1 # SPDX-License-Identifier: GPL-2.0+ 2 # 3 # Copyright (c) 2011 The Chromium OS Authors. 4 5 Device Tree Control in U-Boot 6 ============================= 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 16 The fdt is a convenient vehicle for implementing run-time configuration 17 for three reasons. Firstly it is easy to use, being a simple text file. 18 It is extensible since it consists of nodes and properties in a nice 19 hierarchical format. 20 21 Finally, there is already excellent infrastructure for the fdt: a 22 compiler checks the text file and converts it to a compact binary 23 format, and a library is already available in U-Boot (libfdt) for 24 handling this format. 25 26 The dts directory contains a Makefile for building the device tree blob 27 and embedding it in your U-Boot image. This is useful since it allows 28 U-Boot to configure itself according to what it finds there. If you have 29 a number of similar boards with different peripherals, you can describe 30 the features of each board in the device tree file, and have a single 31 generic source base. 32 33 To enable this feature, add CONFIG_OF_CONTROL to your board config file. 34 35 36 What is a Flat Device Tree? 37 --------------------------- 38 39 An fdt can be specified in source format as a text file. To read about 40 the fdt syntax, take a look at the specification here: 41 42 https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 43 44 You also might find this section of the Linux kernel documentation 45 useful: (access this in the Linux kernel source code) 46 47 Documentation/devicetree/booting-without-of.txt 48 49 There is also a mailing list: 50 51 http://lists.ozlabs.org/listinfo/devicetree-discuss 52 53 In case you are wondering, OF stands for Open Firmware. 54 55 56 Tools 57 ----- 58 59 To use this feature you will need to get the device tree compiler here: 60 61 git://git.kernel.org/pub/scm/utils/dtc/dtc.git 62 63 For example: 64 65 $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git 66 $ cd dtc 67 $ make 68 $ sudo make install 69 70 Then run the compiler (your version will vary): 71 72 $ dtc -v 73 Version: DTC 1.2.0-g2cb4b51f 74 $ make tests 75 $ cd tests 76 $ ./run_tests.sh 77 ********** TEST SUMMARY 78 * Total testcases: 1371 79 * PASS: 1371 80 * FAIL: 0 81 * Bad configuration: 0 82 * Strange test result: 0 83 84 You will also find a useful fdtdump utility for decoding a binary file, as 85 well as fdtget/fdtput for reading and writing properties in a binary file. 86 87 88 Where do I get an fdt file for my board? 89 ---------------------------------------- 90 91 You may find that the Linux kernel has a suitable file. Look in the 92 kernel source in arch/<arch>/boot/dts. 93 94 If not you might find other boards with suitable files that you can 95 modify to your needs. Look in the board directories for files with a 96 .dts extension. 97 98 Failing that, you could write one from scratch yourself! 99 100 101 Configuration 102 ------------- 103 104 Use: 105 106 #define CONFIG_DEFAULT_DEVICE_TREE "<name>" 107 108 to set the filename of the device tree source. Then put your device tree 109 file into 110 111 board/<vendor>/dts/<name>.dts 112 113 This should include your CPU or SOC's device tree file, placed in 114 arch/<arch>/dts, and then make any adjustments required. 115 116 If CONFIG_OF_EMBED is defined, then it will be picked up and built into 117 the U-Boot image (including u-boot.bin). This is suitable for debugging 118 and development only and is not recommended for production devices. 119 120 If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 121 a u-boot.dtb file alongside u-boot.bin. A common approach is then to 122 join the two: 123 124 cat u-boot.bin u-boot.dtb >image.bin 125 126 and then flash image.bin onto your board. Note that U-Boot creates 127 u-boot-dtb.bin which does the above step for you also. If you are using 128 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device 129 tree binary. 130 131 If CONFIG_OF_BOARD is defined, a board-specific routine will provide the 132 device tree at runtime, for example if an earlier bootloader stage creates 133 it and passes it to U-Boot. 134 135 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 136 startup. This is only useful for sandbox. Use the -d flag to U-Boot to 137 specify the file to read. 138 139 You cannot use more than one of these options at the same time. 140 141 To use a device tree file that you have compiled yourself, pass 142 EXT_DTB=<filename> to 'make', as in: 143 144 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb 145 146 Then U-Boot will copy that file to u-boot.dtb, put it in the .img file 147 if used, and u-boot-dtb.bin. 148 149 If you wish to put the fdt at a different address in memory, you can 150 define the "fdtcontroladdr" environment variable. This is the hex 151 address of the fdt binary blob, and will override either of the options. 152 Be aware that this environment variable is checked prior to relocation, 153 when only the compiled-in environment is available. Therefore it is not 154 possible to define this variable in the saved SPI/NAND flash 155 environment, for example (it will be ignored). After relocation, this 156 variable will be set to the address of the newly relocated fdt blob. 157 It is read-only and cannot be changed. It can optionally be used to 158 control the boot process of Linux with bootm/bootz commands. 159 160 To use this, put something like this in your board header file: 161 162 #define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 163 164 Build: 165 166 After board configuration is done, fdt supported u-boot can be build in two ways: 167 1) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 168 $ make 169 2) build the user specified dts file 170 $ make DEVICE_TREE=<dts-file-name> 171 172 173 Limitations 174 ----------- 175 176 U-Boot is designed to build with a single architecture type and CPU 177 type. So for example it is not possible to build a single ARM binary 178 which runs on your AT91 and OMAP boards, relying on an fdt to configure 179 the various features. This is because you must select one of 180 the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 181 time. Similarly you cannot build for multiple cpu types or 182 architectures. 183 184 That said the complexity reduction by using fdt to support variants of 185 boards which use the same SOC / CPU can be substantial. 186 187 It is important to understand that the fdt only selects options 188 available in the platform / drivers. It cannot add new drivers (yet). So 189 you must still have the CONFIG option to enable the driver. For example, 190 you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 191 but can use the fdt to specific the UART clock, peripheral address, etc. 192 In very broad terms, the CONFIG options in general control *what* driver 193 files are pulled in, and the fdt controls *how* those files work. 194 195 -- 196 Simon Glass <sjg (a] chromium.org> 197 1-Sep-11 198
