doc/README.fdt-control

   1 #
   2 # Copyright (c) 2011 The Chromium OS Authors.
   3 #
   4 # SPDX-License-Identifier:      GPL-2.0+
   5 #
   6
   7 Device Tree Control in U-Boot
   8 =============================
   9
  10 This feature provides for run-time configuration of U-Boot via a flat
  11 device tree (fdt). U-Boot configuration has traditionally been done
  12 using CONFIG options in the board config file. This feature aims to
  13 make it possible for a single U-Boot binary to support multiple boards,
  14 with the exact configuration of each board controlled by a flat device
  15 tree (fdt). This is the approach recently taken by the ARM Linux kernel
  16 and has been used by PowerPC for some time.
  17
  18 The fdt is a convenient vehicle for implementing run-time configuration
  19 for three reasons. Firstly it is easy to use, being a simple text file.
  20 It is extensible since it consists of nodes and properties in a nice
  21 hierarchical format.
  22
  23 Finally, there is already excellent infrastructure for the fdt: a
  24 compiler checks the text file and converts it to a compact binary
  25 format, and a library is already available in U-Boot (libfdt) for
  26 handling this format.
  27
  28 The dts directory contains a Makefile for building the device tree blob
  29 and embedding it in your U-Boot image. This is useful since it allows
  30 U-Boot to configure itself according to what it finds there. If you have
  31 a number of similar boards with different peripherals, you can describe
  32 the features of each board in the device tree file, and have a single
  33 generic source base.
  34
  35 To enable this feature, add CONFIG_OF_CONTROL to your board config file.
  36
  37
  38 What is a Flat Device Tree?
  39 ---------------------------
  40
  41 An fdt can be specified in source format as a text file. To read about
  42 the fdt syntax, take a look at the specification here:
  43
  44 https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
  45
  46 You also might find this section of the Linux kernel documentation
  47 useful: (access this in the Linux kernel source code)
  48
  49         Documentation/devicetree/booting-without-of.txt
  50
  51 There is also a mailing list:
  52
  53         http://lists.ozlabs.org/listinfo/devicetree-discuss
  54
  55 In case you are wondering, OF stands for Open Firmware.
  56
  57
  58 Tools
  59 -----
  60
  61 To use this feature you will need to get the device tree compiler here:
  62
  63         git://git.kernel.org/pub/scm/utils/dtc/dtc.git
  64
  65 For example:
  66
  67         $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
  68         $ cd dtc
  69         $ make
  70         $ sudo make install
  71
  72 Then run the compiler (your version will vary):
  73
  74         $ dtc -v
  75         Version: DTC 1.2.0-g2cb4b51f
  76         $ make tests
  77         $ cd tests
  78         $ ./run_tests.sh
  79         ********** TEST SUMMARY
  80         *     Total testcases:  1371
  81         *                PASS:  1371
  82         *                FAIL:  0
  83         *   Bad configuration:  0
  84         * Strange test result:  0
  85
  86 You will also find a useful fdtdump utility for decoding a binary file, as
  87 well as fdtget/fdtput for reading and writing properties in a binary file.
  88
  89
  90 Where do I get an fdt file for my board?
  91 ----------------------------------------
  92
  93 You may find that the Linux kernel has a suitable file. Look in the
  94 kernel source in arch/<arch>/boot/dts.
  95
  96 If not you might find other boards with suitable files that you can
  97 modify to your needs. Look in the board directories for files with a
  98 .dts extension.
  99
 100 Failing that, you could write one from scratch yourself!
 101
 102
 103 Configuration
 104 -------------
 105
 106 Use:
 107
 108 #define CONFIG_DEFAULT_DEVICE_TREE      "<name>"
 109
 110 to set the filename of the device tree source. Then put your device tree
 111 file into
 112
 113         board/<vendor>/dts/<name>.dts
 114
 115 This should include your CPU or SOC's device tree file, placed in
 116 arch/<arch>/dts, and then make any adjustments required.
 117
 118 If CONFIG_OF_EMBED is defined, then it will be picked up and built into
 119 the U-Boot image (including u-boot.bin). This is suitable for debugging
 120 and development only and is not recommended for production devices.
 121
 122 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
 123 a u-boot.dtb file alongside u-boot.bin. A common approach is then to
 124 join the two:
 125
 126         cat u-boot.bin u-boot.dtb >image.bin
 127
 128 and then flash image.bin onto your board. Note that U-Boot creates
 129 u-boot-dtb.bin which does the above step for you also. If you are using
 130 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
 131 tree binary.
 132
 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.
 136
 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.
 140
 141 You cannot use more than one of these options at the same time.
 142
 143 To use a device tree file that you have compiled yourself, pass
 144 EXT_DTB=<filename> to 'make', as in:
 145
 146         make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
 147
 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.
 150
 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.
 161
 162 To use this, put something like this in your board header file:
 163
 164 #define CONFIG_EXTRA_ENV_SETTINGS       "fdtcontroladdr=10000\0"
 165
 166 Build:
 167
 168 After board configuration is done, fdt supported u-boot can be build in two ways:
 169 1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
 170     $ make
 171 2)  build the user specified dts file
 172     $ make DEVICE_TREE=<dts-file-name>
 173
 174
 175 Limitations
 176 -----------
 177
 178 U-Boot is designed to build with a single architecture type and CPU
 179 type. So for example it is not possible to build a single ARM binary
 180 which runs on your AT91 and OMAP boards, relying on an fdt to configure
 181 the various features. This is because you must select one of
 182 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
 183 time. Similarly you cannot build for multiple cpu types or
 184 architectures.
 185
 186 That said the complexity reduction by using fdt to support variants of
 187 boards which use the same SOC / CPU can be substantial.
 188
 189 It is important to understand that the fdt only selects options
 190 available in the platform / drivers. It cannot add new drivers (yet). So
 191 you must still have the CONFIG option to enable the driver. For example,
 192 you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
 193 but can use the fdt to specific the UART clock, peripheral address, etc.
 194 In very broad terms, the CONFIG options in general control *what* driver
 195 files are pulled in, and the fdt controls *how* those files work.
 196
 197 --
 198 Simon Glass <sjg@chromium.org>
 199 1-Sep-11