1# SPDX-License-Identifier: GPL-2.0+ 2# 3# Copyright (c) 2011 The Chromium OS Authors. 4 5Device Tree Control in U-Boot 6============================= 7 8This feature provides for run-time configuration of U-Boot via a flat 9device tree (fdt). U-Boot configuration has traditionally been done 10using CONFIG options in the board config file. This feature aims to 11make it possible for a single U-Boot binary to support multiple boards, 12with the exact configuration of each board controlled by a flat device 13tree (fdt). This is the approach recently taken by the ARM Linux kernel 14and has been used by PowerPC for some time. 15 16The fdt is a convenient vehicle for implementing run-time configuration 17for three reasons. Firstly it is easy to use, being a simple text file. 18It is extensible since it consists of nodes and properties in a nice 19hierarchical format. 20 21Finally, there is already excellent infrastructure for the fdt: a 22compiler checks the text file and converts it to a compact binary 23format, and a library is already available in U-Boot (libfdt) for 24handling this format. 25 26The dts directory contains a Makefile for building the device tree blob 27and embedding it in your U-Boot image. This is useful since it allows 28U-Boot to configure itself according to what it finds there. If you have 29a number of similar boards with different peripherals, you can describe 30the features of each board in the device tree file, and have a single 31generic source base. 32 33To enable this feature, add CONFIG_OF_CONTROL to your board config file. 34 35 36What is a Flat Device Tree? 37--------------------------- 38 39An fdt can be specified in source format as a text file. To read about 40the fdt syntax, take a look at the specification here: 41 42https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 43 44You also might find this section of the Linux kernel documentation 45useful: (access this in the Linux kernel source code) 46 47 Documentation/devicetree/booting-without-of.txt 48 49There is also a mailing list: 50 51 http://lists.ozlabs.org/listinfo/devicetree-discuss 52 53In case you are wondering, OF stands for Open Firmware. 54 55 56Tools 57----- 58 59To use this feature you will need to get the device tree compiler. This is 60provided by U-Boot automatically. If you have a system version of dtc 61(typically in the 'device-tree-compiler' package), it is currently not used. 62 63If you want to build your own dtc, it is kept here: 64 65 git://git.kernel.org/pub/scm/utils/dtc/dtc.git 66 67For example: 68 69 $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git 70 $ cd dtc 71 $ make 72 $ sudo make install 73 74Then run the compiler (your version will vary): 75 76 $ dtc -v 77 Version: DTC 1.2.0-g2cb4b51f 78 $ make tests 79 $ cd tests 80 $ ./run_tests.sh 81 ********** TEST SUMMARY 82 * Total testcases: 1371 83 * PASS: 1371 84 * FAIL: 0 85 * Bad configuration: 0 86 * Strange test result: 0 87 88You will also find a useful fdtdump utility for decoding a binary file, as 89well as fdtget/fdtput for reading and writing properties in a binary file. 90 91 92Where do I get an fdt file for my board? 93---------------------------------------- 94 95You may find that the Linux kernel has a suitable file. Look in the 96kernel source in arch/<arch>/boot/dts. 97 98If not you might find other boards with suitable files that you can 99modify to your needs. Look in the board directories for files with a 100.dts extension. 101 102Failing that, you could write one from scratch yourself! 103 104 105Configuration 106------------- 107 108Use: 109 110#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 111 112to set the filename of the device tree source. Then put your device tree 113file into 114 115 board/<vendor>/dts/<name>.dts 116 117This should include your CPU or SOC's device tree file, placed in 118arch/<arch>/dts, and then make any adjustments required. 119 120If CONFIG_OF_EMBED is defined, then it will be picked up and built into 121the U-Boot image (including u-boot.bin). This is suitable for debugging 122and development only and is not recommended for production devices. 123 124If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 125a u-boot.dtb file alongside u-boot.bin. A common approach is then to 126join the two: 127 128 cat u-boot.bin u-boot.dtb >image.bin 129 130and then flash image.bin onto your board. Note that U-Boot creates 131u-boot-dtb.bin which does the above step for you also. If you are using 132CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device 133tree binary. 134 135If CONFIG_OF_BOARD is defined, a board-specific routine will provide the 136device tree at runtime, for example if an earlier bootloader stage creates 137it and passes it to U-Boot. 138 139If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 140startup. This is only useful for sandbox. Use the -d flag to U-Boot to 141specify the file to read. 142 143You cannot use more than one of these options at the same time. 144 145To use a device tree file that you have compiled yourself, pass 146EXT_DTB=<filename> to 'make', as in: 147 148 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb 149 150Then U-Boot will copy that file to u-boot.dtb, put it in the .img file 151if used, and u-boot-dtb.bin. 152 153If you wish to put the fdt at a different address in memory, you can 154define the "fdtcontroladdr" environment variable. This is the hex 155address of the fdt binary blob, and will override either of the options. 156Be aware that this environment variable is checked prior to relocation, 157when only the compiled-in environment is available. Therefore it is not 158possible to define this variable in the saved SPI/NAND flash 159environment, for example (it will be ignored). After relocation, this 160variable will be set to the address of the newly relocated fdt blob. 161It is read-only and cannot be changed. It can optionally be used to 162control the boot process of Linux with bootm/bootz commands. 163 164To use this, put something like this in your board header file: 165 166#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 167 168Build: 169 170After board configuration is done, fdt supported u-boot can be build in two ways: 1711) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 172 $ make 1732) build the user specified dts file 174 $ make DEVICE_TREE=<dts-file-name> 175 176 177Relocation, SPL and TPL 178----------------------- 179 180U-Boot can be divided into three phases: TPL, SPL and U-Boot proper. 181 182The full device tree is available to U-Boot proper, but normally only a subset 183(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and 184'SPL Support' in doc/driver-model/README.txt for more details. 185 186 187Limitations 188----------- 189 190U-Boot is designed to build with a single architecture type and CPU 191type. So for example it is not possible to build a single ARM binary 192which runs on your AT91 and OMAP boards, relying on an fdt to configure 193the various features. This is because you must select one of 194the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 195time. Similarly you cannot build for multiple cpu types or 196architectures. 197 198That said the complexity reduction by using fdt to support variants of 199boards which use the same SOC / CPU can be substantial. 200 201It is important to understand that the fdt only selects options 202available in the platform / drivers. It cannot add new drivers (yet). So 203you must still have the CONFIG option to enable the driver. For example, 204you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 205but can use the fdt to specific the UART clock, peripheral address, etc. 206In very broad terms, the CONFIG options in general control *what* driver 207files are pulled in, and the fdt controls *how* those files work. 208 209-- 210Simon Glass <sjg@chromium.org> 2111-Sep-11 212