Merge tag 'dm-9oct18' of git://git.denx.de/u-boot-dm

[platform/kernel/u-boot.git] / doc / README.fdt-control

diff --git a/doc/README.fdt-control b/doc/README.fdt-control
index d8fe4a8..d6ab7bf 100644 (file)
--- a/doc/README.fdt-control
+++ b/doc/README.fdt-control
@@ -1,8 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0+
 #
 # Copyright (c) 2011 The Chromium OS Authors.
-#
-# SPDX-License-Identifier:     GPL-2.0+
-#
 
 Device Tree Control in U-Boot
 =============================
@@ -33,12 +31,6 @@ the features of each board in the device tree file, and have a single
 generic source base.
 
 To enable this feature, add CONFIG_OF_CONTROL to your board config file.
-It is currently supported on ARM, x86 and Microblaze - other architectures
-will need to add code to their arch/xxx/lib/board.c file to locate the
-FDT. Alternatively you can enable generic board support on your board
-(with CONFIG_SYS_GENERIC_BOARD) if this is available (as it is for
-PowerPC). For ARM, Tegra and Exynos5 have device trees available for
-common devices.
 
 
 What is a Flat Device Tree?
@@ -64,7 +56,11 @@ In case you are wondering, OF stands for Open Firmware.
 Tools
 -----
 
-To use this feature you will need to get the device tree compiler here:
+To use this feature you will need to get the device tree compiler. This is
+provided by U-Boot automatically. If you have a system version of dtc
+(typically in the 'device-tree-compiler' package), it is currently not used.
+
+If you want to build your own dtc, it is kept here:
 
        git://git.kernel.org/pub/scm/utils/dtc/dtc.git
 
@@ -136,6 +132,10 @@ u-boot-dtb.bin which does the above step for you also. If you are using
 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
 tree binary.
 
+If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
+device tree at runtime, for example if an earlier bootloader stage creates
+it and passes it to U-Boot.
+
 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
 startup. This is only useful for sandbox. Use the -d flag to U-Boot to
 specify the file to read.
@@ -156,7 +156,10 @@ address of the fdt binary blob, and will override either of the options.
 Be aware that this environment variable is checked prior to relocation,
 when only the compiled-in environment is available. Therefore it is not
 possible to define this variable in the saved SPI/NAND flash
-environment, for example (it will be ignored).
+environment, for example (it will be ignored). After relocation, this
+variable will be set to the address of the newly relocated fdt blob.
+It is read-only and cannot be changed. It can optionally be used to
+control the boot process of Linux with bootm/bootz commands.
 
 To use this, put something like this in your board header file:
 
@@ -171,6 +174,16 @@ After board configuration is done, fdt supported u-boot can be build in two ways
     $ make DEVICE_TREE=<dts-file-name>
 
 
+Relocation, SPL and TPL
+-----------------------
+
+U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
+
+The full device tree is available to U-Boot proper, but normally only a subset
+(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
+'SPL Support' in doc/driver-model/README.txt for more details.
+
+
 Limitations
 -----------