doc/README.fdt-control

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  22
  23 Device Tree Control in U-Boot
  24 =============================
  25
  26 This feature provides for run-time configuration of U-Boot via a flat
  27 device tree (fdt). U-Boot configuration has traditionally been done
  28 using CONFIG options in the board config file. This feature aims to
  29 make it possible for a single U-Boot binary to support multiple boards,
  30 with the exact configuration of each board controlled by a flat device
  31 tree (fdt). This is the approach recently taken by the ARM Linux kernel
  32 and has been used by PowerPC for some time.
  33
  34 The fdt is a convenient vehicle for implementing run-time configuration
  35 for three reasons. Firstly it is easy to use, being a simple text file.
  36 It is extensible since it consists of nodes and properties in a nice
  37 hierarchical format.
  38
  39 Finally, there is already excellent infrastructure for the fdt: a
  40 compiler checks the text file and converts it to a compact binary
  41 format, and a library is already available in U-Boot (libfdt) for
  42 handling this format.
  43
  44 The dts directory contains a Makefile for building the device tree blob
  45 and embedding it in your U-Boot image. This is useful since it allows
  46 U-Boot to configure itself according to what it finds there. If you have
  47 a number of similar boards with different peripherals, you can describe
  48 the features of each board in the device tree file, and have a single
  49 generic source base.
  50
  51 To enable this feature, add CONFIG_OF_CONTROL to your board config file.
  52
  53
  54 What is a Flat Device Tree?
  55 ---------------------------
  56
  57 An fdt can be specified in source format as a text file. To read about
  58 the fdt syntax, take a look at the specification here:
  59
  60 https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
  61
  62 You also might find this section of the Linux kernel documentation
  63 useful: (access this in the Linux kernel source code)
  64
  65         Documentation/devicetree/booting-without-of.txt
  66
  67 There is also a mailing list:
  68
  69         http://lists.ozlabs.org/listinfo/devicetree-discuss
  70
  71 In case you are wondering, OF stands for Open Firmware.
  72
  73
  74 Tools
  75 -----
  76
  77 To use this feature you will need to get the device tree compiler here:
  78
  79         git://jdl.com/software/dtc.git
  80
  81 For example:
  82
  83         $ git clone git://jdl.com/software/dtc.git
  84         $ cd dtc
  85         $ make
  86         $ sudo make install
  87
  88 Then run the compiler (your version will vary):
  89
  90         $ dtc -v
  91         Version: DTC 1.2.0-g2cb4b51f
  92         $ make tests
  93         $ cd tests
  94         $ ./run_tests.sh
  95         ********** TEST SUMMARY
  96         *     Total testcases:  1371
  97         *                PASS:  1371
  98         *                FAIL:  0
  99         *   Bad configuration:  0
 100         * Strange test result:  0
 101
 102 You will also find a useful ftdump utility for decoding a binary file.
 103
 104
 105 Where do I get an fdt file for my board?
 106 ----------------------------------------
 107
 108 You may find that the Linux kernel has a suitable file. Look in the
 109 kernel source in arch/<arch>/boot/dts.
 110
 111 If not you might find other boards with suitable files that you can
 112 modify to your needs. Look in the board directories for files with a
 113 .dts extension.
 114
 115 Failing that, you could write one from scratch yourself!
 116
 117
 118 Configuration
 119 -------------
 120
 121 Use:
 122
 123 #define CONFIG_DEFAULT_DEVICE_TREE      "<name>"
 124
 125 to set the filename of the device tree source. Then put your device tree
 126 file into
 127
 128         board/<vendor>/dts/<name>.dts
 129
 130 This should include your CPU or SOC's device tree file, placed in
 131 arch/<arch>/dts, and then make any adjustments required. The name of this
 132 is CONFIG_ARCH_DEVICE_TREE.dts.
 133
 134 If CONFIG_OF_EMBED is defined, then it will be picked up and built into
 135 the U-Boot image (including u-boot.bin).
 136
 137 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
 138 a u-boot.dtb file alongside u-boot.bin. A common approach is then to
 139 join the two:
 140
 141         cat u-boot.bin u-boot.dtb >image.bin
 142
 143 and then flash image.bin onto your board.
 144
 145 You cannot use both of these options at the same time.
 146
 147 If you wish to put the fdt at a different address in memory, you can
 148 define the "fdtcontroladdr" environment variable. This is the hex
 149 address of the fdt binary blob, and will override either of the options.
 150 Be aware that this environment variable is checked prior to relocation,
 151 when only the compiled-in environment is available. Therefore it is not
 152 possible to define this variable in the saved SPI/NAND flash
 153 environment, for example (it will be ignored).
 154
 155 To use this, put something like this in your board header file:
 156
 157 #define CONFIG_EXTRA_ENV_SETTINGS       "fdtcontroladdr=10000\0"
 158
 159
 160 Limitations
 161 -----------
 162
 163 U-Boot is designed to build with a single architecture type and CPU
 164 type. So for example it is not possible to build a single ARM binary
 165 which runs on your AT91 and OMAP boards, relying on an fdt to configure
 166 the various features. This is because you must select one of
 167 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
 168 time. Similarly you cannot build for multiple cpu types or
 169 architectures.
 170
 171 That said the complexity reduction by using fdt to support variants of
 172 boards which use the same SOC / CPU can be substantial.
 173
 174 It is important to understand that the fdt only selects options
 175 available in the platform / drivers. It cannot add new drivers (yet). So
 176 you must still have the CONFIG option to enable the driver. For example,
 177 you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
 178 but can use the fdt to specific the UART clock, peripheral address, etc.
 179 In very broad terms, the CONFIG options in general control *what* driver
 180 files are pulled in, and the fdt controls *how* those files work.
 181
 182 --
 183 Simon Glass <sjg@chromium.org>
 184 1-Sep-11