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 here: 60 61 git://git.kernel.org/pub/scm/utils/dtc/dtc.git 62 63For 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 70Then 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 84You will also find a useful fdtdump utility for decoding a binary file, as 85well as fdtget/fdtput for reading and writing properties in a binary file. 86 87 88Where do I get an fdt file for my board? 89---------------------------------------- 90 91You may find that the Linux kernel has a suitable file. Look in the 92kernel source in arch/<arch>/boot/dts. 93 94If not you might find other boards with suitable files that you can 95modify to your needs. Look in the board directories for files with a 96.dts extension. 97 98Failing that, you could write one from scratch yourself! 99 100 101Configuration 102------------- 103 104Use: 105 106#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 107 108to set the filename of the device tree source. Then put your device tree 109file into 110 111 board/<vendor>/dts/<name>.dts 112 113This should include your CPU or SOC's device tree file, placed in 114arch/<arch>/dts, and then make any adjustments required. 115 116If CONFIG_OF_EMBED is defined, then it will be picked up and built into 117the U-Boot image (including u-boot.bin). This is suitable for debugging 118and development only and is not recommended for production devices. 119 120If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 121a u-boot.dtb file alongside u-boot.bin. A common approach is then to 122join the two: 123 124 cat u-boot.bin u-boot.dtb >image.bin 125 126and then flash image.bin onto your board. Note that U-Boot creates 127u-boot-dtb.bin which does the above step for you also. If you are using 128CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device 129tree binary. 130 131If CONFIG_OF_BOARD is defined, a board-specific routine will provide the 132device tree at runtime, for example if an earlier bootloader stage creates 133it and passes it to U-Boot. 134 135If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 136startup. This is only useful for sandbox. Use the -d flag to U-Boot to 137specify the file to read. 138 139You cannot use more than one of these options at the same time. 140 141To use a device tree file that you have compiled yourself, pass 142EXT_DTB=<filename> to 'make', as in: 143 144 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb 145 146Then U-Boot will copy that file to u-boot.dtb, put it in the .img file 147if used, and u-boot-dtb.bin. 148 149If you wish to put the fdt at a different address in memory, you can 150define the "fdtcontroladdr" environment variable. This is the hex 151address of the fdt binary blob, and will override either of the options. 152Be aware that this environment variable is checked prior to relocation, 153when only the compiled-in environment is available. Therefore it is not 154possible to define this variable in the saved SPI/NAND flash 155environment, for example (it will be ignored). After relocation, this 156variable will be set to the address of the newly relocated fdt blob. 157It is read-only and cannot be changed. It can optionally be used to 158control the boot process of Linux with bootm/bootz commands. 159 160To use this, put something like this in your board header file: 161 162#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 163 164Build: 165 166After board configuration is done, fdt supported u-boot can be build in two ways: 1671) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 168 $ make 1692) build the user specified dts file 170 $ make DEVICE_TREE=<dts-file-name> 171 172 173Limitations 174----------- 175 176U-Boot is designed to build with a single architecture type and CPU 177type. So for example it is not possible to build a single ARM binary 178which runs on your AT91 and OMAP boards, relying on an fdt to configure 179the various features. This is because you must select one of 180the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 181time. Similarly you cannot build for multiple cpu types or 182architectures. 183 184That said the complexity reduction by using fdt to support variants of 185boards which use the same SOC / CPU can be substantial. 186 187It is important to understand that the fdt only selects options 188available in the platform / drivers. It cannot add new drivers (yet). So 189you must still have the CONFIG option to enable the driver. For example, 190you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 191but can use the fdt to specific the UART clock, peripheral address, etc. 192In very broad terms, the CONFIG options in general control *what* driver 193files are pulled in, and the fdt controls *how* those files work. 194 195-- 196Simon Glass <sjg@chromium.org> 1971-Sep-11 198