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-nodtb.bin. A common approach is then to 126join the two: 127 128 cat u-boot-nodtb.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. Resulting 132u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using 133CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device 134tree binary. 135 136If CONFIG_OF_BOARD is defined, a board-specific routine will provide the 137device tree at runtime, for example if an earlier bootloader stage creates 138it and passes it to U-Boot. 139 140If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 141startup. This is only useful for sandbox. Use the -d flag to U-Boot to 142specify the file to read. 143 144You cannot use more than one of these options at the same time. 145 146To use a device tree file that you have compiled yourself, pass 147EXT_DTB=<filename> to 'make', as in: 148 149 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb 150 151Then U-Boot will copy that file to u-boot.dtb, put it in the .img file 152if used, and u-boot-dtb.bin. 153 154If you wish to put the fdt at a different address in memory, you can 155define the "fdtcontroladdr" environment variable. This is the hex 156address of the fdt binary blob, and will override either of the options. 157Be aware that this environment variable is checked prior to relocation, 158when only the compiled-in environment is available. Therefore it is not 159possible to define this variable in the saved SPI/NAND flash 160environment, for example (it will be ignored). After relocation, this 161variable will be set to the address of the newly relocated fdt blob. 162It is read-only and cannot be changed. It can optionally be used to 163control the boot process of Linux with bootm/bootz commands. 164 165To use this, put something like this in your board header file: 166 167#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 168 169Build: 170 171After board configuration is done, fdt supported u-boot can be build in two ways: 1721) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 173 $ make 1742) build the user specified dts file 175 $ make DEVICE_TREE=<dts-file-name> 176 177 178Relocation, SPL and TPL 179----------------------- 180 181U-Boot can be divided into three phases: TPL, SPL and U-Boot proper. 182 183The full device tree is available to U-Boot proper, but normally only a subset 184(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and 185'SPL Support' in doc/driver-model/README.txt for more details. 186 187 188Using several DTBs in the SPL (CONFIG_SPL_MULTI_DTB) 189---------------------------------------------------- 190In some rare cases it is desirable to let SPL be able to select one DTB among 191many. This usually not very useful as the DTB for the SPL is small and usually 192fits several platforms. However the DTB sometimes include information that do 193work on several platforms (like IO tuning parameters). 194In this case it is possible to use CONFIG_SPL_MULTI_DTB. This option appends to 195the SPL a FIT image containing several DTBs listed in SPL_OF_LIST. 196board_fit_config_name_match() is called to select the right DTB. 197 198If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM 199containing the board ID for example), it possible to start with a generic DTB 200and then switch over to the right DTB after the detection. For this purpose, 201the platform code must call fdtdec_resetup(). Based on the returned flag, the 202platform may have to re-initiliaze the DM subusystem using dm_uninit() and 203dm_init_and_scan(). 204 205 206Limitations 207----------- 208 209U-Boot is designed to build with a single architecture type and CPU 210type. So for example it is not possible to build a single ARM binary 211which runs on your AT91 and OMAP boards, relying on an fdt to configure 212the various features. This is because you must select one of 213the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 214time. Similarly you cannot build for multiple cpu types or 215architectures. 216 217That said the complexity reduction by using fdt to support variants of 218boards which use the same SOC / CPU can be substantial. 219 220It is important to understand that the fdt only selects options 221available in the platform / drivers. It cannot add new drivers (yet). So 222you must still have the CONFIG option to enable the driver. For example, 223you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 224but can use the fdt to specific the UART clock, peripheral address, etc. 225In very broad terms, the CONFIG options in general control *what* driver 226files are pulled in, and the fdt controls *how* those files work. 227 228-- 229Simon Glass <sjg@chromium.org> 2301-Sep-11 231