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Tom Rini83d290c2018-05-06 17:58:06 -04001# SPDX-License-Identifier: GPL-2.0+
Simon Glassbbb0b122011-10-15 05:48:21 +00002#
3# Copyright (c) 2011 The Chromium OS Authors.
Simon Glassbbb0b122011-10-15 05:48:21 +00004
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
Simon Glass343864a2018-10-01 12:22:16 -060059To 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:
Simon Glassbbb0b122011-10-15 05:48:21 +000064
Jon Loeliger5f658262014-05-27 09:12:48 -050065 git://git.kernel.org/pub/scm/utils/dtc/dtc.git
Simon Glassbbb0b122011-10-15 05:48:21 +000066
67For example:
68
Jon Loeliger5f658262014-05-27 09:12:48 -050069 $ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
Simon Glassbbb0b122011-10-15 05:48:21 +000070 $ 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
Simon Glass134a6512013-05-07 06:11:46 +000088You 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.
Simon Glassbbb0b122011-10-15 05:48:21 +000090
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
Stephen Warren06520282013-07-24 10:09:22 -0700118arch/<arch>/dts, and then make any adjustments required.
Simon Glassbbb0b122011-10-15 05:48:21 +0000119
120If CONFIG_OF_EMBED is defined, then it will be picked up and built into
Simon Glass63b4b5b2014-06-02 22:04:50 -0600121the U-Boot image (including u-boot.bin). This is suitable for debugging
122and development only and is not recommended for production devices.
Simon Glassbbb0b122011-10-15 05:48:21 +0000123
124If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
125a u-boot.dtb file alongside u-boot.bin. A common approach is then to
126join the two:
127
128 cat u-boot.bin u-boot.dtb >image.bin
129
Simon Glass63b4b5b2014-06-02 22:04:50 -0600130and then flash image.bin onto your board. Note that U-Boot creates
131u-boot-dtb.bin which does the above step for you also. If you are using
132CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
133tree binary.
Simon Glassbbb0b122011-10-15 05:48:21 +0000134
Alex Deymo82f766d2017-04-02 01:25:20 -0700135If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
136device tree at runtime, for example if an earlier bootloader stage creates
137it and passes it to U-Boot.
138
Simon Glassf828bf22013-04-20 08:42:41 +0000139If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
140startup. This is only useful for sandbox. Use the -d flag to U-Boot to
141specify the file to read.
142
143You cannot use more than one of these options at the same time.
Simon Glassbbb0b122011-10-15 05:48:21 +0000144
Simon Glass63b4b5b2014-06-02 22:04:50 -0600145To use a device tree file that you have compiled yourself, pass
Simon Glassd18926a2014-06-12 07:24:43 -0600146EXT_DTB=<filename> to 'make', as in:
Simon Glass63b4b5b2014-06-02 22:04:50 -0600147
Simon Glassd18926a2014-06-12 07:24:43 -0600148 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
Simon Glass63b4b5b2014-06-02 22:04:50 -0600149
150Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
151if used, and u-boot-dtb.bin.
152
Simon Glasseea63e02011-10-24 19:15:34 +0000153If you wish to put the fdt at a different address in memory, you can
154define the "fdtcontroladdr" environment variable. This is the hex
155address of the fdt binary blob, and will override either of the options.
156Be aware that this environment variable is checked prior to relocation,
157when only the compiled-in environment is available. Therefore it is not
158possible to define this variable in the saved SPI/NAND flash
Thomas Chou545dfd12015-10-16 08:44:51 +0800159environment, for example (it will be ignored). After relocation, this
160variable will be set to the address of the newly relocated fdt blob.
161It is read-only and cannot be changed. It can optionally be used to
162control the boot process of Linux with bootm/bootz commands.
Simon Glasseea63e02011-10-24 19:15:34 +0000163
164To use this, put something like this in your board header file:
165
166#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
167
Jagannadha Sutradharudu Teki74de8c92013-02-28 10:20:18 +0000168Build:
169
170After board configuration is done, fdt supported u-boot can be build in two ways:
1711) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
172 $ make
1732) build the user specified dts file
174 $ make DEVICE_TREE=<dts-file-name>
175
Simon Glassbbb0b122011-10-15 05:48:21 +0000176
Simon Glassa15a7aa2018-10-01 12:22:17 -0600177Relocation, SPL and TPL
178-----------------------
179
180U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
181
182The full device tree is available to U-Boot proper, but normally only a subset
183(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
184'SPL Support' in doc/driver-model/README.txt for more details.
185
186
Simon Glassbbb0b122011-10-15 05:48:21 +0000187Limitations
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