README.fdt-control 7.1 KB

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211
  1. #
  2. # Copyright (c) 2011 The Chromium OS Authors.
  3. #
  4. # SPDX-License-Identifier: GPL-2.0+
  5. #
  6. Device Tree Control in U-Boot
  7. =============================
  8. This feature provides for run-time configuration of U-Boot via a flat
  9. device tree (fdt). U-Boot configuration has traditionally been done
  10. using CONFIG options in the board config file. This feature aims to
  11. make it possible for a single U-Boot binary to support multiple boards,
  12. with the exact configuration of each board controlled by a flat device
  13. tree (fdt). This is the approach recently taken by the ARM Linux kernel
  14. 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
  18. hierarchical format.
  19. Finally, there is already excellent infrastructure for the fdt: a
  20. compiler checks the text file and converts it to a compact binary
  21. format, and a library is already available in U-Boot (libfdt) for
  22. handling this format.
  23. The dts directory contains a Makefile for building the device tree blob
  24. and embedding it in your U-Boot image. This is useful since it allows
  25. U-Boot to configure itself according to what it finds there. If you have
  26. a number of similar boards with different peripherals, you can describe
  27. the features of each board in the device tree file, and have a single
  28. generic source base.
  29. To enable this feature, add CONFIG_OF_CONTROL to your board config file.
  30. What is a Flat Device Tree?
  31. ---------------------------
  32. An fdt can be specified in source format as a text file. To read about
  33. the fdt syntax, take a look at the specification here:
  34. https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
  35. You also might find this section of the Linux kernel documentation
  36. useful: (access this in the Linux kernel source code)
  37. Documentation/devicetree/booting-without-of.txt
  38. There is also a mailing list:
  39. http://lists.ozlabs.org/listinfo/devicetree-discuss
  40. In case you are wondering, OF stands for Open Firmware.
  41. Tools
  42. -----
  43. To use this feature you will need to get the device tree compiler here:
  44. git://git.kernel.org/pub/scm/utils/dtc/dtc.git
  45. For example:
  46. $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
  47. $ cd dtc
  48. $ make
  49. $ sudo make install
  50. Then run the compiler (your version will vary):
  51. $ dtc -v
  52. Version: DTC 1.2.0-g2cb4b51f
  53. $ make tests
  54. $ cd tests
  55. $ ./run_tests.sh
  56. ********** TEST SUMMARY
  57. * Total testcases: 1371
  58. * PASS: 1371
  59. * FAIL: 0
  60. * Bad configuration: 0
  61. * Strange test result: 0
  62. You will also find a useful fdtdump utility for decoding a binary file, as
  63. well as fdtget/fdtput for reading and writing properties in a binary file.
  64. Where do I get an fdt file for my board?
  65. ----------------------------------------
  66. You may find that the Linux kernel has a suitable file. Look in the
  67. kernel source in arch/<arch>/boot/dts.
  68. If not you might find other boards with suitable files that you can
  69. modify to your needs. Look in the board directories for files with a
  70. .dts extension.
  71. Failing that, you could write one from scratch yourself!
  72. Configuration
  73. -------------
  74. Use:
  75. #define CONFIG_DEFAULT_DEVICE_TREE "<name>"
  76. to set the filename of the device tree source. Then put your device tree
  77. file into
  78. board/<vendor>/dts/<name>.dts
  79. This should include your CPU or SOC's device tree file, placed in
  80. arch/<arch>/dts, and then make any adjustments required.
  81. If CONFIG_OF_EMBED is defined, then it will be picked up and built into
  82. the U-Boot image (including u-boot.bin). This is suitable for debugging
  83. and development only and is not recommended for production devices.
  84. If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
  85. a u-boot.dtb file alongside u-boot.bin. A common approach is then to
  86. join the two:
  87. cat u-boot.bin u-boot.dtb >image.bin
  88. and then flash image.bin onto your board. Note that U-Boot creates
  89. u-boot-dtb.bin which does the above step for you also. If you are using
  90. CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
  91. tree binary.
  92. If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
  93. startup. This is only useful for sandbox. Use the -d flag to U-Boot to
  94. specify the file to read.
  95. You cannot use more than one of these options at the same time.
  96. To use a device tree file that you have compiled yourself, pass
  97. EXT_DTB=<filename> to 'make', as in:
  98. make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
  99. Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
  100. if used, and u-boot-dtb.bin.
  101. If you wish to put the fdt at a different address in memory, you can
  102. define the "fdtcontroladdr" environment variable. This is the hex
  103. address of the fdt binary blob, and will override either of the options.
  104. Be aware that this environment variable is checked prior to relocation,
  105. when only the compiled-in environment is available. Therefore it is not
  106. possible to define this variable in the saved SPI/NAND flash
  107. environment, for example (it will be ignored). After relocation, this
  108. variable will be set to the address of the newly relocated fdt blob.
  109. It is read-only and cannot be changed. It can optionally be used to
  110. control the boot process of Linux with bootm/bootz commands.
  111. To use this, put something like this in your board header file:
  112. #define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
  113. Build:
  114. After board configuration is done, fdt supported u-boot can be build in two ways:
  115. 1) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
  116. $ make
  117. 2) build the user specified dts file
  118. $ make DEVICE_TREE=<dts-file-name>
  119. Configuration Options
  120. ---------------------
  121. A number of run-time configuration options are provided in the /config node
  122. of the control device tree. You can access these using fdtdec_get_config_int(),
  123. fdtdec_get_config_bool() and fdtdec_get_config_string().
  124. Available options are:
  125. silent-console
  126. If present and non-zero, the console is silenced by default on boot.
  127. no-keyboard
  128. Tells U-Boot not to expect an attached keyboard with a VGA console
  129. Limitations
  130. -----------
  131. U-Boot is designed to build with a single architecture type and CPU
  132. type. So for example it is not possible to build a single ARM binary
  133. which runs on your AT91 and OMAP boards, relying on an fdt to configure
  134. the various features. This is because you must select one of
  135. the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
  136. time. Similarly you cannot build for multiple cpu types or
  137. architectures.
  138. That said the complexity reduction by using fdt to support variants of
  139. boards which use the same SOC / CPU can be substantial.
  140. It is important to understand that the fdt only selects options
  141. available in the platform / drivers. It cannot add new drivers (yet). So
  142. you must still have the CONFIG option to enable the driver. For example,
  143. you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
  144. but can use the fdt to specific the UART clock, peripheral address, etc.
  145. In very broad terms, the CONFIG options in general control *what* driver
  146. files are pulled in, and the fdt controls *how* those files work.
  147. --
  148. Simon Glass <sjg@chromium.org>
  149. 1-Sep-11