).

                default: 4096

                CONFIG_MTD_UBI_BEB_LIMIT
                This option specifies the maximum bad physical eraseblocks UBI
                expects on the MTD device (per 1024 eraseblocks). If the
                underlying flash does not admit of bad eraseblocks (e.g. NOR
                flash), this value is ignored.

                NAND datasheets often specify the minimum and maximum NVM
                (Number of Valid Blocks) for the flashes' endurance lifetime.
                The maximum expected bad eraseblocks per 1024 eraseblocks
                then can be calculated as "1024 * (1 - MinNVB / MaxNVB)",
                which gives 20 for most NANDs (MaxNVB is basically the total
                count of eraseblocks on the chip).

                To put it differently, if this value is 20, UBI will try to
                reserve about 1.9% of physical eraseblocks for bad blocks
                handling. And that will be 1.9% of eraseblocks on the entire
                NAND chip, not just the MTD partition UBI attaches. This means
                that if you have, say, a NAND flash chip admits maximum 40 bad
                eraseblocks, and it is split on two MTD partitions of the same
                size, UBI will reserve 40 eraseblocks when attaching a
                partition.

                default: 20

                CONFIG_MTD_UBI_FASTMAP
                Fastmap is a mechanism which allows attaching an UBI device
                in nearly constant time. Instead of scanning the whole MTD device it
                only has to locate a checkpoint (called fastmap) on the device.
                The on-flash fastmap contains all information needed to attach
                the device. Using fastmap makes only sense on large devices where
                attaching by scanning takes long. UBI will not automatically install
                a fastmap on old images, but you can set the UBI parameter
                CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT to 1 if you want so. Please note
                that fastmap-enabled images are still usable with UBI implementations
                without fastmap support. On typical flash devices the whole fastmap
                fits into one PEB. UBI will reserve PEBs to hold two fastmaps.

                CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT
                Set this parameter to enable fastmap automatically on images
                without a fastmap.
                default: 0

                CONFIG_MTD_UBI_FM_DEBUG
                Enable UBI fastmap debug
                default: 0

- SPL framework
                CONFIG_SPL
                Enable building of SPL globally.

                CONFIG_SPL_MAX_FOOTPRINT
                Maximum size in memory allocated to the SPL, BSS included.
                When defined, the linker checks that the actual memory
                used by SPL from _start to __bss_end does not exceed it.
                CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_BSS_MAX_SIZE
                must not be both defined at the same time.

                CONFIG_SPL_MAX_SIZE
                Maximum size of the SPL image (text, data, rodata, and
                linker lists sections), BSS excluded.
                When defined, the linker checks that the actual size does
                not exceed it.

                CONFIG_SPL_RELOC_TEXT_BASE
                Address to relocate to.  If unspecified, this is equal to
                CONFIG_SPL_TEXT_BASE (i.e. no relocation is done).

                CONFIG_SPL_BSS_START_ADDR
                Link address for the BSS within the SPL binary.

                CONFIG_SPL_BSS_MAX_SIZE
                Maximum size in memory allocated to the SPL BSS.
                When defined, the linker checks that the actual memory used
                by SPL from __bss_start to __bss_end does not exceed it.
                CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_BSS_MAX_SIZE
                must not be both defined at the same time.

                CONFIG_SPL_STACK
                Adress of the start of the stack SPL will use

                CONFIG_SPL_PANIC_ON_RAW_IMAGE
                When defined, SPL will panic() if the image it has
                loaded does not have a signature.
                Defining this is useful when code which loads images
                in SPL cannot guarantee that absolutely all read errors
                will be caught.
                An example is the LPC32XX MLC NAND driver, which will
                consider that a completely unreadable NAND block is bad,
                and thus should be skipped silently.

                CONFIG_SPL_RELOC_STACK
                Adress of the start of the stack SPL will use after
                relocation.  If unspecified, this is equal to
                CONFIG_SPL_STACK.

                CONFIG_SYS_SPL_MALLOC_START
                Starting address of the malloc pool used in SPL.
                When this option is set the full malloc is used in SPL and
                it is set up by spl_init() and before that, the simple malloc()
                can be used if CONFIG_SYS_MALLOC_F is defined.

                CONFIG_SYS_SPL_MALLOC_SIZE
                The size of the malloc pool used in SPL.

                CONFIG_SPL_DISPLAY_PRINT
                For ARM, enable an optional function to print more information
                about the running system.

                CONFIG_SPL_INIT_MINIMAL
                Arch init code should be built for a very small image

                CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION
                Partition on the MMC to load U-Boot from when the MMC is being
                used in raw mode

                CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR
                Sector to load kernel uImage from when MMC is being
                used in raw mode (for Falcon mode)

                CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTOR,
                CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTORS
                Sector and number of sectors to load kernel argument
                parameters from when MMC is being used in raw mode
                (for falcon mode)

                CONFIG_SPL_FS_LOAD_PAYLOAD_NAME
                Filename to read to load U-Boot when reading from filesystem

                CONFIG_SPL_FS_LOAD_KERNEL_NAME
                Filename to read to load kernel uImage when reading
                from filesystem (for Falcon mode)

                CONFIG_SPL_FS_LOAD_ARGS_NAME
                Filename to read to load kernel argument parameters
                when reading from filesystem (for Falcon mode)

                CONFIG_SPL_MPC83XX_WAIT_FOR_NAND
                Set this for NAND SPL on PPC mpc83xx targets, so that
                start.S waits for the rest of the SPL to load before
                continuing (the hardware starts execution after just
                loading the first page rather than the full 4K).

                CONFIG_SPL_SKIP_RELOCATE
                Avoid SPL relocation

                CONFIG_SPL_NAND_IDENT
                SPL uses the chip ID list to identify the NAND flash.
                Requires CONFIG_SPL_NAND_BASE.

                CONFIG_SPL_UBI
                Support for a lightweight UBI (fastmap) scanner and
                loader

                CONFIG_SPL_NAND_RAW_ONLY
                Support to boot only raw u-boot.bin images. Use this only
                if you need to save space.

                CONFIG_SPL_COMMON_INIT_DDR
                Set for common ddr init with serial presence detect in
                SPL binary.

                CONFIG_SYS_NAND_5_ADDR_CYCLE, CONFIG_SYS_NAND_PAGE_COUNT,
                CONFIG_SYS_NAND_PAGE_SIZE, CONFIG_SYS_NAND_OOBSIZE,
                CONFIG_SYS_NAND_BLOCK_SIZE, CONFIG_SYS_NAND_BAD_BLOCK_POS,
                CONFIG_SYS_NAND_ECCPOS, CONFIG_SYS_NAND_ECCSIZE,
                CONFIG_SYS_NAND_ECCBYTES
                Defines the size and behavior of the NAND that SPL uses
                to read U-Boot

                CONFIG_SYS_NAND_U_BOOT_DST
                Location in memory to load U-Boot to

                CONFIG_SYS_NAND_U_BOOT_SIZE
                Size of image to load

                CONFIG_SYS_NAND_U_BOOT_START
                Entry point in loaded image to jump to

                CONFIG_SYS_NAND_HW_ECC_OOBFIRST
                Define this if you need to first read the OOB and then the
                data. This is used, for example, on davinci platforms.

                CONFIG_SPL_RAM_DEVICE
                Support for running image already present in ram, in SPL binary

                CONFIG_SPL_PAD_TO
                Image offset to which the SPL should be padded before appending
                the SPL payload. By default, this is defined as
                CONFIG_SPL_MAX_SIZE, or 0 if CONFIG_SPL_MAX_SIZE is undefined.
                CONFIG_SPL_PAD_TO must be either 0, meaning to append the SPL
                payload without any padding, or >= CONFIG_SPL_MAX_SIZE.

                CONFIG_SPL_TARGET
                Final target image containing SPL and payload.  Some SPLs
                use an arch-specific makefile fragment instead, for
                example if more than one image needs to be produced.

                CONFIG_SPL_FIT_PRINT
                Printing information about a FIT image adds quite a bit of
                code to SPL. So this is normally disabled in SPL. Use this
                option to re-enable it. This will affect the output of the
                bootm command when booting a FIT image.

- TPL framework
                CONFIG_TPL
                Enable building of TPL globally.

                CONFIG_TPL_PAD_TO
                Image offset to which the TPL should be padded before appending
                the TPL payload. By default, this is defined as
                CONFIG_SPL_MAX_SIZE, or 0 if CONFIG_SPL_MAX_SIZE is undefined.
                CONFIG_SPL_PAD_TO must be either 0, meaning to append the SPL
                payload without any padding, or >= CONFIG_SPL_MAX_SIZE.

- Interrupt support (PPC):

                There are common interrupt_init() and timer_interrupt()
                for all PPC archs. interrupt_init() calls interrupt_init_cpu()
                for CPU specific initialization. interrupt_init_cpu()
                should set decrementer_count to appropriate value. If
                CPU resets decrementer automatically after interrupt
                (ppc4xx) it should set decrementer_count to zero.
                timer_interrupt() calls timer_interrupt_cpu() for CPU
                specific handling. If board has watchdog / status_led
                / other_activity_monitor it works automatically from
                general timer_interrupt().


Board initialization settings:
------------------------------

During Initialization u-boot calls a number of board specific functions
to allow the preparation of board specific prerequisites, e.g. pin setup
before drivers are initialized. To enable these callbacks the
following configuration macros have to be defined. Currently this is
architecture specific, so please check arch/your_architecture/lib/board.c
typically in board_init_f() and board_init_r().

- CONFIG_BOARD_EARLY_INIT_F: Call board_early_init_f()
- CONFIG_BOARD_EARLY_INIT_R: Call board_early_init_r()
- CONFIG_BOARD_LATE_INIT: Call board_late_init()
- CONFIG_BOARD_POSTCLK_INIT: Call board_postclk_init()

Configuration Settings:
-----------------------

- MEM_SUPPORT_64BIT_DATA: Defined automatically if compiled as 64-bit.
                Optionally it can be defined to support 64-bit memory commands.

- CONFIG_SYS_LONGHELP: Defined when you want long help messages included;
                undefine this when you're short of memory.

- CONFIG_SYS_HELP_CMD_WIDTH: Defined when you want to override the default
                width of the commands listed in the 'help' command output.

- CONFIG_SYS_PROMPT:    This is what U-Boot prints on the console to
                prompt for user input.

- CONFIG_SYS_CBSIZE:    Buffer size for input from the Console

- CONFIG_SYS_PBSIZE:    Buffer size for Console output

- CONFIG_SYS_MAXARGS:   max. Number of arguments accepted for monitor commands

- CONFIG_SYS_BARGSIZE: Buffer size for Boot Arguments which are passed to
                the application (usually a Linux kernel) when it is
                booted

- CONFIG_SYS_BAUDRATE_TABLE:
                List of legal baudrate settings for this board.

- CONFIG_SYS_MEM_RESERVE_SECURE
                Only implemented for ARMv8 for now.
                If defined, the size of CONFIG_SYS_MEM_RESERVE_SECURE memory
                is substracted from total RAM and won't be reported to OS.
                This memory can be used as secure memory. A variable
                gd->arch.secure_ram is used to track the location. In systems
                the RAM base is not zero, or RAM is divided into banks,
                this variable needs to be recalcuated to get the address.

- CONFIG_SYS_MEM_TOP_HIDE:
                If CONFIG_SYS_MEM_TOP_HIDE is defined in the board config header,
                this specified memory area will get subtracted from the top
                (end) of RAM and won't get "touched" at all by U-Boot. By
                fixing up gd->ram_size the Linux kernel should gets passed
                the now "corrected" memory size and won't touch it either.
                This should work for arch/ppc and arch/powerpc. Only Linux
                board ports in arch/powerpc with bootwrapper support that
                recalculate the memory size from the SDRAM controller setup
                will have to get fixed in Linux additionally.

                This option can be used as a workaround for the 440EPx/GRx
                CHIP 11 errata where the last 256 bytes in SDRAM shouldn't
                be touched.

                WARNING: Please make sure that this value is a multiple of
                the Linux page size (normally 4k). If this is not the case,
                then the end address of the Linux memory will be located at a
                non page size aligned address and this could cause major
                problems.

- CONFIG_SYS_LOADS_BAUD_CHANGE:
                Enable temporary baudrate change while serial download

- CONFIG_SYS_SDRAM_BASE:
                Physical start address of SDRAM. _Must_ be 0 here.

- CONFIG_SYS_FLASH_BASE:
                Physical start address of Flash memory.

- CONFIG_SYS_MONITOR_BASE:
                Physical start address of boot monitor code (set by
                make config files to be same as the text base address
                (CONFIG_SYS_TEXT_BASE) used when linking) - same as
                CONFIG_SYS_FLASH_BASE when booting from flash.

- CONFIG_SYS_MONITOR_LEN:
                Size of memory reserved for monitor code, used to
                determine _at_compile_time_ (!) if the environment is
                embedded within the U-Boot image, or in a separate
                flash sector.

- CONFIG_SYS_MALLOC_LEN:
                Size of DRAM reserved for malloc() use.

- CONFIG_SYS_MALLOC_F_LEN
                Size of the malloc() pool for use before relocation. If
                this is defined, then a very simple malloc() implementation
                will become available before relocation. The address is just
                below the global data, and the stack is moved down to make
                space.

                This feature allocates regions with increasing addresses
                within the region. calloc() is supported, but realloc()
                is not available. free() is supported but does nothing.
                The memory will be freed (or in fact just forgotten) when
                U-Boot relocates itself.

- CONFIG_SYS_MALLOC_SIMPLE
                Provides a simple and small malloc() and calloc() for those
                boards which do not use the full malloc in SPL (which is
                enabled with CONFIG_SYS_SPL_MALLOC_START).

- CONFIG_SYS_NONCACHED_MEMORY:
                Size of non-cached memory area. This area of memory will be
                typically located right below the malloc() area and mapped
                uncached in the MMU. This is useful for drivers that would
                otherwise require a lot of explicit cache maintenance. For
                some drivers it's also impossible to properly maintain the
                cache. For example if the regions that need to be flushed
                are not a multiple of the cache-line size, *and* padding
                cannot be allocated between the regions to align them (i.e.
                if the HW requires a contiguous array of regions, and the
                size of each region is not cache-aligned), then a flush of
                one region may result in overwriting data that hardware has
                written to another region in the same cache-line. This can
                happen for example in network drivers where descriptors for
                buffers are typically smaller than the CPU cache-line (e.g.
                16 bytes vs. 32 or 64 bytes).

                Non-cached memory is only supported on 32-bit ARM at present.

- CONFIG_SYS_BOOTM_LEN:
                Normally compressed uImages are limited to an
                uncompressed size of 8 MBytes. If this is not enough,
                you can define CONFIG_SYS_BOOTM_LEN in your board config file
                to adjust this setting to your needs.

- CONFIG_SYS_BOOTMAPSZ:
                Maximum size of memory mapped by the startup code of
                the Linux kernel; all data that must be processed by
                the Linux kernel (bd_info, boot arguments, FDT blob if
                used) must be put below this limit, unless "bootm_low"
                environment variable is defined and non-zero. In such case
                all data for the Linux kernel must be between "bootm_low"
                and "bootm_low" + CONFIG_SYS_BOOTMAPSZ.  The environment
                variable "bootm_mapsize" will override the value of
                CONFIG_SYS_BOOTMAPSZ.  If CONFIG_SYS_BOOTMAPSZ is undefined,
                then the value in "bootm_size" will be used instead.

- CONFIG_SYS_BOOT_RAMDISK_HIGH:
                Enable initrd_high functionality.  If defined then the
                initrd_high feature is enabled and the bootm ramdisk subcommand
                is enabled.

- CONFIG_SYS_BOOT_GET_CMDLINE:
                Enables allocating and saving kernel cmdline in space between
                "bootm_low" and "bootm_low" + BOOTMAPSZ.

- CONFIG_SYS_BOOT_GET_KBD:
                Enables allocating and saving a kernel copy of the bd_info in
                space between "bootm_low" and "bootm_low" + BOOTMAPSZ.

- CONFIG_SYS_MAX_FLASH_BANKS:
                Max number of Flash memory banks

- CONFIG_SYS_MAX_FLASH_SECT:
                Max number of sectors on a Flash chip

- CONFIG_SYS_FLASH_ERASE_TOUT:
                Timeout for Flash erase operations (in ms)

- CONFIG_SYS_FLASH_WRITE_TOUT:
                Timeout for Flash write operations (in ms)

- CONFIG_SYS_FLASH_LOCK_TOUT
                Timeout for Flash set sector lock bit operation (in ms)

- CONFIG_SYS_FLASH_UNLOCK_TOUT
                Timeout for Flash clear lock bits operation (in ms)

- CONFIG_SYS_FLASH_PROTECTION
                If defined, hardware flash sectors protection is used
                instead of U-Boot software protection.

- CONFIG_SYS_DIRECT_FLASH_TFTP:

                Enable TFTP transfers directly to flash memory;
                without this option such a download has to be
                performed in two steps: (1) download to RAM, and (2)
                copy from RAM to flash.

                The two-step approach is usually more reliable, since
                you can check if the download worked before you erase
                the flash, but in some situations (when system RAM is
                too limited to allow for a temporary copy of the
                downloaded image) this option may be very useful.

- CONFIG_SYS_FLASH_CFI:
                Define if the flash driver uses extra elements in the
                common flash structure for storing flash geometry.

- CONFIG_FLASH_CFI_DRIVER
                This option also enables the building of the cfi_flash driver
                in the drivers directory

- CONFIG_FLASH_CFI_MTD
                This option enables the building of the cfi_mtd driver
                in the drivers directory. The driver exports CFI flash
                to the MTD layer.

- CONFIG_SYS_FLASH_USE_BUFFER_WRITE
                Use buffered writes to flash.

- CONFIG_FLASH_SPANSION_S29WS_N
                s29ws-n MirrorBit flash has non-standard addresses for buffered
                write commands.

- CONFIG_SYS_FLASH_QUIET_TEST
                If this option is defined, the common CFI flash doesn't
                print it's warning upon not recognized FLASH banks. This
                is useful, if some of the configured banks are only
                optionally available.

- CONFIG_FLASH_SHOW_PROGRESS
                If defined (must be an integer), print out countdown
                digits and dots.  Recommended value: 45 (9..1) for 80
                column displays, 15 (3..1) for 40 column displays.

- CONFIG_FLASH_VERIFY
                If defined, the content of the flash (destination) is compared
                against the source after the write operation. An error message
                will be printed when the contents are not identical.
                Please note that this option is useless in nearly all cases,
                since such flash programming errors usually are detected earlier
                while unprotecting/erasing/programming. Please only enable
                this option if you really know what you are doing.

- CONFIG_SYS_RX_ETH_BUFFER:
                Defines the number of Ethernet receive buffers. On some
                Ethernet controllers it is recommended to set this value
                to 8 or even higher (EEPRO100 or 405 EMAC), since all
                buffers can be full shortly after enabling the interface
                on high Ethernet traffic.
                Defaults to 4 if not defined.

- CONFIG_ENV_MAX_ENTRIES

        Maximum number of entries in the hash table that is used
        internally to store the environment settings. The default
        setting is supposed to be generous and should work in most
        cases. This setting can be used to tune behaviour; see
        lib/hashtable.c for details.

- CONFIG_ENV_FLAGS_LIST_DEFAULT
- CONFIG_ENV_FLAGS_LIST_STATIC
        Enable validation of the values given to environment variables when
        calling env set.  Variables can be restricted to only decimal,
        hexadecimal, or boolean.  If CONFIG_CMD_NET is also defined,
        the variables can also be restricted to IP address or MAC address.

        The format of the list is:
                type_attribute = [s|d|x|b|i|m]
                access_attribute = [a|r|o|c]
                attributes = type_attribute[access_attribute]
                entry = variable_name[:attributes]
                list = entry[,list]

        The type attributes are:
                s - String (default)
                d - Decimal
                x - Hexadecimal
                b - Boolean ([1yYtT|0nNfF])
                i - IP address
                m - MAC address

        The access attributes are:
                a - Any (default)
                r - Read-only
                o - Write-once
                c - Change-default

        - CONFIG_ENV_FLAGS_LIST_DEFAULT
                Define this to a list (string) to define the ".flags"
                environment variable in the default or embedded environment.

        - CONFIG_ENV_FLAGS_LIST_STATIC
                Define this to a list (string) to define validation that
                should be done if an entry is not found in the ".flags"
                environment variable.  To override a setting in the static
                list, simply add an entry for the same variable name to the
                ".flags" variable.

        If CONFIG_REGEX is defined, the variable_name above is evaluated as a
        regular expression. This allows multiple variables to define the same
        flags without explicitly listing them for each variable.

The following definitions that deal with the placement and management
of environment data (variable area); in general, we support the
following configurations:

- CONFIG_BUILD_ENVCRC:

        Builds up envcrc with the target environment so that external utils
        may easily extract it and embed it in final U-Boot images.

BE CAREFUL! The first access to the environment happens quite early
in U-Boot initialization (when we try to get the setting of for the
console baudrate). You *MUST* have mapped your NVRAM area then, or
U-Boot will hang.

Please note that even with NVRAM we still use a copy of the
environment in RAM: we could work on NVRAM directly, but we want to
keep settings there always unmodified except somebody uses "saveenv"
to save the current settings.

BE CAREFUL! For some special cases, the local device can not use
"saveenv" command. For example, the local device will get the
environment stored in a remote NOR flash by SRIO or PCIE link,
but it can not erase, write this NOR flash by SRIO or PCIE interface.

- CONFIG_NAND_ENV_DST

        Defines address in RAM to which the nand_spl code should copy the
        environment. If redundant environment is used, it will be copied to
        CONFIG_NAND_ENV_DST + CONFIG_ENV_SIZE.

Please note that the environment is read-only until the monitor
has been relocated to RAM and a RAM copy of the environment has been
created; also, when using EEPROM you will have to use env_get_f()
until then to read environment variables.

The environment is protected by a CRC32 checksum. Before the monitor
is relocated into RAM, as a result of a bad CRC you will be working
with the compiled-in default environment - *silently*!!! [This is
necessary, because the first environment variable we need is the
"baudrate" setting for the console - if we have a bad CRC, we don't
have any device yet where we could complain.]

Note: once the monitor has been relocated, then it will complain if
the default environment is used; a new CRC is computed as soon as you
use the "saveenv" command to store a valid environment.

- CONFIG_SYS_FAULT_ECHO_LINK_DOWN:
                Echo the inverted Ethernet link state to the fault LED.

                Note: If this option is active, then CONFIG_SYS_FAULT_MII_ADDR
                      also needs to be defined.

- CONFIG_SYS_FAULT_MII_ADDR:
                MII address of the PHY to check for the Ethernet link state.

- CONFIG_NS16550_MIN_FUNCTIONS:
                Define this if you desire to only have use of the NS16550_init
                and NS16550_putc functions for the serial driver located at
                drivers/serial/ns16550.c.  This option is useful for saving
                space for already greatly restricted images, including but not
                limited to NAND_SPL configurations.

- CONFIG_DISPLAY_BOARDINFO
                Display information about the board that U-Boot is running on
                when U-Boot starts up. The board function checkboard() is called
                to do this.

- CONFIG_DISPLAY_BOARDINFO_LATE
                Similar to the previous option, but display this information
                later, once stdio is running and output goes to the LCD, if
                present.

- CONFIG_BOARD_SIZE_LIMIT:
                Maximum size of the U-Boot image. When defined, the
                build system checks that the actual size does not
                exceed it.

Low Level (hardware related) configuration options:
---------------------------------------------------

- CONFIG_SYS_CACHELINE_SIZE:
                Cache Line Size of the CPU.

- CONFIG_SYS_CCSRBAR_DEFAULT:
                Default (power-on reset) physical address of CCSR on Freescale
                PowerPC SOCs.

- CONFIG_SYS_CCSRBAR:
                Virtual address of CCSR.  On a 32-bit build, this is typically
                the same value as CONFIG_SYS_CCSRBAR_DEFAULT.

- CONFIG_SYS_CCSRBAR_PHYS:
                Physical address of CCSR.  CCSR can be relocated to a new
                physical address, if desired.  In this case, this macro should
                be set to that address.  Otherwise, it should be set to the
                same value as CONFIG_SYS_CCSRBAR_DEFAULT.  For example, CCSR
                is typically relocated on 36-bit builds.  It is recommended
                that this macro be defined via the _HIGH and _LOW macros:

                #define CONFIG_SYS_CCSRBAR_PHYS ((CONFIG_SYS_CCSRBAR_PHYS_HIGH
                        * 1ull) << 32 | CONFIG_SYS_CCSRBAR_PHYS_LOW)

- CONFIG_SYS_CCSRBAR_PHYS_HIGH:
                Bits 33-36 of CONFIG_SYS_CCSRBAR_PHYS.  This value is typically
                either 0 (32-bit build) or 0xF (36-bit build).  This macro is
                used in assembly code, so it must not contain typecasts or
                integer size suffixes (e.g. "ULL").

- CONFIG_SYS_CCSRBAR_PHYS_LOW:
                Lower 32-bits of CONFIG_SYS_CCSRBAR_PHYS.  This macro is
                used in assembly code, so it must not contain typecasts or
                integer size suffixes (e.g. "ULL").

- CONFIG_SYS_CCSR_DO_NOT_RELOCATE:
                If this macro is defined, then CONFIG_SYS_CCSRBAR_PHYS will be
                forced to a value that ensures that CCSR is not relocated.

- CONFIG_IDE_AHB:
                Most IDE controllers were designed to be connected with PCI
                interface. Only few of them were designed for AHB interface.
                When software is doing ATA command and data transfer to
                IDE devices through IDE-AHB controller, some additional
                registers accessing to these kind of IDE-AHB controller
                is required.

- CONFIG_SYS_IMMR:      Physical address of the Internal Memory.
                DO NOT CHANGE unless you know exactly what you're
                doing! (11-4) [MPC8xx systems only]

- CONFIG_SYS_INIT_RAM_ADDR:

                Start address of memory area that can be used for
                initial data and stack; please note that this must be
                writable memory that is working WITHOUT special
                initialization, i. e. you CANNOT use normal RAM which
                will become available only after programming the
                memory controller and running certain initialization
                sequences.

                U-Boot uses the following memory types:
                - MPC8xx: IMMR (internal memory of the CPU)

- CONFIG_SYS_GBL_DATA_OFFSET:

                Offset of the initial data structure in the memory
                area defined by CONFIG_SYS_INIT_RAM_ADDR. Usually
                CONFIG_SYS_GBL_DATA_OFFSET is chosen such that the initial
                data is located at the end of the available space
                (sometimes written as (CONFIG_SYS_INIT_RAM_SIZE -
                GENERATED_GBL_DATA_SIZE), and the initial stack is just
                below that area (growing from (CONFIG_SYS_INIT_RAM_ADDR +
                CONFIG_SYS_GBL_DATA_OFFSET) downward.

        Note:
                On the MPC824X (or other systems that use the data
                cache for initial memory) the address chosen for
                CONFIG_SYS_INIT_RAM_ADDR is basically arbitrary - it must
                point to an otherwise UNUSED address space between
                the top of RAM and the start of the PCI space.

- CONFIG_SYS_SCCR:      System Clock and reset Control Register (15-27)

- CONFIG_SYS_OR_TIMING_SDRAM:
                SDRAM timing

- CONFIG_SYS_MAMR_PTA:
                periodic timer for refresh

- FLASH_BASE0_PRELIM, FLASH_BASE1_PRELIM, CONFIG_SYS_REMAP_OR_AM,
  CONFIG_SYS_PRELIM_OR_AM, CONFIG_SYS_OR_TIMING_FLASH, CONFIG_SYS_OR0_REMAP,
  CONFIG_SYS_OR0_PRELIM, CONFIG_SYS_BR0_PRELIM, CONFIG_SYS_OR1_REMAP, CONFIG_SYS_OR1_PRELIM,
  CONFIG_SYS_BR1_PRELIM:
                Memory Controller Definitions: BR0/1 and OR0/1 (FLASH)

- SDRAM_BASE2_PRELIM, SDRAM_BASE3_PRELIM, SDRAM_MAX_SIZE,
  CONFIG_SYS_OR_TIMING_SDRAM, CONFIG_SYS_OR2_PRELIM, CONFIG_SYS_BR2_PRELIM,
  CONFIG_SYS_OR3_PRELIM, CONFIG_SYS_BR3_PRELIM:
                Memory Controller Definitions: BR2/3 and OR2/3 (SDRAM)

- CONFIG_SYS_SRIO:
                Chip has SRIO or not

- CONFIG_SRIO1:
                Board has SRIO 1 port available

- CONFIG_SRIO2:
                Board has SRIO 2 port available

- CONFIG_SRIO_PCIE_BOOT_MASTER
                Board can support master function for Boot from SRIO and PCIE

- CONFIG_SYS_SRIOn_MEM_VIRT:
                Virtual Address of SRIO port 'n' memory region

- CONFIG_SYS_SRIOn_MEM_PHYxS:
                Physical Address of SRIO port 'n' memory region

- CONFIG_SYS_SRIOn_MEM_SIZE:
                Size of SRIO port 'n' memory region

- CONFIG_SYS_NAND_BUSWIDTH_16BIT
                Defined to tell the NAND controller that the NAND chip is using
                a 16 bit bus.
                Not all NAND drivers use this symbol.
                Example of drivers that use it:
                - drivers/mtd/nand/raw/ndfc.c
                - drivers/mtd/nand/raw/mxc_nand.c

- CONFIG_SYS_NDFC_EBC0_CFG
                Sets the EBC0_CFG register for the NDFC. If not defined
                a default value will be used.

- CONFIG_SPD_EEPROM
                Get DDR timing information from an I2C EEPROM. Common
                with pluggable memory modules such as SODIMMs

  SPD_EEPROM_ADDRESS
                I2C address of the SPD EEPROM

- CONFIG_SYS_SPD_BUS_NUM
                If SPD EEPROM is on an I2C bus other than the first
                one, specify here. Note that the value must resolve
                to something your driver can deal with.

- CONFIG_SYS_DDR_RAW_TIMING
                Get DDR timing information from other than SPD. Common with
                soldered DDR chips onboard without SPD. DDR raw timing
                parameters are extracted from datasheet and hard-coded into
                header files or board specific files.

- CONFIG_FSL_DDR_INTERACTIVE
                Enable interactive DDR debugging. See doc/README.fsl-ddr.

- CONFIG_FSL_DDR_SYNC_REFRESH
                Enable sync of refresh for multiple controllers.

- CONFIG_FSL_DDR_BIST
                Enable built-in memory test for Freescale DDR controllers.

- CONFIG_SYS_83XX_DDR_USES_CS0
                Only for 83xx systems. If specified, then DDR should
                be configured using CS0 and CS1 instead of CS2 and CS3.

- CONFIG_RMII
                Enable RMII mode for all FECs.
                Note that this is a global option, we can't
                have one FEC in standard MII mode and another in RMII mode.

- CONFIG_CRC32_VERIFY
                Add a verify option to the crc32 command.
                The syntax is:

                => crc32 -v <address> <count> <crc32>

                Where address/count indicate a memory area
                and crc32 is the correct crc32 which the
                area should have.

- CONFIG_LOOPW
                Add the "loopw" memory command. This only takes effect if
                the memory commands are activated globally (CONFIG_CMD_MEMORY).

- CONFIG_CMD_MX_CYCLIC
                Add the "mdc" and "mwc" memory commands. These are cyclic
                "md/mw" commands.
                Examples:

                => mdc.b 10 4 500
                This command will print 4 bytes (10,11,12,13) each 500 ms.

                => mwc.l 100 12345678 10
                This command will write 12345678 to address 100 all 10 ms.

                This only takes effect if the memory commands are activated
                globally (CONFIG_CMD_MEMORY).

- CONFIG_SPL_BUILD
                Set when the currently-running compilation is for an artifact
                that will end up in the SPL (as opposed to the TPL or U-Boot
                proper). Code that needs stage-specific behavior should check
                this.

- CONFIG_TPL_BUILD
                Set when the currently-running compilation is for an artifact
                that will end up in the TPL (as opposed to the SPL or U-Boot
                proper). Code that needs stage-specific behavior should check
                this.

- CONFIG_SYS_MPC85XX_NO_RESETVEC
                Only for 85xx systems. If this variable is specified, the section
                .resetvec is not kept and the section .bootpg is placed in the
                previous 4k of the .text section.

- CONFIG_ARCH_MAP_SYSMEM
                Generally U-Boot (and in particular the md command) uses
                effective address. It is therefore not necessary to regard
                U-Boot address as virtual addresses that need to be translated
                to physical addresses. However, sandbox requires this, since
                it maintains its own little RAM buffer which contains all
                addressable memory. This option causes some memory accesses
                to be mapped through map_sysmem() / unmap_sysmem().

- CONFIG_X86_RESET_VECTOR
                If defined, the x86 reset vector code is included. This is not
                needed when U-Boot is running from Coreboot.

- CONFIG_SYS_NAND_NO_SUBPAGE_WRITE
                Option to disable subpage write in NAND driver
                driver that uses this:
                drivers/mtd/nand/raw/davinci_nand.c

Freescale QE/FMAN Firmware Support:
-----------------------------------

The Freescale QUICCEngine (QE) and Frame Manager (FMAN) both support the
loading of "firmware", which is encoded in the QE firmware binary format.
This firmware often needs to be loaded during U-Boot booting, so macros
are used to identify the storage device (NOR flash, SPI, etc) and the address
within that device.

- CONFIG_SYS_FMAN_FW_ADDR
        The address in the storage device where the FMAN microcode is located.  The
        meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
        is also specified.

- CONFIG_SYS_QE_FW_ADDR
        The address in the storage device where the QE microcode is located.  The
        meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
        is also specified.

- CONFIG_SYS_QE_FMAN_FW_LENGTH
        The maximum possible size of the firmware.  The firmware binary format
        has a field that specifies the actual size of the firmware, but it
        might not be possible to read any part of the firmware unless some
        local storage is allocated to hold the entire firmware first.

- CONFIG_SYS_QE_FMAN_FW_IN_NOR
        Specifies that QE/FMAN firmware is located in NOR flash, mapped as
        normal addressable memory via the LBC.  CONFIG_SYS_FMAN_FW_ADDR is the
        virtual address in NOR flash.

- CONFIG_SYS_QE_FMAN_FW_IN_NAND
        Specifies that QE/FMAN firmware is located in NAND flash.
        CONFIG_SYS_FMAN_FW_ADDR is the offset within NAND flash.

- CONFIG_SYS_QE_FMAN_FW_IN_MMC
        Specifies that QE/FMAN firmware is located on the primary SD/MMC
        device.  CONFIG_SYS_FMAN_FW_ADDR is the byte offset on that device.

- CONFIG_SYS_QE_FMAN_FW_IN_REMOTE
        Specifies that QE/FMAN firmware is located in the remote (master)
        memory space.   CONFIG_SYS_FMAN_FW_ADDR is a virtual address which
        can be mapped from slave TLB->slave LAW->slave SRIO or PCIE outbound
        window->master inbound window->master LAW->the ucode address in
        master's memory space.

Freescale Layerscape Management Complex Firmware Support:
---------------------------------------------------------
The Freescale Layerscape Management Complex (MC) supports the loading of
"firmware".
This firmware often needs to be loaded during U-Boot booting, so macros
are used to identify the storage device (NOR flash, SPI, etc) and the address
within that device.

- CONFIG_FSL_MC_ENET
        Enable the MC driver for Layerscape SoCs.

Freescale Layerscape Debug Server Support:
-------------------------------------------
The Freescale Layerscape Debug Server Support supports the loading of
"Debug Server firmware" and triggering SP boot-rom.
This firmware often needs to be loaded during U-Boot booting.

- CONFIG_SYS_MC_RSV_MEM_ALIGN
        Define alignment of reserved memory MC requires

Reproducible builds
-------------------

In order to achieve reproducible builds, timestamps used in the U-Boot build
process have to be set to a fixed value.

This is done using the SOURCE_DATE_EPOCH environment variable.
SOURCE_DATE_EPOCH is to be set on the build host's shell, not as a configuration
option for U-Boot or an environment variable in U-Boot.

SOURCE_DATE_EPOCH should be set to a number of seconds since the epoch, in UTC.

Building the Software:
======================

Building U-Boot has been tested in several native build environments
and in many different cross environments. Of course we cannot support
all possibly existing versions of cross development tools in all
(potentially obsolete) versions. In case of tool chain problems we
recommend to use the ELDK (see https://www.denx.de/wiki/DULG/ELDK)
which is extensively used to build and test U-Boot.

If you are not using a native environment, it is assumed that you
have GNU cross compiling tools available in your path. In this case,
you must set the environment variable CROSS_COMPILE in your shell.
Note that no changes to the Makefile or any other source files are
necessary. For example using the ELDK on a 4xx CPU, please enter:

        $ CROSS_COMPILE=ppc_4xx-
        $ export CROSS_COMPILE

U-Boot is intended to be simple to build. After installing the
sources you must configure U-Boot for one specific board type. This
is done by typing:

        make NAME_defconfig

where "NAME_defconfig" is the name of one of the existing configu-
rations; see configs/*_defconfig for supported names.

Note: for some boards special configuration names may exist; check if
      additional information is available from the board vendor; for
      instance, the TQM823L systems are available without (standard)
      or with LCD support. You can select such additional "features"
      when choosing the configuration, i. e.

      make TQM823L_defconfig
        - will configure for a plain TQM823L, i. e. no LCD support

      make TQM823L_LCD_defconfig
        - will configure for a TQM823L with U-Boot console on LCD

      etc.


Finally, type "make all", and you should get some working U-Boot
images ready for download to / installation on your system:

- "u-boot.bin" is a raw binary image
- "u-boot" is an image in ELF binary format
- "u-boot.srec" is in Motorola S-Record format

By default the build is performed locally and the objects are saved
in the source directory. One of the two methods can be used to change
this behavior and build U-Boot to some external directory:

1. Add O= to the make command line invocations:

        make O=/tmp/build distclean
        make O=/tmp/build NAME_defconfig
        make O=/tmp/build all

2. Set environment variable KBUILD_OUTPUT to point to the desired location:

        export KBUILD_OUTPUT=/tmp/build
        make distclean
        make NAME_defconfig
        make all

Note that the command line "O=" setting overrides the KBUILD_OUTPUT environment
variable.

User specific CPPFLAGS, AFLAGS and CFLAGS can be passed to the compiler by
setting the according environment variables KCPPFLAGS, KAFLAGS and KCFLAGS.
For example to treat all compiler warnings as errors:

        make KCFLAGS=-Werror

Please be aware that the Makefiles assume you are using GNU make, so
for instance on NetBSD you might need to use "gmake" instead of
native "make".


If the system board that you have is not listed, then you will need
to port U-Boot to your hardware platform. To do this, follow these
steps:

1.  Create a new directory to hold your board specific code. Add any
    files you need. In your board directory, you will need at least
    the "Makefile" and a "<board>.c".
2.  Create a new configuration file "include/configs/<board>.h" for
    your board.
3.  If you're porting U-Boot to a new CPU, then also create a new
    directory to hold your CPU specific code. Add any files you need.
4.  Run "make <board>_defconfig" with your new name.
5.  Type "make", and you should get a working "u-boot.srec" file
    to be installed on your target system.
6.  Debug and solve any problems that might arise.
    [Of course, this last step is much harder than it sounds.]


Testing of U-Boot Modifications, Ports to New Hardware, etc.:
==============================================================

If you have modified U-Boot sources (for instance added a new board
or support for new devices, a new CPU, etc.) you are expected to
provide feedback to the other developers. The feedback normally takes
the form of a "patch", i.e. a context diff against a certain (latest
official or latest in the git repository) version of U-Boot sources.

But before you submit such a patch, please verify that your modifi-
cation did not break existing code. At least make sure that *ALL* of
the supported boards compile WITHOUT ANY compiler warnings. To do so,
just run the buildman script (tools/buildman/buildman), which will
configure and build U-Boot for ALL supported system. Be warned, this
will take a while. Please see the buildman README, or run 'buildman -H'
for documentation.


See also "U-Boot Porting Guide" below.


Monitor Commands - Overview:
============================

go      - start application at address 'addr'
run     - run commands in an environment variable
bootm   - boot application image from memory
bootp   - boot image via network using BootP/TFTP protocol
bootz   - boot zImage from memory
tftpboot- boot image via network using TFTP protocol
               and env variables "ipaddr" and "serverip"
               (and eventually "gatewayip")
tftpput - upload a file via network using TFTP protocol
rarpboot- boot image via network using RARP/TFTP protocol
diskboot- boot from IDE devicebootd   - boot default, i.e., run 'bootcmd'
loads   - load S-Record file over serial line
loadb   - load binary file over serial line (kermit mode)
md      - memory display
mm      - memory modify (auto-incrementing)
nm      - memory modify (constant address)
mw      - memory write (fill)
ms      - memory search
cp      - memory copy
cmp     - memory compare
crc32   - checksum calculation
i2c     - I2C sub-system
sspi    - SPI utility commands
base    - print or set address offset
printenv- print environment variables
pwm     - control pwm channels
setenv  - set environment variables
saveenv - save environment variables to persistent storage
protect - enable or disable FLASH write protection
erase   - erase FLASH memory
flinfo  - print FLASH memory information
nand    - NAND memory operations (see doc/README.nand)
bdinfo  - print Board Info structure
iminfo  - print header information for application image
coninfo - print console devices and informations
ide     - IDE sub-system
loop    - infinite loop on address range
loopw   - infinite write loop on address range
mtest   - simple RAM test
icache  - enable or disable instruction cache
dcache  - enable or disable data cache
reset   - Perform RESET of the CPU
echo    - echo args to console
version - print monitor version
help    - print online help
?       - alias for 'help'


Monitor Commands - Detailed Description:
========================================

TODO.

For now: just type "help <command>".


Environment Variables:
======================

U-Boot supports user configuration using Environment Variables which
can be made persistent by saving to Flash memory.

Environment Variables are set using "setenv", printed using
"printenv", and saved to Flash using "saveenv". Using "setenv"
without a value can be used to delete a variable from the
environment. As long as you don't save the environment you are
working with an in-memory copy. In case the Flash area containing the
environment is erased by accident, a default environment is provided.

Some configuration options can be set using Environment Variables.

List of environment variables (most likely not complete):

  baudrate      - see CONFIG_BAUDRATE

  bootdelay     - see CONFIG_BOOTDELAY

  bootcmd       - see CONFIG_BOOTCOMMAND

  bootargs      - Boot arguments when booting an RTOS image

  bootfile      - Name of the image to load with TFTP

  bootm_low     - Memory range available for image processing in the bootm
                  command can be restricted. This variable is given as
                  a hexadecimal number and defines lowest address allowed
                  for use by the bootm command. See also "bootm_size"
                  environment variable. Address defined by "bootm_low" is
                  also the base of the initial memory mapping for the Linux
                  kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
                  bootm_mapsize.

  bootm_mapsize - Size of the initial memory mapping for the Linux kernel.
                  This variable is given as a hexadecimal number and it
                  defines the size of the memory region starting at base
                  address bootm_low that is accessible by the Linux kernel
                  during early boot.  If unset, CONFIG_SYS_BOOTMAPSZ is used
                  as the default value if it is defined, and bootm_size is
                  used otherwise.

  bootm_size    - Memory range available for image processing in the bootm
                  command can be restricted. This variable is given as
                  a hexadecimal number and defines the size of the region
                  allowed for use by the bootm command. See also "bootm_low"
                  environment variable.

  bootstopkeysha256, bootdelaykey, bootstopkey  - See README.autoboot

  updatefile    - Location of the software update file on a TFTP server, used
                  by the automatic software update feature. Please refer to
                  documentation in doc/README.update for more details.

  autoload      - if set to "no" (any string beginning with 'n'),
                  "bootp" will just load perform a lookup of the
                  configuration from the BOOTP server, but not try to
                  load any image using TFTP

  autostart     - if set to "yes", an image loaded using the "bootp",
                  "rarpboot", "tftpboot" or "diskboot" commands will
                  be automatically started (by internally calling
                  "bootm")

                  If set to "no", a standalone image passed to the
                  "bootm" command will be copied to the load address
                  (and eventually uncompressed), but NOT be started.
                  This can be used to load and uncompress arbitrary
                  data.

  fdt_high      - if set this restricts the maximum address that the
                  flattened device tree will be copied into upon boot.
                  For example, if you have a system with 1 GB memory
                  at physical address 0x10000000, while Linux kernel
                  only recognizes the first 704 MB as low memory, you
                  may need to set fdt_high as 0x3C000000 to have the
                  device tree blob be copied to the maximum address
                  of the 704 MB low memory, so that Linux kernel can
                  access it during the boot procedure.

                  If this is set to the special value 0xFFFFFFFF then
                  the fdt will not be copied at all on boot.  For this
                  to work it must reside in writable memory, have
                  sufficient padding on the end of it for u-boot to
                  add the information it needs into it, and the memory
                  must be accessible by the kernel.

  fdtcontroladdr- if set this is the address of the control flattened
                  device tree used by U-Boot when CONFIG_OF_CONTROL is
                  defined.

  i2cfast       - (PPC405GP|PPC405EP only)
                  if set to 'y' configures Linux I2C driver for fast
                  mode (400kHZ). This environment variable is used in
                  initialization code. So, for changes to be effective
                  it must be saved and board must be reset.

  initrd_high   - restrict positioning of initrd images:
                  If this variable is not set, initrd images will be
                  copied to the highest possible address in RAM; this
                  is usually what you want since it allows for
                  maximum initrd size. If for some reason you want to
                  make sure that the initrd image is loaded below the
                  CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
                  variable to a value of "no" or "off" or "0".
                  Alternatively, you can set it to a maximum upper
                  address to use (U-Boot will still check that it
                  does not overwrite the U-Boot stack and data).

                  For instance, when you have a system with 16 MB
                  RAM, and want to reserve 4 MB from use by Linux,
                  you can do this by adding "mem=12M" to the value of
                  the "bootargs" variable. However, now you must make
                  sure that the initrd image is placed in the first
                  12 MB as well - this can be done with

                  setenv initrd_high 00c00000

                  If you set initrd_high to 0xFFFFFFFF, this is an
                  indication to U-Boot that all addresses are legal
                  for the Linux kernel, including addresses in flash
                  memory. In this case U-Boot will NOT COPY the
                  ramdisk at all. This may be useful to reduce the
                  boot time on your system, but requires that this
                  feature is supported by your Linux kernel.

  ipaddr        - IP address; needed for tftpboot command

  loadaddr      - Default load address for commands like "bootp",
                  "rarpboot", "tftpboot", "loadb" or "diskboot"

  loads_echo    - see CONFIG_LOADS_ECHO

  serverip      - TFTP server IP address; needed for tftpboot command

  bootretry     - see CONFIG_BOOT_RETRY_TIME

  bootdelaykey  - see CONFIG_AUTOBOOT_DELAY_STR

  bootstopkey   - see CONFIG_AUTOBOOT_STOP_STR

  ethprime      - controls which interface is used first.

  ethact        - controls which interface is currently active.
                  For example you can do the following

                  => setenv ethact FEC
                  => ping 192.168.0.1 # traffic sent on FEC
                  => setenv ethact SCC
                  => ping 10.0.0.1 # traffic sent on SCC

  ethrotate     - When set to "no" U-Boot does not go through all
                  available network interfaces.
                  It just stays at the currently selected interface.

  netretry      - When set to "no" each network operation will
                  either succeed or fail without retrying.
                  When set to "once" the network operation will
                  fail when all the available network interfaces
                  are tried once without success.
                  Useful on scripts which control the retry operation
                  themselves.

  npe_ucode     - set load address for the NPE microcode

  silent_linux  - If set then Linux will be told to boot silently, by
                  changing the console to be empty. If "yes" it will be
                  made silent. If "no" it will not be made silent. If
                  unset, then it will be made silent if the U-Boot console
                  is silent.

  tftpsrcp      - If this is set, the value is used for TFTP's
                  UDP source port.

  tftpdstp      - If this is set, the value is used for TFTP's UDP
                  destination port instead of the Well Know Port 69.

  tftpblocksize - Block size to use for TFTP transfers; if not set,
                  we use the TFTP server's default block size

  tftptimeout   - Retransmission timeout for TFTP packets (in milli-
                  seconds, minimum value is 1000 = 1 second). Defines
                  when a packet is considered to be lost so it has to
                  be retransmitted. The default is 5000 = 5 seconds.
                  Lowering this value may make downloads succeed
                  faster in networks with high packet loss rates or
                  with unreliable TFTP servers.

  tftptimeoutcountmax   - maximum count of TFTP timeouts (no
                  unit, minimum value = 0). Defines how many timeouts
                  can happen during a single file transfer before that
                  transfer is aborted. The default is 10, and 0 means
                  'no timeouts allowed'. Increasing this value may help
                  downloads succeed with high packet loss rates, or with
                  unreliable TFTP servers or client hardware.

  tftpwindowsize        - if this is set, the value is used for TFTP's
                  window size as described by RFC 7440.
                  This means the count of blocks we can receive before
                  sending ack to server.

  vlan          - When set to a value < 4095 the traffic over
                  Ethernet is encapsulated/received over 802.1q
                  VLAN tagged frames.

  bootpretryperiod      - Period during which BOOTP/DHCP sends retries.
                  Unsigned value, in milliseconds. If not set, the period will
                  be either the default (28000), or a value based on
                  CONFIG_NET_RETRY_COUNT, if defined. This value has
                  precedence over the valu based on CONFIG_NET_RETRY_COUNT.

  memmatches    - Number of matches found by the last 'ms' command, in hex

  memaddr       - Address of the last match found by the 'ms' command, in hex,
                  or 0 if none

  mempos        - Index position of the last match found by the 'ms' command,
                  in units of the size (.b, .w, .l) of the search

  zbootbase     - (x86 only) Base address of the bzImage 'setup' block

  zbootaddr     - (x86 only) Address of the loaded bzImage, typically
                  BZIMAGE_LOAD_ADDR which is 0x100000

The following image location variables contain the location of images
used in booting. The "Image" column gives the role of the image and is
not an environment variable name. The other columns are environment
variable names. "File Name" gives the name of the file on a TFTP
server, "RAM Address" gives the location in RAM the image will be
loaded to, and "Flash Location" gives the image's address in NOR
flash or offset in NAND flash.

*Note* - these variables don't have to be defined for all boards, some
boards currently use other variables for these purposes, and some
boards use these variables for other purposes.

Image               File Name        RAM Address       Flash Location
-----               ---------        -----------       --------------
u-boot              u-boot           u-boot_addr_r     u-boot_addr
Linux kernel        bootfile         kernel_addr_r     kernel_addr
device tree blob    fdtfile          fdt_addr_r        fdt_addr
ramdisk             ramdiskfile      ramdisk_addr_r    ramdisk_addr

The following environment variables may be used and automatically
updated by the network boot commands ("bootp" and "rarpboot"),
depending the information provided by your boot server:

  bootfile      - see above
  dnsip         - IP address of your Domain Name Server
  dnsip2        - IP address of your secondary Domain Name Server
  gatewayip     - IP address of the Gateway (Router) to use
  hostname      - Target hostname
  ipaddr        - see above
  netmask       - Subnet Mask
  rootpath      - Pathname of the root filesystem on the NFS server
  serverip      - see above


There are two special Environment Variables:

  serial#       - contains hardware identification information such
                  as type string and/or serial number
  ethaddr       - Ethernet address

These variables can be set only once (usually during manufacturing of
the board). U-Boot refuses to delete or overwrite these variables
once they have been set once.


Further special Environment Variables:

  ver           - Contains the U-Boot version string as printed
                  with the "version" command. This variable is
                  readonly (see CONFIG_VERSION_VARIABLE).


Please note that changes to some configuration parameters may take
only effect after the next boot (yes, that's just like Windoze :-).


Callback functions for environment variables:
---------------------------------------------

For some environment variables, the behavior of u-boot needs to change
when their values are changed.  This functionality allows functions to
be associated with arbitrary variables.  On creation, overwrite, or
deletion, the callback will provide the opportunity for some side
effect to happen or for the change to be rejected.

The callbacks are named and associated with a function using the
U_BOOT_ENV_CALLBACK macro in your board or driver code.

These callbacks are associated with variables in one of two ways.  The
static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
in the board configuration to a string that defines a list of
associations.  The list must be in the following format:

        entry = variable_name[:callback_name]
        list = entry[,list]

If the callback name is not specified, then the callback is deleted.
Spaces are also allowed anywhere in the list.

Callbacks can also be associated by defining the ".callbacks" variable
with the same list format above.  Any association in ".callbacks" will
override any association in the static list. You can define
CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
".callbacks" environment variable in the default or embedded environment.

If CONFIG_REGEX is defined, the variable_name above is evaluated as a
regular expression. This allows multiple variables to be connected to
the same callback without explicitly listing them all out.

The signature of the callback functions is:

    int callback(const char *name, const char *value, enum env_op op, int flags)

* name - changed environment variable
* value - new value of the environment variable
* op - operation (create, overwrite, or delete)
* flags - attributes of the environment variable change, see flags H_* in
  include/search.h

The return value is 0 if the variable change is accepted and 1 otherwise.


Note for Redundant Ethernet Interfaces:
=======================================

Some boards come with redundant Ethernet interfaces; U-Boot supports
such configurations and is capable of automatic selection of a
"working" interface when needed. MAC assignment works as follows:

Network interfaces are numbered eth0, eth1, eth2, ... Corresponding
MAC addresses can be stored in the environment as "ethaddr" (=>eth0),
"eth1addr" (=>eth1), "eth2addr", ...

If the network interface stores some valid MAC address (for instance
in SROM), this is used as default address if there is NO correspon-
ding setting in the environment; if the corresponding environment
variable is set, this overrides the settings in the card; that means:

o If the SROM has a valid MAC address, and there is no address in the
  environment, the SROM's address is used.

o If there is no valid address in the SROM, and a definition in the
  environment exists, then the value from the environment variable is
  used.

o If both the SROM and the environment contain a MAC address, and
  both addresses are the same, this MAC address is used.

o If both the SROM and the environment contain a MAC address, and the
  addresses differ, the value from the environment is used and a
  warning is printed.

o If neither SROM nor the environment contain a MAC address, an error
  is raised. If CONFIG_NET_RANDOM_ETHADDR is defined, then in this case
  a random, locally-assigned MAC is used.

If Ethernet drivers implement the 'write_hwaddr' function, valid MAC addresses
will be programmed into hardware as part of the initialization process.  This
may be skipped by setting the appropriate 'ethmacskip' environment variable.
The naming convention is as follows:
"ethmacskip" (=>eth0), "eth1macskip" (=>eth1) etc.

Image Formats:
==============

U-Boot is capable of booting (and performing other auxiliary operations on)
images in two formats:

New uImage format (FIT)
-----------------------

Flexible and powerful format based on Flattened Image Tree -- FIT (similar
to Flattened Device Tree). It allows the use of images with multiple
components (several kernels, ramdisks, etc.), with contents protected by
SHA1, MD5 or CRC32. More details are found in the doc/uImage.FIT directory.


Old uImage format
-----------------

Old image format is based on binary files which can be basically anything,
preceded by a special header; see the definitions in include/image.h for
details; basically, the header defines the following image properties:

* Target Operating System (Provisions for OpenBSD, NetBSD, FreeBSD,
  4.4BSD, Linux, SVR4, Esix, Solaris, Irix, SCO, Dell, NCR, VxWorks,
  LynxOS, pSOS, QNX, RTEMS, INTEGRITY;
  Currently supported: Linux, NetBSD, VxWorks, QNX, RTEMS, LynxOS,
  INTEGRITY).
* Target CPU Architecture (Provisions for Alpha, ARM, Intel x86,
  IA64, MIPS, NDS32, Nios II, PowerPC, IBM S390, SuperH, Sparc, Sparc 64 Bit;
  Currently supported: ARM, Intel x86, MIPS, NDS32, Nios II, PowerPC).
* Compression Type (uncompressed, gzip, bzip2)
* Load Address
* Entry Point
* Image Name
* Image Timestamp

The header is marked by a special Magic Number, and both the header
and the data portions of the image are secured against corruption by
CRC32 checksums.


Linux Support:
==============

Although U-Boot should support any OS or standalone application
easily, the main focus has always been on Linux during the design of
U-Boot.

U-Boot includes many features that so far have been part of some
special "boot loader" code within the Linux kernel. Also, any
"initrd" images to be used are no longer part of one big Linux image;
instead, kernel and "initrd" are separate images. This implementation
serves several purposes:

- the same features can be used for other OS or standalone
  applications (for instance: using compressed images to reduce the
  Flash memory footprint)

- it becomes much easier to port new Linux kernel versions because
  lots of low-level, hardware dependent stuff are done by U-Boot

- the same Linux kernel image can now be used with different "initrd"
  images; of course this also means that different kernel images can
  be run with the same "initrd". This makes testing easier (you don't
  have to build a new "zImage.initrd" Linux image when you just
  change a file in your "initrd"). Also, a field-upgrade of the
  software is easier now.


Linux HOWTO:
============

Porting Linux to U-Boot based systems:
---------------------------------------

U-Boot cannot save you from doing all the necessary modifications to
configure the Linux device drivers for use with your target hardware
(no, we don't intend to provide a full virtual machine interface to
Linux :-).

But now you can ignore ALL boot loader code (in arch/powerpc/mbxboot).

Just make sure your machine specific header file (for instance
include/asm-ppc/tqm8xx.h) includes the same definition of the Board
Information structure as we define in include/asm-<arch>/u-boot.h,
and make sure that your definition of IMAP_ADDR uses the same value
as your U-Boot configuration in CONFIG_SYS_IMMR.

Note that U-Boot now has a driver model, a unified model for drivers.
If you are adding a new driver, plumb it into driver model. If there
is no uclass available, you are encouraged to create one. See
doc/driver-model.


Configuring the Linux kernel:
-----------------------------

No specific requirements for U-Boot. Make sure you have some root
device (initial ramdisk, NFS) for your target system.


Building a Linux Image:
-----------------------

With U-Boot, "normal" build targets like "zImage" or "bzImage" are
not used. If you use recent kernel source, a new build target
"uImage" will exist which automatically builds an image usable by
U-Boot. Most older kernels also have support for a "pImage" target,
which was introduced for our predecessor project PPCBoot and uses a
100% compatible format.

Example:

        make TQM850L_defconfig
        make oldconfig
        make dep
        make uImage

The "uImage" build target uses a special tool (in 'tools/mkimage') to
encapsulate a compressed Linux kernel image with header  information,
CRC32 checksum etc. for use with U-Boot. This is what we are doing:

* build a standard "vmlinux" kernel image (in ELF binary format):

* convert the kernel into a raw binary image:

        ${CROSS_COMPILE}-objcopy -O binary \
                                 -R .note -R .comment \
                                 -S vmlinux linux.bin

* compress the binary image:

        gzip -9 linux.bin

* package compressed binary image for U-Boot:

        mkimage -A ppc -O linux -T kernel -C gzip \
                -a 0 -e 0 -n "Linux Kernel Image" \
                -d linux.bin.gz uImage


The "mkimage" tool can also be used to create ramdisk images for use
with U-Boot, either separated from the Linux kernel image, or
combined into one file. "mkimage" encapsulates the images with a 64
byte header containing information about target architecture,
operating system, image type, compression method, entry points, time
stamp, CRC32 checksums, etc.

"mkimage" can be called in two ways: to verify existing images and
print the header information, or to build new images.

In the first form (with "-l" option) mkimage lists the information
contained in the header of an existing U-Boot image; this includes
checksum verification:

        tools/mkimage -l image
          -l ==> list image header information

The second form (with "-d" option) is used to build a U-Boot image
from a "data file" which is used as image payload:

        tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
                      -n name -d data_file image
          -A ==> set architecture to 'arch'
          -O ==> set operating system to 'os'
          -T ==> set image type to 'type'
          -C ==> set compression type 'comp'
          -a ==> set load address to 'addr' (hex)
          -e ==> set entry point to 'ep' (hex)
          -n ==> set image name to 'name'
          -d ==> use image data from 'datafile'

Right now, all Linux kernels for PowerPC systems use the same load
address (0x00000000), but the entry point address depends on the
kernel version:

- 2.2.x kernels have the entry point at 0x0000000C,
- 2.3.x and later kernels have the entry point at 0x00000000.

So a typical call to build a U-Boot image would read:

        -> tools/mkimage -n '2.4.4 kernel for TQM850L' \
        > -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
        > -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz \
        > examples/uImage.TQM850L
        Image Name:   2.4.4 kernel for TQM850L
        Created:      Wed Jul 19 02:34:59 2000
        Image Type:   PowerPC Linux Kernel Image (gzip compressed)
        Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
        Load Address: 0x00000000
        Entry Point:  0x00000000

To verify the contents of the image (or check for corruption):

        -> tools/mkimage -l examples/uImage.TQM850L
        Image Name:   2.4.4 kernel for TQM850L
        Created:      Wed Jul 19 02:34:59 2000
        Image Type:   PowerPC Linux Kernel Image (gzip compressed)
        Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
        Load Address: 0x00000000
        Entry Point:  0x00000000

NOTE: for embedded systems where boot time is critical you can trade
speed for memory and install an UNCOMPRESSED image instead: this
needs more space in Flash, but boots much faster since it does not
need to be uncompressed:

        -> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz
        -> tools/mkimage -n '2.4.4 kernel for TQM850L' \
        > -A ppc -O linux -T kernel -C none -a 0 -e 0 \
        > -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux \
        > examples/uImage.TQM850L-uncompressed
        Image Name:   2.4.4 kernel for TQM850L
        Created:      Wed Jul 19 02:34:59 2000
        Image Type:   PowerPC Linux Kernel Image (uncompressed)
        Data Size:    792160 Bytes = 773.59 kB = 0.76 MB
        Load Address: 0x00000000
        Entry Point:  0x00000000


Similar you can build U-Boot images from a 'ramdisk.image.gz' file
when your kernel is intended to use an initial ramdisk:

        -> tools/mkimage -n 'Simple Ramdisk Image' \
        > -A ppc -O linux -T ramdisk -C gzip \
        > -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
        Image Name:   Simple Ramdisk Image
        Created:      Wed Jan 12 14:01:50 2000
        Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
        Data Size:    566530 Bytes = 553.25 kB = 0.54 MB
        Load Address: 0x00000000
        Entry Point:  0x00000000

The "dumpimage" tool can be used to disassemble or list the contents of images
built by mkimage. See dumpimage's help output (-h) for details.

Installing a Linux Image:
-------------------------

To downloading a U-Boot image over the serial (console) interface,
you must convert the image to S-Record format:

        objcopy -I binary -O srec examples/image examples/image.srec

The 'objcopy' does not understand the information in the U-Boot
image header, so the resulting S-Record file will be relative to
address 0x00000000. To load it to a given address, you need to
specify the target address as 'offset' parameter with the 'loads'
command.

Example: install the image to address 0x40100000 (which on the
TQM8xxL is in the first Flash bank):

        => erase 40100000 401FFFFF

        .......... done
        Erased 8 sectors

        => loads 40100000
        ## Ready for S-Record download ...
        ~>examples/image.srec
        1 2 3 4 5 6 7 8 9 10 11 12 13 ...
        ...
        15989 15990 15991 15992
        [file transfer complete]
        [connected]
        ## Start Addr = 0x00000000


You can check the success of the download using the 'iminfo' command;
this includes a checksum verification so you can be sure no data
corruption happened:

        => imi 40100000

        ## Checking Image at 40100000 ...
           Image Name:   2.2.13 for initrd on TQM850L
           Image Type:   PowerPC Linux Kernel Image (gzip compressed)
           Data Size:    335725 Bytes = 327 kB = 0 MB
           Load Address: 00000000
           Entry Point:  0000000c
           Verifying Checksum ... OK


Boot Linux:
-----------

The "bootm" command is used to boot an application that is stored in
memory (RAM or Flash). In case of a Linux kernel image, the contents
of the "bootargs" environment variable is passed to the kernel as
parameters. You can check and modify this variable using the
"printenv" and "setenv" commands:


        => printenv bootargs
        bootargs=root=/dev/ram

        => setenv bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

        => printenv bootargs
        bootargs=root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

        => bootm 40020000
        ## Booting Linux kernel at 40020000 ...
           Image Name:   2.2.13 for NFS on TQM850L
           Image Type:   PowerPC Linux Kernel Image (gzip compressed)
           Data Size:    381681 Bytes = 372 kB = 0 MB
           Load Address: 00000000
           Entry Point:  0000000c
           Verifying Checksum ... OK
           Uncompressing Kernel Image ... OK
        Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:35:17 MEST 2000
        Boot arguments: root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
        time_init: decrementer frequency = 187500000/60
        Calibrating delay loop... 49.77 BogoMIPS
        Memory: 15208k available (700k kernel code, 444k data, 32k init) [c0000000,c1000000]
        ...

If you want to boot a Linux kernel with initial RAM disk, you pass
the memory addresses of both the kernel and the initrd image (PPBCOOT
format!) to the "bootm" command:

        => imi 40100000 40200000

        ## Checking Image at 40100000 ...
           Image Name:   2.2.13 for initrd on TQM850L
           Image Type:   PowerPC Linux Kernel Image (gzip compressed)
           Data Size:    335725 Bytes = 327 kB = 0 MB
           Load Address: 00000000
           Entry Point:  0000000c
           Verifying Checksum ... OK

        ## Checking Image at 40200000 ...
           Image Name:   Simple Ramdisk Image
           Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
           Data Size:    566530 Bytes = 553 kB = 0 MB
           Load Address: 00000000
           Entry Point:  00000000
           Verifying Checksum ... OK

        => bootm 40100000 40200000
        ## Booting Linux kernel at 40100000 ...
           Image Name:   2.2.13 for initrd on TQM850L
           Image Type:   PowerPC Linux Kernel Image (gzip compressed)
           Data Size:    335725 Bytes = 327 kB = 0 MB
           Load Address: 00000000
           Entry Point:  0000000c
           Verifying Checksum ... OK
           Uncompressing Kernel Image ... OK
        ## Loading RAMDisk Image at 40200000 ...
           Image Name:   Simple Ramdisk Image
           Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
           Data Size:    566530 Bytes = 553 kB = 0 MB
           Load Address: 00000000
           Entry Point:  00000000
           Verifying Checksum ... OK
           Loading Ramdisk ... OK
        Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:32:08 MEST 2000
        Boot arguments: root=/dev/ram
        time_init: decrementer frequency = 187500000/60
        Calibrating delay loop... 49.77 BogoMIPS
        ...
        RAMDISK: Compressed image found at block 0
        VFS: Mounted root (ext2 filesystem).

        bash#

Boot Linux and pass a flat device tree:
-----------

First, U-Boot must be compiled with the appropriate defines. See the section
titled "Linux Kernel Interface" above for a more in depth explanation. The
following is an example of how to start a kernel and pass an updated
flat device tree:

=> print oftaddr
oftaddr=0x300000
=> print oft
oft=oftrees/mpc8540ads.dtb
=> tftp $oftaddr $oft
Speed: 1000, full duplex
Using TSEC0 device
TFTP from server 192.168.1.1; our IP address is 192.168.1.101
Filename 'oftrees/mpc8540ads.dtb'.
Load address: 0x300000
Loading: #
done
Bytes transferred = 4106 (100a hex)
=> tftp $loadaddr $bootfile
Speed: 1000, full duplex
Using TSEC0 device
TFTP from server 192.168.1.1; our IP address is 192.168.1.2
Filename 'uImage'.
Load address: 0x200000
Loading:############
done
Bytes transferred = 1029407 (fb51f hex)
=> print loadaddr
loadaddr=200000
=> print oftaddr
oftaddr=0x300000
=> bootm $loadaddr - $oftaddr
## Booting image at 00200000 ...
   Image Name:   Linux-2.6.17-dirty
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    1029343 Bytes = 1005.2 kB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
   Uncompressing Kernel Image ... OK
Booting using flat device tree at 0x300000
Using MPC85xx ADS machine description
Memory CAM mapping: CAM0=256Mb, CAM1=256Mb, CAM2=0Mb residual: 0Mb
[snip]


More About U-Boot Image Types:
------------------------------

U-Boot supports the following image types:

   "Standalone Programs" are directly runnable in the environment
        provided by U-Boot; it is expected that (if they behave
        well) you can continue to work in U-Boot after return from
        the Standalone Program.
   "OS Kernel Images" are usually images of some Embedded OS which
        will take over control completely. Usually these programs
        will install their own set of exception handlers, device
        drivers, set up the MMU, etc. - this means, that you cannot
        expect to re-enter U-Boot except by resetting the CPU.
   "RAMDisk Images" are more or less just data blocks, and their
        parameters (address, size) are passed to an OS kernel that is
        being started.
   "Multi-File Images" contain several images, typically an OS
        (Linux) kernel image and one or more data images like
        RAMDisks. This construct is useful for instance when you want
        to boot over the network using BOOTP etc., where the boot
        server provides just a single image file, but you want to get
        for instance an OS kernel and a RAMDisk image.

        "Multi-File Images" start with a list of image sizes, each
        image size (in bytes) specified by an "uint32_t" in network
        byte order. This list is terminated by an "(uint32_t)0".
        Immediately after the terminating 0 follow the images, one by
        one, all aligned on "uint32_t" boundaries (size rounded up to
        a multiple of 4 bytes).

   "Firmware Images" are binary images containing firmware (like
        U-Boot or FPGA images) which usually will be programmed to
        flash memory.

   "Script files" are command sequences that will be executed by
        U-Boot's command interpreter; this feature is especially
        useful when you configure U-Boot to use a real shell (hush)
        as command interpreter.

Booting the Linux zImage:
-------------------------

On some platforms, it's possible to boot Linux zImage. This is done
using the "bootz" command. The syntax of "bootz" command is the same
as the syntax of "bootm" command.

Note, defining the CONFIG_SUPPORT_RAW_INITRD allows user to supply
kernel with raw initrd images. The syntax is slightly different, the
address of the initrd must be augmented by it's size, in the following
format: "<initrd addres>:<initrd size>".


Standalone HOWTO:
=================

One of the features of U-Boot is that you can dynamically load and
run "standalone" applications, which can use some resources of
U-Boot like console I/O functions or interrupt services.

Two simple examples are included with the sources:

"Hello World" Demo:
-------------------

'examples/hello_world.c' contains a small "Hello World" Demo
application; it is automatically compiled when you build U-Boot.
It's configured to run at address 0x00040004, so you can play with it
like that:

        => loads
        ## Ready for S-Record download ...
        ~>examples/hello_world.srec
        1 2 3 4 5 6 7 8 9 10 11 ...
        [file transfer complete]
        [connected]
        ## Start Addr = 0x00040004

        => go 40004 Hello World! This is a test.
        ## Starting application at 0x00040004 ...
        Hello World
        argc = 7
        argv[0] = "40004"
        argv[1] = "Hello"
        argv[2] = "World!"
        argv[3] = "This"
        argv[4] = "is"
        argv[5] = "a"
        argv[6] = "test."
        argv[7] = "<NULL>"
        Hit any key to exit ...

        ## Application terminated, rc = 0x0

Another example, which demonstrates how to register a CPM interrupt
handler with the U-Boot code, can be found in 'examples/timer.c'.
Here, a CPM timer is set up to generate an interrupt every second.
The interrupt service routine is trivial, just printing a '.'
character, but this is just a demo program. The application can be
controlled by the following keys:

        ? - print current values og the CPM Timer registers
        b - enable interrupts and start timer
        e - stop timer and disable interrupts
        q - quit application

        => loads
        ## Ready for S-Record download ...
        ~>examples/timer.srec
        1 2 3 4 5 6 7 8 9 10 11 ...
        [file transfer complete]
        [connected]
        ## Start Addr = 0x00040004

        => go 40004
        ## Starting application at 0x00040004 ...
        TIMERS=0xfff00980
        Using timer 1
          tgcr @ 0xfff00980, tmr @ 0xfff00990, trr @ 0xfff00994, tcr @ 0xfff00998, tcn @ 0xfff0099c, ter @ 0xfff009b0

Hit 'b':
        [q, b, e, ?] Set interval 1000000 us
        Enabling timer
Hit '?':
        [q, b, e, ?] ........
        tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0xef6, ter=0x0
Hit '?':
        [q, b, e, ?] .
        tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x2ad4, ter=0x0
Hit '?':
        [q, b, e, ?] .
        tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x1efc, ter=0x0
Hit '?':
        [q, b, e, ?] .
        tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x169d, ter=0x0
Hit 'e':
        [q, b, e, ?] ...Stopping timer
Hit 'q':
        [q, b, e, ?] ## Application terminated, rc = 0x0


Minicom warning:
================

Over time, many people have reported problems when trying to use the
"minicom" terminal emulation program for serial download. I (wd)
consider minicom to be broken, and recommend not to use it. Under
Unix, I recommend to use C-Kermit for general purpose use (and
especially for kermit binary protocol download ("loadb" command), and
use "cu" for S-Record download ("loads" command).  See
https://www.denx.de/wiki/view/DULG/SystemSetup#Section_4.3.
for help with kermit.


Nevertheless, if you absolutely want to use it try adding this
configuration to your "File transfer protocols" section:

           Name    Program                      Name U/D FullScr IO-Red. Multi
        X  kermit  /usr/bin/kermit -i -l %l -s   Y    U    Y       N      N
        Y  kermit  /usr/bin/kermit -i -l %l -r   N    D    Y       N      N


NetBSD Notes:
=============

Starting at version 0.9.2, U-Boot supports NetBSD both as host
(build U-Boot) and target system (boots NetBSD/mpc8xx).

Building requires a cross environment; it is known to work on
NetBSD/i386 with the cross-powerpc-netbsd-1.3 package (you will also
need gmake since the Makefiles are not compatible with BSD make).
Note that the cross-powerpc package does not install include files;
attempting to build U-Boot will fail because <machine/ansi.h> is
missing.  This file has to be installed and patched manually:

        # cd /usr/pkg/cross/powerpc-netbsd/include
        # mkdir powerpc
        # ln -s powerpc machine
        # cp /usr/src/sys/arch/powerpc/include/ansi.h powerpc/ansi.h
        # ${EDIT} powerpc/ansi.h        ## must remove __va_list, _BSD_VA_LIST

Native builds *don't* work due to incompatibilities between native
and U-Boot include files.

Booting assumes that (the first part of) the image booted is a
stage-2 loader which in turn loads and then invokes the kernel
proper. Loader sources will eventually appear in the NetBSD source
tree (probably in sys/arc/mpc8xx/stand/u-boot_stage2/); in the
meantime, see ftp://ftp.denx.de/pub/u-boot/ppcboot_stage2.tar.gz


Implementation Internals:
=========================

The following is not intended to be a complete description of every
implementation detail. However, it should help to understand the
inner workings of U-Boot and make it easier to port it to custom
hardware.


Initial Stack, Global Data:
---------------------------

The implementation of U-Boot is complicated by the fact that U-Boot
starts running out of ROM (flash memory), usually without access to
system RAM (because the memory controller is not initialized yet).
This means that we don't have writable Data or BSS segments, and BSS
is not initialized as zero. To be able to get a C environment working
at all, we have to allocate at least a minimal stack. Implementation
options for this are defined and restricted by the CPU used: Some CPU
models provide on-chip memory (like the IMMR area on MPC8xx and
MPC826x processors), on others (parts of) the data cache can be
locked as (mis-) used as memory, etc.

        Chris Hallinan posted a good summary of these issues to the
        U-Boot mailing list:

        Subject: RE: [U-Boot-Users] RE: More On Memory Bank x (nothingness)?
        From: "Chris Hallinan" <clh@net1plus.com>
        Date: Mon, 10 Feb 2003 16:43:46 -0500 (22:43 MET)
        ...

        Correct me if I'm wrong, folks, but the way I understand it
        is this: Using DCACHE as initial RAM for Stack, etc, does not
        require any physical RAM backing up the cache. The cleverness
        is that the cache is being used as a temporary supply of
        necessary storage before the SDRAM controller is setup. It's
        beyond the scope of this list to explain the details, but you
        can see how this works by studying the cache architecture and
        operation in the architecture and processor-specific manuals.

        OCM is On Chip Memory, which I believe the 405GP has 4K. It
        is another option for the system designer to use as an
        initial stack/RAM area prior to SDRAM being available. Either
        option should work for you. Using CS 4 should be fine if your
        board designers haven't used it for something that would
        cause you grief during the initial boot! It is frequently not
        used.

        CONFIG_SYS_INIT_RAM_ADDR should be somewhere that won't interfere
        with your processor/board/system design. The default value
        you will find in any recent u-boot distribution in
        walnut.h should work for you. I'd set it to a value larger
        than your SDRAM module. If you have a 64MB SDRAM module, set
        it above 400_0000. Just make sure your board has no resources
        that are supposed to respond to that address! That code in
        start.S has been around a while and should work as is when
        you get the config right.

        -Chris Hallinan
        DS4.COM, Inc.

It is essential to remember this, since it has some impact on the C
code for the initialization procedures:

* Initialized global data (data segment) is read-only. Do not attempt
  to write it.

* Do not use any uninitialized global data (or implicitly initialized
  as zero data - BSS segment) at all - this is undefined, initiali-
  zation is performed later (when relocating to RAM).

* Stack space is very limited. Avoid big data buffers or things like
  that.

Having only the stack as writable memory limits means we cannot use
normal global data to share information between the code. But it
turned out that the implementation of U-Boot can be greatly
simplified by making a global data structure (gd_t) available to all
functions. We could pass a pointer to this data as argument to _all_
functions, but this would bloat the code. Instead we use a feature of
the GCC compiler (Global Register Variables) to share the data: we
place a pointer (gd) to the global data into a register which we
reserve for this purpose.

When choosing a register for such a purpose we are restricted by the
relevant  (E)ABI  specifications for the current architecture, and by
GCC's implementation.

For PowerPC, the following registers have specific use:
        R1:     stack pointer
        R2:     reserved for system use
        R3-R4:  parameter passing and return values
        R5-R10: parameter passing
        R13:    small data area pointer
        R30:    GOT pointer
        R31:    frame pointer

        (U-Boot also uses R12 as internal GOT pointer. r12
        is a volatile register so r12 needs to be reset when
        going back and forth between asm and C)

    ==> U-Boot will use R2 to hold a pointer to the global data

    Note: on PPC, we could use a static initializer (since the
    address of the global data structure is known at compile time),
    but it turned out that reserving a register results in somewhat
    smaller code - although the code savings are not that big (on
    average for all boards 752 bytes for the whole U-Boot image,
    624 text + 127 data).

On ARM, the following registers are used:

        R0:     function argument word/integer result
        R1-R3:  function argument word
        R9:     platform specific
        R10:    stack limit (used only if stack checking is enabled)
        R11:    argument (frame) pointer
        R12:    temporary workspace
        R13:    stack pointer
        R14:    link register
        R15:    program counter

    ==> U-Boot will use R9 to hold a pointer to the global data

    Note: on ARM, only R_ARM_RELATIVE relocations are supported.

On Nios II, the ABI is documented here:
        https://www.altera.com/literature/hb/nios2/n2cpu_nii51016.pdf

    ==> U-Boot will use gp to hold a pointer to the global data

    Note: on Nios II, we give "-G0" option to gcc and don't use gp
    to access small data sections, so gp is free.

On NDS32, the following registers are used:

        R0-R1:  argument/return
        R2-R5:  argument
        R15:    temporary register for assembler
        R16:    trampoline register
        R28:    frame pointer (FP)
        R29:    global pointer (GP)
        R30:    link register (LP)
        R31:    stack pointer (SP)
        PC:     program counter (PC)

    ==> U-Boot will use R10 to hold a pointer to the global data

NOTE: DECLARE_GLOBAL_DATA_PTR must be used with file-global scope,
or current versions of GCC may "optimize" the code too much.

On RISC-V, the following registers are used:

        x0: hard-wired zero (zero)
        x1: return address (ra)
        x2:     stack pointer (sp)
        x3:     global pointer (gp)
        x4:     thread pointer (tp)
        x5:     link register (t0)
        x8:     frame pointer (fp)
        x10-x11:        arguments/return values (a0-1)
        x12-x17:        arguments (a2-7)
        x28-31:  temporaries (t3-6)
        pc:     program counter (pc)

    ==> U-Boot will use gp to hold a pointer to the global data

Memory Management:
------------------

U-Boot runs in system state and uses physical addresses, i.e. the
MMU is not used either for address mapping nor for memory protection.

The available memory is mapped to fixed addresses using the memory
controller. In this process, a contiguous block is formed for each
memory type (Flash, SDRAM, SRAM), even when it consists of several
physical memory banks.

U-Boot is installed in the first 128 kB of the first Flash bank (on
TQM8xxL modules this is the range 0x40000000 ... 0x4001FFFF). After
booting and sizing and initializing DRAM, the code relocates itself
to the upper end of DRAM. Immediately below the U-Boot code some
memory is reserved for use by malloc() [see CONFIG_SYS_MALLOC_LEN
configuration setting]. Below that, a structure with global Board
Info data is placed, followed by the stack (growing downward).

Additionally, some exception handler code is copied to the low 8 kB
of DRAM (0x00000000 ... 0x00001FFF).

So a typical memory configuration with 16 MB of DRAM could look like
this:

        0x0000 0000     Exception Vector code
              :
        0x0000 1FFF
        0x0000 2000     Free for Application Use
              :
              :

              :
              :
        0x00FB FF20     Monitor Stack (Growing downward)
        0x00FB FFAC     Board Info Data and permanent copy of global data
        0x00FC 0000     Malloc Arena
              :
        0x00FD FFFF
        0x00FE 0000     RAM Copy of Monitor Code
        ...             eventually: LCD or video framebuffer
        ...             eventually: pRAM (Protected RAM - unchanged by reset)
        0x00FF FFFF     [End of RAM]


System Initialization:
----------------------

In the reset configuration, U-Boot starts at the reset entry point
(on most PowerPC systems at address 0x00000100). Because of the reset
configuration for CS0# this is a mirror of the on board Flash memory.
To be able to re-map memory U-Boot then jumps to its link address.
To be able to implement the initialization code in C, a (small!)
initial stack is set up in the internal Dual Ported RAM (in case CPUs
which provide such a feature like), or in a locked part of the data
cache. After that, U-Boot initializes the CPU core, the caches and
the SIU.

Next, all (potentially) available memory banks are mapped using a
preliminary mapping. For example, we put them on 512 MB boundaries
(multiples of 0x20000000: SDRAM on 0x00000000 and 0x20000000, Flash
on 0x40000000 and 0x60000000, SRAM on 0x80000000). Then UPM A is
programmed for SDRAM access. Using the temporary configuration, a
simple memory test is run that determines the size of the SDRAM
banks.

When there is more than one SDRAM bank, and the banks are of
different size, the largest is mapped first. For equal size, the first
bank (CS2#) is mapped first. The first mapping is always for address
0x00000000, with any additional banks following immediately to create
contiguous memory starting from 0.

Then, the monitor installs itself at the upper end of the SDRAM area
and allocates memory for use by malloc() and for the global Board
Info data; also, the exception vector code is copied to the low RAM
pages, and the final stack is set up.

Only after this relocation will you have a "normal" C environment;
until that you are restricted in several ways, mostly because you are
running from ROM, and because the code will have to be relocated to a
new address in RAM.


U-Boot Porting Guide:
----------------------

[Based on messages by Jerry Van Baren in the U-Boot-Users mailing
list, October 2002]


int main(int argc, char *argv[])
{
        sighandler_t no_more_time;

        signal(SIGALRM, no_more_time);
        alarm(PROJECT_DEADLINE - toSec (3 * WEEK));

        if (available_money > available_manpower) {
                Pay consultant to port U-Boot;
                return 0;
        }

        Download latest U-Boot source;

        Subscribe to u-boot mailing list;

        if (clueless)
                email("Hi, I am new to U-Boot, how do I get started?");

        while (learning) {
                Read the README file in the top level directory;
                Read https://www.denx.de/wiki/bin/view/DULG/Manual;
                Read applicable doc/README.*;
                Read the source, Luke;
                /* find . -name "*.[chS]" | xargs grep -i <keyword> */
        }

        if (available_money > toLocalCurrency ($2500))
                Buy a BDI3000;
        else
                Add a lot of aggravation and time;

        if (a similar board exists) {   /* hopefully... */
                cp -a board/<similar> board/<myboard>
                cp include/configs/<similar>.h include/configs/<myboard>.h
        } else {
                Create your own board support subdirectory;
                Create your own board include/configs/<myboard>.h file;
        }
        Edit new board/<myboard> files
        Edit new include/configs/<myboard>.h

        while (!accepted) {
                while (!running) {
                        do {
                                Add / modify source code;
                        } until (compiles);
                        Debug;
                        if (clueless)
                                email("Hi, I am having problems...");
                }
                Send patch file to the U-Boot email list;
                if (reasonable critiques)
                        Incorporate improvements from email list code review;
                else
                        Defend code as written;
        }

        return 0;
}

void no_more_time (int sig)
{
      hire_a_guru();
}


Coding Standards:
-----------------

All contributions to U-Boot should conform to the Linux kernel
coding style; see the kernel coding style guide at
https://www.kernel.org/doc/html/latest/process/coding-style.html, and the
script "scripts/Lindent" in your Linux kernel source directory.

Source files originating from a different project (for example the
MTD subsystem) are generally exempt from these guidelines and are not
reformatted to ease subsequent migration to newer versions of those
sources.

Please note that U-Boot is implemented in C (and to some small parts in
Assembler); no C++ is used, so please do not use C++ style comments (//)
in your code.

Please also stick to the following formatting rules:
- remove any trailing white space
- use TAB characters for indentation and vertical alignment, not spaces
- make sure NOT to use DOS '\r\n' line feeds
- do not add more than 2 consecutive empty lines to source files
- do not add trailing empty lines to source files

Submissions which do not conform to the standards may be returned
with a request to reformat the changes.


Submitting Patches:
-------------------

Since the number of patches for U-Boot is growing, we need to
establish some rules. Submissions which do not conform to these rules
may be rejected, even when they contain important and valuable stuff.

Please see https://www.denx.de/wiki/U-Boot/Patches for details.

Patches shall be sent to the u-boot mailing list <u-boot@lists.denx.de>;
see https://lists.denx.de/listinfo/u-boot

When you send a patch, please include the following information with
it:

* For bug fixes: a description of the bug and how your patch fixes
  this bug. Please try to include a way of demonstrating that the
  patch actually fixes something.

* For new features: a description of the feature and your
  implementation.

* For major contributions, add a MAINTAINERS file with your
  information and associated file and directory references.

* When you add support for a new board, don't forget to add a
  maintainer e-mail address to the boards.cfg file, too.

* If your patch adds new configuration options, don't forget to
  document these in the README file.

* The patch itself. If you are using git (which is *strongly*
  recommended) you can easily generate the patch using the
  "git format-patch". If you then use "git send-email" to send it to
  the U-Boot mailing list, you will avoid most of the common problems
  with some other mail clients.

  If you cannot use git, use "diff -purN OLD NEW". If your version of
  diff does not support these options, then get the latest version of
  GNU diff.

  The current directory when running this command shall be the parent
  directory of the U-Boot source tree (i. e. please make sure that
  your patch includes sufficient directory information for the
  affected files).

  We prefer patches as plain text. MIME attachments are discouraged,
  and compressed attachments must not be used.

* If one logical set of modifications affects or creates several
  files, all these changes shall be submitted in a SINGLE patch file.

* Changesets that contain different, unrelated modifications shall be
  submitted as SEPARATE patches, one patch per changeset.


Notes:

* Before sending the patch, run the buildman script on your patched
  source tree and make sure that no errors or warnings are reported
  for any of the boards.

* Keep your modifications to the necessary minimum: A patch
  containing several unrelated changes or arbitrary reformats will be
  returned with a request to re-formatting / split it.

* If you modify existing code, make sure that your new code does not
  add to the memory footprint of the code ;-) Small is beautiful!
  When adding new features, these should compile conditionally only
  (using #ifdef), and the resulting code with the new feature
  disabled must not need more memory than the old code without your
  modification.

* Remember that there is a size limit of 100 kB per message on the
  u-boot mailing list. Bigger patches will be moderated. If they are
  reasonable and not too big, they will be acknowledged. But patches
  bigger than the size limit should be avoided.