1# 2# Copyright (c) 2011 The Chromium OS Authors. 3# 4# See file CREDITS for list of people who contributed to this 5# project. 6# 7# This program is free software; you can redistribute it and/or 8# modify it under the terms of the GNU General Public License as 9# published by the Free Software Foundatio; either version 2 of 10# the License, or (at your option) any later version. 11# 12# This program is distributed in the hope that it will be useful, 13# but WITHOUT ANY WARRANTY; without even the implied warranty of 14# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15# GNU General Public License for more details. 16# 17# You should have received a copy of the GNU General Public License 18# along with this program; if not, write to the Free Software 19# Foundation, Inc., 59 Temple Place, Suite 330, Boston, 20# MA 02111-1307 USA 21# 22 23Device Tree Control in U-Boot 24============================= 25 26This feature provides for run-time configuration of U-Boot via a flat 27device tree (fdt). U-Boot configuration has traditionally been done 28using CONFIG options in the board config file. This feature aims to 29make it possible for a single U-Boot binary to support multiple boards, 30with the exact configuration of each board controlled by a flat device 31tree (fdt). This is the approach recently taken by the ARM Linux kernel 32and has been used by PowerPC for some time. 33 34The fdt is a convenient vehicle for implementing run-time configuration 35for three reasons. Firstly it is easy to use, being a simple text file. 36It is extensible since it consists of nodes and properties in a nice 37hierarchical format. 38 39Finally, there is already excellent infrastructure for the fdt: a 40compiler checks the text file and converts it to a compact binary 41format, and a library is already available in U-Boot (libfdt) for 42handling this format. 43 44The dts directory contains a Makefile for building the device tree blob 45and embedding it in your U-Boot image. This is useful since it allows 46U-Boot to configure itself according to what it finds there. If you have 47a number of similar boards with different peripherals, you can describe 48the features of each board in the device tree file, and have a single 49generic source base. 50 51To enable this feature, add CONFIG_OF_CONTROL to your board config file. 52It is currently supported on ARM, x86 and Microblaze - other architectures 53will need to add code to their arch/xxx/lib/board.c file to locate the 54FDT. Alternatively you can enable generic board support on your board 55(with CONFIG_SYS_GENERIC_BOARD) if this is available (as it is for 56PowerPC). For ARM, Tegra and Exynos5 have device trees available for 57common devices. 58 59 60What is a Flat Device Tree? 61--------------------------- 62 63An fdt can be specified in source format as a text file. To read about 64the fdt syntax, take a look at the specification here: 65 66https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 67 68You also might find this section of the Linux kernel documentation 69useful: (access this in the Linux kernel source code) 70 71 Documentation/devicetree/booting-without-of.txt 72 73There is also a mailing list: 74 75 http://lists.ozlabs.org/listinfo/devicetree-discuss 76 77In case you are wondering, OF stands for Open Firmware. 78 79 80Tools 81----- 82 83To use this feature you will need to get the device tree compiler here: 84 85 git://jdl.com/software/dtc.git 86 87For example: 88 89 $ git clone git://jdl.com/software/dtc.git 90 $ cd dtc 91 $ make 92 $ sudo make install 93 94Then run the compiler (your version will vary): 95 96 $ dtc -v 97 Version: DTC 1.2.0-g2cb4b51f 98 $ make tests 99 $ cd tests 100 $ ./run_tests.sh 101 ********** TEST SUMMARY 102 * Total testcases: 1371 103 * PASS: 1371 104 * FAIL: 0 105 * Bad configuration: 0 106 * Strange test result: 0 107 108You will also find a useful fdtdump utility for decoding a binary file, as 109well as fdtget/fdtput for reading and writing properties in a binary file. 110 111 112Where do I get an fdt file for my board? 113---------------------------------------- 114 115You may find that the Linux kernel has a suitable file. Look in the 116kernel source in arch/<arch>/boot/dts. 117 118If not you might find other boards with suitable files that you can 119modify to your needs. Look in the board directories for files with a 120.dts extension. 121 122Failing that, you could write one from scratch yourself! 123 124 125Configuration 126------------- 127 128Use: 129 130#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 131 132to set the filename of the device tree source. Then put your device tree 133file into 134 135 board/<vendor>/dts/<name>.dts 136 137This should include your CPU or SOC's device tree file, placed in 138arch/<arch>/dts, and then make any adjustments required. The name of this 139is CONFIG_ARCH_DEVICE_TREE.dts. 140 141If CONFIG_OF_EMBED is defined, then it will be picked up and built into 142the U-Boot image (including u-boot.bin). 143 144If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 145a u-boot.dtb file alongside u-boot.bin. A common approach is then to 146join the two: 147 148 cat u-boot.bin u-boot.dtb >image.bin 149 150and then flash image.bin onto your board. 151 152If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on 153startup. This is only useful for sandbox. Use the -d flag to U-Boot to 154specify the file to read. 155 156You cannot use more than one of these options at the same time. 157 158If you wish to put the fdt at a different address in memory, you can 159define the "fdtcontroladdr" environment variable. This is the hex 160address of the fdt binary blob, and will override either of the options. 161Be aware that this environment variable is checked prior to relocation, 162when only the compiled-in environment is available. Therefore it is not 163possible to define this variable in the saved SPI/NAND flash 164environment, for example (it will be ignored). 165 166To use this, put something like this in your board header file: 167 168#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 169 170Build: 171 172After board configuration is done, fdt supported u-boot can be build in two ways: 1731) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE 174 $ make 1752) build the user specified dts file 176 $ make DEVICE_TREE=<dts-file-name> 177 178 179Limitations 180----------- 181 182U-Boot is designed to build with a single architecture type and CPU 183type. So for example it is not possible to build a single ARM binary 184which runs on your AT91 and OMAP boards, relying on an fdt to configure 185the various features. This is because you must select one of 186the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 187time. Similarly you cannot build for multiple cpu types or 188architectures. 189 190That said the complexity reduction by using fdt to support variants of 191boards which use the same SOC / CPU can be substantial. 192 193It is important to understand that the fdt only selects options 194available in the platform / drivers. It cannot add new drivers (yet). So 195you must still have the CONFIG option to enable the driver. For example, 196you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 197but can use the fdt to specific the UART clock, peripheral address, etc. 198In very broad terms, the CONFIG options in general control *what* driver 199files are pulled in, and the fdt controls *how* those files work. 200 201-- 202Simon Glass <sjg@chromium.org> 2031-Sep-11 204