blob: 85bda035043497d8d23aed8a436e24d630c60937 [file] [log] [blame]
Simon Glassbbb0b122011-10-15 05:48:21 +00001#
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
Simon Glasseea63e02011-10-24 19:15:34 +0000147If 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
Simon Glassbbb0b122011-10-15 05:48:21 +0000159
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