1 .. SPDX-License-Identifier: GPL-2.0+
2 .. sectionauthor:: Copyright 2011 The Chromium OS Authors
4 Device Tree Control in U-Boot
5 =============================
7 This feature provides for run-time configuration of U-Boot via a flat
8 device tree (fdt). U-Boot configuration has traditionally been done
9 using CONFIG options in the board config file. This feature aims to
10 make it possible for a single U-Boot binary to support multiple boards,
11 with the exact configuration of each board controlled by a flat device
12 tree (fdt). This is the approach recently taken by the ARM Linux kernel
13 and has been used by PowerPC for some time.
15 The fdt is a convenient vehicle for implementing run-time configuration
16 for three reasons. Firstly it is easy to use, being a simple text file.
17 It is extensible since it consists of nodes and properties in a nice
20 Finally, there is already excellent infrastructure for the fdt: a
21 compiler checks the text file and converts it to a compact binary
22 format, and a library is already available in U-Boot (libfdt) for
25 The dts directory contains a Makefile for building the device tree blob
26 and embedding it in your U-Boot image. This is useful since it allows
27 U-Boot to configure itself according to what it finds there. If you have
28 a number of similar boards with different peripherals, you can describe
29 the features of each board in the device tree file, and have a single
32 To enable this feature, add CONFIG_OF_CONTROL to your board config file.
35 What is a Flat Device Tree?
36 ---------------------------
38 An fdt can be specified in source format as a text file. To read about
39 the fdt syntax, take a look at the specification (dtspec_).
41 You also might find this section of the Linux kernel documentation
42 useful: (access this in the Linux kernel source code)
44 Documentation/devicetree/booting-without-of.txt
46 There is also a mailing list:
48 http://lists.ozlabs.org/listinfo/devicetree-discuss
50 In case you are wondering, OF stands for Open Firmware.
56 To use this feature you will need to get the device tree compiler. This is
57 provided by U-Boot automatically. If you have a system version of dtc
58 (typically in the 'device-tree-compiler' package), it is currently not used.
60 If you want to build your own dtc, it is kept here::
62 git://git.kernel.org/pub/scm/utils/dtc/dtc.git
66 $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
71 Then run the compiler (your version will vary)::
74 Version: DTC 1.2.0-g2cb4b51f
78 ********** TEST SUMMARY
79 * Total testcases: 1371
82 * Bad configuration: 0
83 * Strange test result: 0
85 You will also find a useful fdtdump utility for decoding a binary file, as
86 well as fdtget/fdtput for reading and writing properties in a binary file.
89 Where do I get an fdt file for my board?
90 ----------------------------------------
92 You may find that the Linux kernel has a suitable file. Look in the
93 kernel source in arch/<arch>/boot/dts.
95 If not you might find other boards with suitable files that you can
96 modify to your needs. Look in the board directories for files with a
99 Failing that, you could write one from scratch yourself!
107 #define CONFIG_DEFAULT_DEVICE_TREE "<name>"
109 to set the filename of the device tree source. Then put your device tree
112 board/<vendor>/dts/<name>.dts
114 This should include your CPU or SOC's device tree file, placed in
115 arch/<arch>/dts, and then make any adjustments required.
117 If CONFIG_OF_EMBED is defined, then it will be picked up and built into
118 the U-Boot image (including u-boot.bin). This is suitable for debugging
119 and development only and is not recommended for production devices.
121 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
122 a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
125 cat u-boot-nodtb.bin u-boot.dtb >image.bin
127 and then flash image.bin onto your board. Note that U-Boot creates
128 u-boot-dtb.bin which does the above step for you also. Resulting
129 u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
130 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
133 If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
134 device tree at runtime, for example if an earlier bootloader stage creates
135 it and passes it to U-Boot.
137 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
138 startup. This is only useful for sandbox. Use the -d flag to U-Boot to
139 specify the file to read.
141 You cannot use more than one of these options at the same time.
143 To use a device tree file that you have compiled yourself, pass
144 EXT_DTB=<filename> to 'make', as in::
146 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
148 Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
149 if used, and u-boot-dtb.bin.
151 If you wish to put the fdt at a different address in memory, you can
152 define the "fdtcontroladdr" environment variable. This is the hex
153 address of the fdt binary blob, and will override either of the options.
154 Be aware that this environment variable is checked prior to relocation,
155 when only the compiled-in environment is available. Therefore it is not
156 possible to define this variable in the saved SPI/NAND flash
157 environment, for example (it will be ignored). After relocation, this
158 variable will be set to the address of the newly relocated fdt blob.
159 It is read-only and cannot be changed. It can optionally be used to
160 control the boot process of Linux with bootm/bootz commands.
162 To use this, put something like this in your board header file::
164 #define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
168 After board configuration is done, fdt supported u-boot can be build in two
171 # build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
175 # build the user specified dts file::
177 $ make DEVICE_TREE=<dts-file-name>
180 Relocation, SPL and TPL
181 -----------------------
183 U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
185 The full device tree is available to U-Boot proper, but normally only a subset
186 (or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
187 'SPL Support' in doc/driver-model/design.rst for more details.
190 Using several DTBs in the SPL (CONFIG_SPL_MULTI_DTB)
191 ----------------------------------------------------
192 In some rare cases it is desirable to let SPL be able to select one DTB among
193 many. This usually not very useful as the DTB for the SPL is small and usually
194 fits several platforms. However the DTB sometimes include information that do
195 work on several platforms (like IO tuning parameters).
196 In this case it is possible to use CONFIG_SPL_MULTI_DTB. This option appends to
197 the SPL a FIT image containing several DTBs listed in SPL_OF_LIST.
198 board_fit_config_name_match() is called to select the right DTB.
200 If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
201 containing the board ID for example), it possible to start with a generic DTB
202 and then switch over to the right DTB after the detection. For this purpose,
203 the platform code must call fdtdec_resetup(). Based on the returned flag, the
204 platform may have to re-initiliaze the DM subusystem using dm_uninit() and
211 U-Boot is designed to build with a single architecture type and CPU
212 type. So for example it is not possible to build a single ARM binary
213 which runs on your AT91 and OMAP boards, relying on an fdt to configure
214 the various features. This is because you must select one of
215 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
216 time. Similarly you cannot build for multiple cpu types or
219 That said the complexity reduction by using fdt to support variants of
220 boards which use the same SOC / CPU can be substantial.
222 It is important to understand that the fdt only selects options
223 available in the platform / drivers. It cannot add new drivers (yet). So
224 you must still have the CONFIG option to enable the driver. For example,
225 you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
226 but can use the fdt to specific the UART clock, peripheral address, etc.
227 In very broad terms, the CONFIG options in general control *what* driver
228 files are pulled in, and the fdt controls *how* those files work.
230 .. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf