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. 52 53 54What is a Flat Device Tree? 55--------------------------- 56 57An fdt can be specified in source format as a text file. To read about 58the fdt syntax, take a look at the specification here: 59 60https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf 61 62You also might find this section of the Linux kernel documentation 63useful: (access this in the Linux kernel source code) 64 65 Documentation/devicetree/booting-without-of.txt 66 67There is also a mailing list: 68 69 http://lists.ozlabs.org/listinfo/devicetree-discuss 70 71In case you are wondering, OF stands for Open Firmware. 72 73 74Tools 75----- 76 77To use this feature you will need to get the device tree compiler here: 78 79 git://jdl.com/software/dtc.git 80 81For example: 82 83 $ git clone git://jdl.com/software/dtc.git 84 $ cd dtc 85 $ make 86 $ sudo make install 87 88Then run the compiler (your version will vary): 89 90 $ dtc -v 91 Version: DTC 1.2.0-g2cb4b51f 92 $ make tests 93 $ cd tests 94 $ ./run_tests.sh 95 ********** TEST SUMMARY 96 * Total testcases: 1371 97 * PASS: 1371 98 * FAIL: 0 99 * Bad configuration: 0 100 * Strange test result: 0 101 102You will also find a useful ftdump utility for decoding a binary file. 103 104 105Where do I get an fdt file for my board? 106---------------------------------------- 107 108You may find that the Linux kernel has a suitable file. Look in the 109kernel source in arch/<arch>/boot/dts. 110 111If not you might find other boards with suitable files that you can 112modify to your needs. Look in the board directories for files with a 113.dts extension. 114 115Failing that, you could write one from scratch yourself! 116 117 118Configuration 119------------- 120 121Use: 122 123#define CONFIG_DEFAULT_DEVICE_TREE "<name>" 124 125to set the filename of the device tree source. Then put your device tree 126file into 127 128 board/<vendor>/dts/<name>.dts 129 130This should include your CPU or SOC's device tree file, placed in 131arch/<arch>/dts, and then make any adjustments required. The name of this 132is CONFIG_ARCH_DEVICE_TREE.dts. 133 134If CONFIG_OF_EMBED is defined, then it will be picked up and built into 135the U-Boot image (including u-boot.bin). 136 137If CONFIG_OF_SEPARATE is defined, then it will be built and placed in 138a u-boot.dtb file alongside u-boot.bin. A common approach is then to 139join the two: 140 141 cat u-boot.bin u-boot.dtb >image.bin 142 143and then flash image.bin onto your board. 144 145You cannot use both of these options at the same time. 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). 154 155To use this, put something like this in your board header file: 156 157#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0" 158 159 160Limitations 161----------- 162 163U-Boot is designed to build with a single architecture type and CPU 164type. So for example it is not possible to build a single ARM binary 165which runs on your AT91 and OMAP boards, relying on an fdt to configure 166the various features. This is because you must select one of 167the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build 168time. Similarly you cannot build for multiple cpu types or 169architectures. 170 171That said the complexity reduction by using fdt to support variants of 172boards which use the same SOC / CPU can be substantial. 173 174It is important to understand that the fdt only selects options 175available in the platform / drivers. It cannot add new drivers (yet). So 176you must still have the CONFIG option to enable the driver. For example, 177you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver, 178but can use the fdt to specific the UART clock, peripheral address, etc. 179In very broad terms, the CONFIG options in general control *what* driver 180files are pulled in, and the fdt controls *how* those files work. 181 182-- 183Simon Glass <sjg@chromium.org> 1841-Sep-11 185