doc/develop/devicetree/control.rst - filogic/uboot - Gitiles

 .. SPDX-License-Identifier: GPL-2.0+
 .. sectionauthor:: Copyright 2011 The Chromium OS Authors
 .. Copyright 2023-2024 Linaro Ltd.

 Devicetree Control in U-Boot
 ============================

 This feature provides for run-time configuration of U-Boot via a flattened
 devicetree (fdt).

 This feature aims to make it possible for a single U-Boot binary to support
 multiple boards, with the exact configuration of each board controlled by
 a flattened devicetree (fdt). This is the approach  taken by Linux kernel for
 ARM and RISC-V and has been used by PowerPC for some time.

 The fdt is a convenient vehicle for implementing run-time configuration
 for three reasons:

 - There is already excellent infrastructure for the fdt: a compiler checks
   the text file and converts it to a compact binary format, and a library
   is already available in U-Boot (libfdt) for handling this format
 - It is extensible since it consists of nodes and properties in a nice
   hierarchical format
 - It is fairly efficient to read incrementally

 The U-Boot Makefile infrastructure allows for building the devicetree blob
 and embedding it in the U-Boot image. This is useful since it allows U-Boot
 to configure itself according to what it finds there. If you have a number
 of similar boards with different peripherals, you can describe the features
 of each board in the devicetree file, and have a single generic source base.

 To enable this feature, select `OF_CONTROL` via Kconfig.


 What is a Flattened Devicetree?
 -------------------------------

 An fdt can be specified in source format as a text file. To read about
 the fdt syntax, take a look at `the devicetree specification`_.

 There is also a `devicetree compiler mailing list`_ for the compiler and
 associated tools.

 In case you are wondering, OF stands for Open Firmware. This follows the
 convention used in Linux.


 Tools
 -----

 To create flattened device trees the device tree compiler is used. This is
 provided by U-Boot automatically. If you have a system version of dtc
 (typically in the 'device-tree-compiler' package), that system version 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

 You can decode a binary file with::

     dtc -I dtb -O dts <filename.dtb>

 That repo also includes `fdtget`/`fdtput` for reading and writing properties in
 a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.


 Where do I get a devicetree file for my board?
 ----------------------------------------------

 The devicetree files and devicetree bindings are maintained as part of the Linux
 kernel git repository. Traditionally, U-Boot placed copies of devicetree source
 files from the Linux kernel into `arch/<arch>/dts/<name>.dts`. However, this
 required each board maintainer to manually keep their devicetree in sync with
 the Linux kernel and often led to divergence between these copies.

 U-Boot rather maintains a Git subtree as `dts/upstream/` sub-directory. It is
 regularly synced with the Linux kernel and hence no need for manual devicetree
 sync. You may find that the `dts/upstream/` already has a suitable devicetree
 file for your board. Look in `dts/upstream/src/<arch>/<vendor>`.

 If not you might find other boards with suitable files that you can
 modify to your needs. Look in the board directories for files with a
 .dts extension.

 Failing that, you could write one from scratch yourself!


 Resyncing with devicetree-rebasing
 ----------------------------------

 The `devicetree-rebasing repository`_ maintains a mirror copy of
 devicetree files along with the bindings synced at every Linux kernel major
 release or intermediate release candidates. The U-Boot maintainers regularly
 sync the `dts/upstream/` subtree from the devicetree-rebasing repo whenever
 the next branch opens (refer: :doc:`../release_cycle`) with the latest mainline
 Linux kernel release. To sync the `dts/upstream/` subtree, run::

     ./tools/update-subtree.sh pull dts <devicetree-rebasing-release-tag>

 If required it is also possible to cherry-pick fixes from the
 devicetree-rebasing repository prior to next sync, usage::

     ./tools/update-subtree.sh pick dts <devicetree-rebasing-commit-id>


 Configuration
 -------------

 SoC/board maintainers are encouraged to migrate to use synced copies from
 `dts/upstream/src/<arch>/<vendor>`. To do that add `imply OF_UPSTREAM` for the
 SoC being used via Kconfig and set `DEFAULT_DEVICE_TREE=<vendor>/<name>` when
 prompted by Kconfig.

 However, if `dts/upstream/` hasn't yet received devicetree source file for your
 newly added board support then one option is that you can add the corresponding
 devicetree source file as `arch/<arch>/dts/<name>.dts`. To select that add `#
 CONFIG_OF_UPSTREAM is not set` and set `DEFAULT_DEVICE_TREE=<name>` when
 prompted by Kconfig. Another option is that you can use the "pick" option of
 `tools/update-subtree.sh` mentioned above to bring in the commits that you
 need.

 This should include your CPU or SoC's devicetree file. On top of that any U-Boot
 specific tweaks (see: :ref:`dttweaks`) can be made for your board.

 If `OF_EMBED` is selected by Kconfig, then it will be picked up and built into
 the U-Boot image (including u-boot.bin). This is suitable for debugging
 and development only and is not recommended for production devices.

 If `OF_SEPARATE` is selected by Kconfig, then it will be built and placed in
 a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
 in u-boot.bin so you can still just flash u-boot.bin onto your board. If Kconfig
 option `SPL_FRAMEWORK` is enabled, then u-boot.img will be built to include the
 device tree binary.

 If `OF_BOARD` is selected by Kconfig, a board-specific routine will provide the
 devicetree at runtime, for example if an earlier bootloader stage creates
 it and passes it to U-Boot.

 If `BLOBLIST` is selected by Kconfig, the devicetree may come from a bloblist
 passed from a previous stage, if present.

 If `SANDBOX` is selected by Kconfig, then it will be read from a file on
 startup. Use the -d flag to U-Boot to specify the file to read, -D for the
 default and -T for the test devicetree, used to run sandbox unit tests.

 You cannot use more than one of these options at the same time.

 To use a devicetree file that you have compiled yourself, pass
 EXT_DTB=<filename> to 'make', as in::

    make EXT_DTB=boot/am335x-boneblack-pubkey.dtb

 Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
 if used, and u-boot-dtb.bin.

 If you wish to put the fdt at a different address in memory, you can
 define the "fdtcontroladdr" environment variable. This is the hex
 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). 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::

    #define CFG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"

 Build:

 After the board configuration is done, fdt supported u-boot can be built in two
 ways:

 #  build the default dts which is selected by DEFAULT_DEVICE_TREE Kconfig::

     $ make

 #  build the user specified dts file::

     $ make DEVICE_TREE=<dts-file-name>


 .. _dttweaks:

 Adding tweaks for U-Boot
 ------------------------

 With `dts/upstream` Git subtree, it is ensured that devicetree files in U-Boot
 are an exact copy of those in Linux kernel available under
 `dts/upstream/src/<arch>/<vendor>`.

 U-Boot is of course a very different project from Linux, e.g. it operates under
 much more restrictive memory and code-size constraints. Where Linux may use a
 full clock driver with Common Clock Format (CCF) to find the input clock to the
 UART, U-Boot typically wants to output a banner as early as possible before too
 much code has run.

 A second difference is that U-Boot includes different phases. For SPL,
 constraints are even more extreme and the devicetree is shrunk to remove
 unwanted nodes, or even turned into C code to avoid access overhead.

 U-Boot automatically looks for and includes a file with updates to the standard
 devicetree for your board, searching for them in `arch/<arch>/dts/` in this
 order::

    <orig_filename>-u-boot.dtsi
    <CONFIG_SYS_SOC>-u-boot.dtsi
    <CONFIG_SYS_CPU>-u-boot.dtsi
    <CONFIG_SYS_VENDOR>-u-boot.dtsi
    u-boot.dtsi

 Only one of these is selected but of course you can #include another one within
 that file, to create a hierarchy of shared files.


 External .dtsi fragments
 ------------------------

 Apart from describing the hardware present, U-Boot also uses its
 control dtb for various configuration purposes. For example, the
 public key(s) used for Verified Boot are embedded in a specific format
 in a /signature node.

 As mentioned above, the U-Boot build system automatically includes a
 `*-u-boot.dtsi` file, if found, containing U-Boot specific
 quirks. However, some data, such as the mentioned public keys, are not
 appropriate for upstream U-Boot but are better kept and maintained
 outside the U-Boot repository. You can use `DEVICE_TREE_INCLUDES` Kconfig
 option to specify a list of .dtsi files that will also be included when
 building .dtb files.


 Devicetree bindings schema checks
 ---------------------------------

 With devicetee-rebasing Git subtree, the devicetree bindings are also regularly
 synced with Linux kernel as `dts/upstream/Bindings/` sub-directory. This
 allows U-Boot to run devicetree bindings schema checks which will bring
 compliance to U-Boot core/drivers regarding usage of devicetree.

 Dependencies
 ~~~~~~~~~~~~

 The DT schema project must be installed in order to validate the DT schema
 binding documents and validate DTS files using the DT schema. For installation
 instructions, refer to the `DT schema project page`_.

 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
 installed. Ensure they are in your PATH (~/.local/bin by default).

 You should also install yamllint (used by dtschema when present). On Debian/
 Ubuntu systems::

     apt install yamllint

 Running checks
 ~~~~~~~~~~~~~~

 In order to perform validation of DTB files, use the ``dtbs_check`` target::

     make dtbs_check

 It is also possible to run checks with a subset of matching schema files by
 setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or
 patterns (partial match of a fixed string). Each file or pattern should be
 separated by ':'.

 ::

     make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
     make dtbs_check DT_SCHEMA_FILES=/gpio/
     make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml


 Relocation, SPL and TPL
 -----------------------

 U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.

 The full devicetree 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/design.rst for more details.


 Using several DTBs in the SPL (SPL_MULTI_DTB_FIT Kconfig option)
 ----------------------------------------------------------------
 In some rare cases it is desirable to let SPL be able to select one DTB among
 many. This usually not very useful as the DTB for the SPL is small and usually
 fits several platforms. However the DTB sometimes include information that do
 work on several platforms (like IO tuning parameters).
 In this case it is possible to use SPL_MULTI_DTB_FIT Kconfig option. This option
 appends to the SPL a FIT image containing several DTBs listed in SPL_OF_LIST.
 board_fit_config_name_match() is called to select the right DTB.

 If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
 containing the board ID for example), it possible to start with a generic DTB
 and then switch over to the right DTB after the detection. For this purpose,
 the platform code must call fdtdec_resetup(). Based on the returned flag, the
 platform may have to re-initialise the DM subsystem using dm_uninit() and
 dm_init_and_scan().


 Limitations
 -----------

 Devicetrees can help reduce the complexity of supporting variants of boards
 which use the same SOC / CPU.

 However U-Boot is designed to build for a single architecture type and CPU
 type. So for example it is not possible to build a single ARM binary
 which runs on your AT91 and OMAP boards, relying on an fdt to configure
 the various features. This is because you must select one of
 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
 time. Similarly U-Boot cannot be built for multiple cpu types or
 architectures.

 It is important to understand that the fdt only selects options
 available in the platform / drivers. It cannot add new drivers (yet). So
 you must still have the Kconfig option to enable the driver. For example,
 you need to enable SYS_NS16550 Kconfig option to bring in the NS16550 driver,
 but can use the fdt to specific the UART clock, peripheral address, etc.
 In very broad terms, the Kconfig options in general control *what* driver
 files are pulled in, and the fdt controls *how* those files work.

 History
 -------

 U-Boot configuration was previous done using CONFIG options in the board
 config file. This eventually got out of hand with nearly 10,000 options.

 U-Boot adopted devicetrees around the same time as Linux and early boards
 used it before Linux (e.g. snow). The two projects developed in parallel
 and there are still some differences in the bindings for certain boards.
 While there has been discussion of having a separate repository for devicetree
 files, in practice the Linux kernel Git repository has become the place where
 these are stored, with U-Boot taking copies via
 `devicetree-rebasing repository`_ and adding tweaks with u-boot.dtsi files.

 .. _the devicetree specification: https://www.devicetree.org/specifications/
 .. _devicetree compiler mailing list: https://www.spinics.net/lists/devicetree-compiler/
 .. _devicetree-rebasing repository: https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git
 .. _DT schema project page: https://github.com/devicetree-org/dt-schema/tree/main
	.. SPDX-License-Identifier: GPL-2.0+
	.. sectionauthor:: Copyright 2011 The Chromium OS Authors
	.. Copyright 2023-2024 Linaro Ltd.

	Devicetree Control in U-Boot
	============================

	This feature provides for run-time configuration of U-Boot via a flattened
	devicetree (fdt).

	This feature aims to make it possible for a single U-Boot binary to support
	multiple boards, with the exact configuration of each board controlled by
	a flattened devicetree (fdt). This is the approach taken by Linux kernel for
	ARM and RISC-V and has been used by PowerPC for some time.

	The fdt is a convenient vehicle for implementing run-time configuration
	for three reasons:

	- There is already excellent infrastructure for the fdt: a compiler checks
	the text file and converts it to a compact binary format, and a library
	is already available in U-Boot (libfdt) for handling this format
	- It is extensible since it consists of nodes and properties in a nice
	hierarchical format
	- It is fairly efficient to read incrementally

	The U-Boot Makefile infrastructure allows for building the devicetree blob
	and embedding it in the U-Boot image. This is useful since it allows U-Boot
	to configure itself according to what it finds there. If you have a number
	of similar boards with different peripherals, you can describe the features
	of each board in the devicetree file, and have a single generic source base.

	To enable this feature, select `OF_CONTROL` via Kconfig.


	What is a Flattened Devicetree?
	-------------------------------

	An fdt can be specified in source format as a text file. To read about
	the fdt syntax, take a look at `the devicetree specification`_.

	There is also a `devicetree compiler mailing list`_ for the compiler and
	associated tools.

	In case you are wondering, OF stands for Open Firmware. This follows the
	convention used in Linux.


	Tools
	-----

	To create flattened device trees the device tree compiler is used. This is
	provided by U-Boot automatically. If you have a system version of dtc
	(typically in the 'device-tree-compiler' package), that system version 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

	You can decode a binary file with::

	dtc -I dtb -O dts <filename.dtb>

	That repo also includes `fdtget`/`fdtput` for reading and writing properties in
	a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.


	Where do I get a devicetree file for my board?
	----------------------------------------------

	The devicetree files and devicetree bindings are maintained as part of the Linux
	kernel git repository. Traditionally, U-Boot placed copies of devicetree source
	files from the Linux kernel into `arch/<arch>/dts/<name>.dts`. However, this
	required each board maintainer to manually keep their devicetree in sync with
	the Linux kernel and often led to divergence between these copies.

	U-Boot rather maintains a Git subtree as `dts/upstream/` sub-directory. It is
	regularly synced with the Linux kernel and hence no need for manual devicetree
	sync. You may find that the `dts/upstream/` already has a suitable devicetree
	file for your board. Look in `dts/upstream/src/<arch>/<vendor>`.

	If not you might find other boards with suitable files that you can
	modify to your needs. Look in the board directories for files with a
	.dts extension.

	Failing that, you could write one from scratch yourself!


	Resyncing with devicetree-rebasing
	----------------------------------

	The `devicetree-rebasing repository`_ maintains a mirror copy of
	devicetree files along with the bindings synced at every Linux kernel major
	release or intermediate release candidates. The U-Boot maintainers regularly
	sync the `dts/upstream/` subtree from the devicetree-rebasing repo whenever
	the next branch opens (refer: :doc:`../release_cycle`) with the latest mainline
	Linux kernel release. To sync the `dts/upstream/` subtree, run::

	./tools/update-subtree.sh pull dts <devicetree-rebasing-release-tag>

	If required it is also possible to cherry-pick fixes from the
	devicetree-rebasing repository prior to next sync, usage::

	./tools/update-subtree.sh pick dts <devicetree-rebasing-commit-id>


	Configuration
	-------------

	SoC/board maintainers are encouraged to migrate to use synced copies from
	`dts/upstream/src/<arch>/<vendor>`. To do that add `imply OF_UPSTREAM` for the
	SoC being used via Kconfig and set `DEFAULT_DEVICE_TREE=<vendor>/<name>` when
	prompted by Kconfig.

	However, if `dts/upstream/` hasn't yet received devicetree source file for your
	newly added board support then one option is that you can add the corresponding
	devicetree source file as `arch/<arch>/dts/<name>.dts`. To select that add `#
	CONFIG_OF_UPSTREAM is not set` and set `DEFAULT_DEVICE_TREE=<name>` when
	prompted by Kconfig. Another option is that you can use the "pick" option of
	`tools/update-subtree.sh` mentioned above to bring in the commits that you
	need.

	This should include your CPU or SoC's devicetree file. On top of that any U-Boot
	specific tweaks (see: :ref:`dttweaks`) can be made for your board.

	If `OF_EMBED` is selected by Kconfig, then it will be picked up and built into
	the U-Boot image (including u-boot.bin). This is suitable for debugging
	and development only and is not recommended for production devices.

	If `OF_SEPARATE` is selected by Kconfig, then it will be built and placed in
	a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
	in u-boot.bin so you can still just flash u-boot.bin onto your board. If Kconfig
	option `SPL_FRAMEWORK` is enabled, then u-boot.img will be built to include the
	device tree binary.

	If `OF_BOARD` is selected by Kconfig, a board-specific routine will provide the
	devicetree at runtime, for example if an earlier bootloader stage creates
	it and passes it to U-Boot.

	If `BLOBLIST` is selected by Kconfig, the devicetree may come from a bloblist
	passed from a previous stage, if present.

	If `SANDBOX` is selected by Kconfig, then it will be read from a file on
	startup. Use the -d flag to U-Boot to specify the file to read, -D for the
	default and -T for the test devicetree, used to run sandbox unit tests.

	You cannot use more than one of these options at the same time.

	To use a devicetree file that you have compiled yourself, pass
	EXT_DTB=<filename> to 'make', as in::

	make EXT_DTB=boot/am335x-boneblack-pubkey.dtb

	Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
	if used, and u-boot-dtb.bin.

	If you wish to put the fdt at a different address in memory, you can
	define the "fdtcontroladdr" environment variable. This is the hex
	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). 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::

	#define CFG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"

	Build:

	After the board configuration is done, fdt supported u-boot can be built in two
	ways:

	# build the default dts which is selected by DEFAULT_DEVICE_TREE Kconfig::

	$ make

	# build the user specified dts file::

	$ make DEVICE_TREE=<dts-file-name>


	.. _dttweaks:

	Adding tweaks for U-Boot
	------------------------

	With `dts/upstream` Git subtree, it is ensured that devicetree files in U-Boot
	are an exact copy of those in Linux kernel available under
	`dts/upstream/src/<arch>/<vendor>`.

	U-Boot is of course a very different project from Linux, e.g. it operates under
	much more restrictive memory and code-size constraints. Where Linux may use a
	full clock driver with Common Clock Format (CCF) to find the input clock to the
	UART, U-Boot typically wants to output a banner as early as possible before too
	much code has run.

	A second difference is that U-Boot includes different phases. For SPL,
	constraints are even more extreme and the devicetree is shrunk to remove
	unwanted nodes, or even turned into C code to avoid access overhead.

	U-Boot automatically looks for and includes a file with updates to the standard
	devicetree for your board, searching for them in `arch/<arch>/dts/` in this
	order::

	<orig_filename>-u-boot.dtsi
	<CONFIG_SYS_SOC>-u-boot.dtsi
	<CONFIG_SYS_CPU>-u-boot.dtsi
	<CONFIG_SYS_VENDOR>-u-boot.dtsi
	u-boot.dtsi

	Only one of these is selected but of course you can #include another one within
	that file, to create a hierarchy of shared files.


	External .dtsi fragments
	------------------------

	Apart from describing the hardware present, U-Boot also uses its
	control dtb for various configuration purposes. For example, the
	public key(s) used for Verified Boot are embedded in a specific format
	in a /signature node.

	As mentioned above, the U-Boot build system automatically includes a
	`*-u-boot.dtsi` file, if found, containing U-Boot specific
	quirks. However, some data, such as the mentioned public keys, are not
	appropriate for upstream U-Boot but are better kept and maintained
	outside the U-Boot repository. You can use `DEVICE_TREE_INCLUDES` Kconfig
	option to specify a list of .dtsi files that will also be included when
	building .dtb files.


	Devicetree bindings schema checks
	---------------------------------

	With devicetee-rebasing Git subtree, the devicetree bindings are also regularly
	synced with Linux kernel as `dts/upstream/Bindings/` sub-directory. This
	allows U-Boot to run devicetree bindings schema checks which will bring
	compliance to U-Boot core/drivers regarding usage of devicetree.

	Dependencies
	~~~~~~~~~~~~

	The DT schema project must be installed in order to validate the DT schema
	binding documents and validate DTS files using the DT schema. For installation
	instructions, refer to the `DT schema project page`_.

	Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
	installed. Ensure they are in your PATH (~/.local/bin by default).

	You should also install yamllint (used by dtschema when present). On Debian/
	Ubuntu systems::

	apt install yamllint

	Running checks
	~~~~~~~~~~~~~~

	In order to perform validation of DTB files, use the ``dtbs_check`` target::

	make dtbs_check

	It is also possible to run checks with a subset of matching schema files by
	setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or
	patterns (partial match of a fixed string). Each file or pattern should be
	separated by ':'.

	::

	make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
	make dtbs_check DT_SCHEMA_FILES=/gpio/
	make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml


	Relocation, SPL and TPL
	-----------------------

	U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.

	The full devicetree 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/design.rst for more details.


	Using several DTBs in the SPL (SPL_MULTI_DTB_FIT Kconfig option)
	----------------------------------------------------------------
	In some rare cases it is desirable to let SPL be able to select one DTB among
	many. This usually not very useful as the DTB for the SPL is small and usually
	fits several platforms. However the DTB sometimes include information that do
	work on several platforms (like IO tuning parameters).
	In this case it is possible to use SPL_MULTI_DTB_FIT Kconfig option. This option
	appends to the SPL a FIT image containing several DTBs listed in SPL_OF_LIST.
	board_fit_config_name_match() is called to select the right DTB.

	If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
	containing the board ID for example), it possible to start with a generic DTB
	and then switch over to the right DTB after the detection. For this purpose,
	the platform code must call fdtdec_resetup(). Based on the returned flag, the
	platform may have to re-initialise the DM subsystem using dm_uninit() and
	dm_init_and_scan().


	Limitations
	-----------

	Devicetrees can help reduce the complexity of supporting variants of boards
	which use the same SOC / CPU.

	However U-Boot is designed to build for a single architecture type and CPU
	type. So for example it is not possible to build a single ARM binary
	which runs on your AT91 and OMAP boards, relying on an fdt to configure
	the various features. This is because you must select one of
	the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
	time. Similarly U-Boot cannot be built for multiple cpu types or
	architectures.

	It is important to understand that the fdt only selects options
	available in the platform / drivers. It cannot add new drivers (yet). So
	you must still have the Kconfig option to enable the driver. For example,
	you need to enable SYS_NS16550 Kconfig option to bring in the NS16550 driver,
	but can use the fdt to specific the UART clock, peripheral address, etc.
	In very broad terms, the Kconfig options in general control what driver
	files are pulled in, and the fdt controls how those files work.

	History
	-------

	U-Boot configuration was previous done using CONFIG options in the board
	config file. This eventually got out of hand with nearly 10,000 options.

	U-Boot adopted devicetrees around the same time as Linux and early boards
	used it before Linux (e.g. snow). The two projects developed in parallel
	and there are still some differences in the bindings for certain boards.
	While there has been discussion of having a separate repository for devicetree
	files, in practice the Linux kernel Git repository has become the place where
	these are stored, with U-Boot taking copies via
	`devicetree-rebasing repository`_ and adding tweaks with u-boot.dtsi files.

	.. _the devicetree specification: https://www.devicetree.org/specifications/
	.. _devicetree compiler mailing list: https://www.spinics.net/lists/devicetree-compiler/
	.. _devicetree-rebasing repository: https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git
	.. _DT schema project page: https://github.com/devicetree-org/dt-schema/tree/main