1# 2# Copyright (c) 2011 The Chromium OS Authors. 3# 4# SPDX-License-Identifier: GPL-2.0+ 5# 6 7Device Tree Control in U-Boot 8============================= 9 10This feature provides for run-time configuration of U-Boot via a flat 11device tree (fdt). U-Boot configuration has traditionally been done 12using CONFIG options in the board config file. This feature aims to 13make it possible for a single U-Boot binary to support multiple boards, 14with the exact configuration of each board controlled by a flat device 15tree (fdt). This is the approach recently taken by the ARM Linux kernel 16and has been used by PowerPC for some time. 17 18The fdt is a convenient vehicle for implementing run-time configuration 19for three reasons. Firstly it is easy to use, being a simple text file. 20It is extensible since it consists of nodes and properties in a nice 21hierarchical format. 22 23Finally, there is already excellent infrastructure for the fdt: a 24compiler checks the text file and converts it to a compact binary 25format, and a library is already available in U-Boot (libfdt) for 26handling this format. 27 28The dts directory contains a Makefile for building the device tree blob 29and embedding it in your U-Boot image. This is useful since it allows 30U-Boot to configure itself according to what it finds there. If you have 31a number of similar boards with different peripherals, you can describe 32the features of each board in the device tree file, and have a single 33generic source base. 34 35To enable this feature, add CONFIG_OF_CONTROL to your board config file. 36It is currently supported on ARM, x86 and Microblaze - other architectures 37will need to add code to their arch/xxx/lib/board.c file to locate the 38FDT. Alternatively you can enable generic board support on your board 39(with CONFIG_SYS_GENERIC_BOARD) if this is available (as it is for 40PowerPC). For ARM, Tegra and Exynos5 have device trees available for 41common devices. 42 43 44What is a Flat Device Tree? 45--------------------------- 46 47An fdt can be specified in source format as a text file. To read about 48the fdt syntax, take a look at the specification here: 49 50https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 51 52You also might find this section of the Linux kernel documentation 53useful: (access this in the Linux kernel source code) 54 55 Documentation/devicetree/booting-without-of.txt 56 57There is also a mailing list: 58 59 http://lists.ozlabs.org/listinfo/devicetree-discuss 60 61In case you are wondering, OF stands for Open Firmware. 62 63 64Tools 65----- 66 67To use this feature you will need to get the device tree compiler here: 68 69 git://jdl.com/software/dtc.git 70 71For example: 72 73 $ git clone git://jdl.com/software/dtc.git 74 $ cd dtc 75 $ make 76 $ sudo make install 77 78Then run the compiler (your version will vary): 79 80 $ dtc -v 81 Version: DTC 1.2.0-g2cb4b51f 82 $ make tests 83 $ cd tests 84 $ ./run_tests.sh 85 ********** TEST SUMMARY 86 * Total testcases: 1371 87 * PASS: 1371 88 * FAIL: 0 89 * Bad configuration: 0 90 * Strange test result: 0 91 92You will also find a useful fdtdump utility for decoding a binary file, as 93well as fdtget/fdtput for reading and writing properties in a binary file. 94 95 96Where do I get an fdt file for my board? 97---------------------------------------- 98 99You may find that the Linux kernel has a suitable file. Look in the 100kernel source in arch/<arch>/boot/dts. 101 102If not you might find other boards with suitable files that you can 103modify to your needs. Look in the board directories for files with a 104.dts extension. 105 106Failing that, you could write one from scratch yourself! 107 108 109Configuration 110------------- 111 112Use: 113 114#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 115 116to set the filename of the device tree source. Then put your device tree 117file into 118 119 board/<vendor>/dts/<name>.dts 120 121This should include your CPU or SOC's device tree file, placed in 122arch/<arch>/dts, and then make any adjustments required. The name of this 123is CONFIG_ARCH_DEVICE_TREE.dts. 124 125If CONFIG_OF_EMBED is defined, then it will be picked up and built into 126the U-Boot image (including u-boot.bin). 127 128If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 129a u-boot.dtb file alongside u-boot.bin. A common approach is then to 130join the two: 131 132 cat u-boot.bin u-boot.dtb >image.bin 133 134and then flash image.bin onto your board. 135 136If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 137startup. This is only useful for sandbox. Use the -d flag to U-Boot to 138specify the file to read. 139 140You cannot use more than one of these options at the same time. 141 142If you wish to put the fdt at a different address in memory, you can 143define the "fdtcontroladdr" environment variable. This is the hex 144address of the fdt binary blob, and will override either of the options. 145Be aware that this environment variable is checked prior to relocation, 146when only the compiled-in environment is available. Therefore it is not 147possible to define this variable in the saved SPI/NAND flash 148environment, for example (it will be ignored). 149 150To use this, put something like this in your board header file: 151 152#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 153 154Build: 155 156After board configuration is done, fdt supported u-boot can be build in two ways: 1571) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 158 $ make 1592) build the user specified dts file 160 $ make DEVICE_TREE=<dts-file-name> 161 162 163Limitations 164----------- 165 166U-Boot is designed to build with a single architecture type and CPU 167type. So for example it is not possible to build a single ARM binary 168which runs on your AT91 and OMAP boards, relying on an fdt to configure 169the various features. This is because you must select one of 170the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 171time. Similarly you cannot build for multiple cpu types or 172architectures. 173 174That said the complexity reduction by using fdt to support variants of 175boards which use the same SOC / CPU can be substantial. 176 177It is important to understand that the fdt only selects options 178available in the platform / drivers. It cannot add new drivers (yet). So 179you must still have the CONFIG option to enable the driver. For example, 180you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 181but can use the fdt to specific the UART clock, peripheral address, etc. 182In very broad terms, the CONFIG options in general control *what* driver 183files are pulled in, and the fdt controls *how* those files work. 184 185-- 186Simon Glass <sjg@chromium.org> 1871-Sep-11 188