#
  # 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