doc/README.fdt-control - u-boot - Git at Google

 #
 # Copyright (c) 2011 The Chromium OS Authors.
 #
 # SPDX-License-Identifier:	GPL-2.0+
 #

 Device Tree Control in U-Boot
 =============================

 This feature provides for run-time configuration of U-Boot via a flat
 device tree (fdt). U-Boot configuration has traditionally been done
 using CONFIG options in the board config file. 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 flat device
 tree (fdt). This is the approach recently taken by the ARM Linux kernel
 and has been used by PowerPC for some time.

 The fdt is a convenient vehicle for implementing run-time configuration
 for three reasons. Firstly it is easy to use, being a simple text file.
 It is extensible since it consists of nodes and properties in a nice
 hierarchical format.

 Finally, 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.

 The dts directory contains a Makefile for building the device tree blob
 and embedding it in your 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 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?
 ---------------------------

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

 https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf

 You also might find this section of the Linux kernel documentation
 useful: (access this in the Linux kernel source code)

 	Documentation/devicetree/booting-without-of.txt

 There is also a mailing list:

 	http://lists.ozlabs.org/listinfo/devicetree-discuss

 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:

 	git://git.kernel.org/pub/scm/utils/dtc/dtc.git

 For example:

 	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
 	$ cd dtc
 	$ make
 	$ sudo make install

 Then run the compiler (your version will vary):

 	$ dtc -v
 	Version: DTC 1.2.0-g2cb4b51f
 	$ make tests
 	$ cd tests
 	$ ./run_tests.sh
 	********** TEST SUMMARY
 	*     Total testcases:	1371
 	*                PASS:	1371
 	*                FAIL:	0
 	*   Bad configuration:	0
 	* Strange test result:	0

 You will also find a useful fdtdump utility for decoding a binary file, as
 well as fdtget/fdtput for reading and writing properties in a binary file.


 Where do I get an fdt file for my board?
 ----------------------------------------

 You may find that the Linux kernel has a suitable file. Look in the
 kernel source in arch/<arch>/boot/dts.

 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!


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

 Use:

 #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"

 to set the filename of the device tree source. Then put your device tree
 file into

 	board/<vendor>/dts/<name>.dts

 This should include your CPU or SOC's device tree file, placed in
 arch/<arch>/dts, and then make any adjustments required.

 If CONFIG_OF_EMBED is defined, 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 CONFIG_OF_SEPARATE is defined, then it will be built and placed in
 a u-boot.dtb file alongside u-boot.bin. A common approach is then to
 join the two:

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

 and then flash image.bin onto your board. Note that U-Boot creates
 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_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.

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

 To use a device tree 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).

 To use this, put something like this in your board header file:

 #define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"

 Build:

 After board configuration is done, fdt supported u-boot can be build in two ways:
 1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
     $ make
 2)  build the user specified dts file
     $ make DEVICE_TREE=<dts-file-name>


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

 U-Boot is designed to build with 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 you cannot build for multiple cpu types or
 architectures.

 That said the complexity reduction by using fdt to support variants of
 boards which use the same SOC / CPU can be substantial.

 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 CONFIG option to enable the driver. For example,
 you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
 but can use the fdt to specific the UART clock, peripheral address, etc.
 In very broad terms, the CONFIG options in general control *what* driver
 files are pulled in, and the fdt controls *how* those files work.

 --
 Simon Glass <sjg@chromium.org>
 1-Sep-11
	#
	# Copyright (c) 2011 The Chromium OS Authors.
	#
	# SPDX-License-Identifier: GPL-2.0+
	#

	Device Tree Control in U-Boot
	=============================

	This feature provides for run-time configuration of U-Boot via a flat
	device tree (fdt). U-Boot configuration has traditionally been done
	using CONFIG options in the board config file. 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 flat device
	tree (fdt). This is the approach recently taken by the ARM Linux kernel
	and has been used by PowerPC for some time.

	The fdt is a convenient vehicle for implementing run-time configuration
	for three reasons. Firstly it is easy to use, being a simple text file.
	It is extensible since it consists of nodes and properties in a nice
	hierarchical format.

	Finally, 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.

	The dts directory contains a Makefile for building the device tree blob
	and embedding it in your 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 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?
	---------------------------

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

	https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf

	You also might find this section of the Linux kernel documentation
	useful: (access this in the Linux kernel source code)

	Documentation/devicetree/booting-without-of.txt

	There is also a mailing list:

	http://lists.ozlabs.org/listinfo/devicetree-discuss

	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:

	git://git.kernel.org/pub/scm/utils/dtc/dtc.git

	For example:

	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
	$ cd dtc
	$ make
	$ sudo make install

	Then run the compiler (your version will vary):

	$ dtc -v
	Version: DTC 1.2.0-g2cb4b51f
	$ make tests
	$ cd tests
	$ ./run_tests.sh
	********** TEST SUMMARY
	* Total testcases: 1371
	* PASS: 1371
	* FAIL: 0
	* Bad configuration: 0
	* Strange test result: 0

	You will also find a useful fdtdump utility for decoding a binary file, as
	well as fdtget/fdtput for reading and writing properties in a binary file.


	Where do I get an fdt file for my board?
	----------------------------------------

	You may find that the Linux kernel has a suitable file. Look in the
	kernel source in arch/<arch>/boot/dts.

	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!


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

	Use:

	#define CONFIG_DEFAULT_DEVICE_TREE "<name>"

	to set the filename of the device tree source. Then put your device tree
	file into

	board/<vendor>/dts/<name>.dts

	This should include your CPU or SOC's device tree file, placed in
	arch/<arch>/dts, and then make any adjustments required.

	If CONFIG_OF_EMBED is defined, 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 CONFIG_OF_SEPARATE is defined, then it will be built and placed in
	a u-boot.dtb file alongside u-boot.bin. A common approach is then to
	join the two:

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

	and then flash image.bin onto your board. Note that U-Boot creates
	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_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.

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

	To use a device tree 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).

	To use this, put something like this in your board header file:

	#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"

	Build:

	After board configuration is done, fdt supported u-boot can be build in two ways:
	1) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
	$ make
	2) build the user specified dts file
	$ make DEVICE_TREE=<dts-file-name>


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

	U-Boot is designed to build with 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 you cannot build for multiple cpu types or
	architectures.

	That said the complexity reduction by using fdt to support variants of
	boards which use the same SOC / CPU can be substantial.

	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 CONFIG option to enable the driver. For example,
	you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
	but can use the fdt to specific the UART clock, peripheral address, etc.
	In very broad terms, the CONFIG options in general control what driver
	files are pulled in, and the fdt controls how those files work.

	--
	Simon Glass <sjg@chromium.org>
	1-Sep-11