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. 36 37 38What is a Flat Device Tree? 39--------------------------- 40 41An fdt can be specified in source format as a text file. To read about 42the fdt syntax, take a look at the specification here: 43 44https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 45 46You also might find this section of the Linux kernel documentation 47useful: (access this in the Linux kernel source code) 48 49 Documentation/devicetree/booting-without-of.txt 50 51There is also a mailing list: 52 53 http://lists.ozlabs.org/listinfo/devicetree-discuss 54 55In case you are wondering, OF stands for Open Firmware. 56 57 58Tools 59----- 60 61To use this feature you will need to get the device tree compiler here: 62 63 git://git.kernel.org/pub/scm/utils/dtc/dtc.git 64 65For example: 66 67 $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git 68 $ cd dtc 69 $ make 70 $ sudo make install 71 72Then run the compiler (your version will vary): 73 74 $ dtc -v 75 Version: DTC 1.2.0-g2cb4b51f 76 $ make tests 77 $ cd tests 78 $ ./run_tests.sh 79 ********** TEST SUMMARY 80 * Total testcases: 1371 81 * PASS: 1371 82 * FAIL: 0 83 * Bad configuration: 0 84 * Strange test result: 0 85 86You will also find a useful fdtdump utility for decoding a binary file, as 87well as fdtget/fdtput for reading and writing properties in a binary file. 88 89 90Where do I get an fdt file for my board? 91---------------------------------------- 92 93You may find that the Linux kernel has a suitable file. Look in the 94kernel source in arch/<arch>/boot/dts. 95 96If not you might find other boards with suitable files that you can 97modify to your needs. Look in the board directories for files with a 98.dts extension. 99 100Failing that, you could write one from scratch yourself! 101 102 103Configuration 104------------- 105 106Use: 107 108#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 109 110to set the filename of the device tree source. Then put your device tree 111file into 112 113 board/<vendor>/dts/<name>.dts 114 115This should include your CPU or SOC's device tree file, placed in 116arch/<arch>/dts, and then make any adjustments required. 117 118If CONFIG_OF_EMBED is defined, then it will be picked up and built into 119the U-Boot image (including u-boot.bin). This is suitable for debugging 120and development only and is not recommended for production devices. 121 122If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 123a u-boot.dtb file alongside u-boot.bin. A common approach is then to 124join the two: 125 126 cat u-boot.bin u-boot.dtb >image.bin 127 128and then flash image.bin onto your board. Note that U-Boot creates 129u-boot-dtb.bin which does the above step for you also. If you are using 130CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device 131tree binary. 132 133If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 134startup. This is only useful for sandbox. Use the -d flag to U-Boot to 135specify the file to read. 136 137You cannot use more than one of these options at the same time. 138 139To use a device tree file that you have compiled yourself, pass 140EXT_DTB=<filename> to 'make', as in: 141 142 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb 143 144Then U-Boot will copy that file to u-boot.dtb, put it in the .img file 145if used, and u-boot-dtb.bin. 146 147If you wish to put the fdt at a different address in memory, you can 148define the "fdtcontroladdr" environment variable. This is the hex 149address of the fdt binary blob, and will override either of the options. 150Be aware that this environment variable is checked prior to relocation, 151when only the compiled-in environment is available. Therefore it is not 152possible to define this variable in the saved SPI/NAND flash 153environment, for example (it will be ignored). After relocation, this 154variable will be set to the address of the newly relocated fdt blob. 155It is read-only and cannot be changed. It can optionally be used to 156control the boot process of Linux with bootm/bootz commands. 157 158To use this, put something like this in your board header file: 159 160#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 161 162Build: 163 164After board configuration is done, fdt supported u-boot can be build in two ways: 1651) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 166 $ make 1672) build the user specified dts file 168 $ make DEVICE_TREE=<dts-file-name> 169 170 171Configuration Options 172--------------------- 173 174A number of run-time configuration options are provided in the /config node 175of the control device tree. You can access these using fdtdec_get_config_int(), 176fdtdec_get_config_bool() and fdtdec_get_config_string(). 177 178Available options are: 179 180silent-console 181 If present and non-zero, the console is silenced by default on boot. 182 183no-keyboard 184 Tells U-Boot not to expect an attached keyboard with a VGA console 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