xref: /openbmc/u-boot/doc/README.fdt-control (revision b2153075)
1#
2# Copyright (c) 2011 The Chromium OS Authors.
3#
4# SPDX-License-Identifier:	GPL-2.0+
5#
6
7Device Tree Control in U-Boot
8=============================
9
10This feature provides for run-time configuration of U-Boot via a flat
11device tree (fdt). U-Boot configuration has traditionally been done
12using CONFIG options in the board config file. This feature aims to
13make it possible for a single U-Boot binary to support multiple boards,
14with the exact configuration of each board controlled by a flat device
15tree (fdt). This is the approach recently taken by the ARM Linux kernel
16and has been used by PowerPC for some time.
17
18The fdt is a convenient vehicle for implementing run-time configuration
19for three reasons. Firstly it is easy to use, being a simple text file.
20It is extensible since it consists of nodes and properties in a nice
21hierarchical format.
22
23Finally, there is already excellent infrastructure for the fdt: a
24compiler checks the text file and converts it to a compact binary
25format, and a library is already available in U-Boot (libfdt) for
26handling this format.
27
28The dts directory contains a Makefile for building the device tree blob
29and embedding it in your U-Boot image. This is useful since it allows
30U-Boot to configure itself according to what it finds there. If you have
31a number of similar boards with different peripherals, you can describe
32the features of each board in the device tree file, and have a single
33generic source base.
34
35To enable this feature, add CONFIG_OF_CONTROL to your board config file.
36
37
38What is a Flat Device Tree?
39---------------------------
40
41An fdt can be specified in source format as a text file. To read about
42the fdt syntax, take a look at the specification here:
43
44https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
45
46You also might find this section of the Linux kernel documentation
47useful: (access this in the Linux kernel source code)
48
49	Documentation/devicetree/booting-without-of.txt
50
51There is also a mailing list:
52
53	http://lists.ozlabs.org/listinfo/devicetree-discuss
54
55In case you are wondering, OF stands for Open Firmware.
56
57
58Tools
59-----
60
61To use this feature you will need to get the device tree compiler here:
62
63	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
64
65For example:
66
67	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
68	$ cd dtc
69	$ make
70	$ sudo make install
71
72Then run the compiler (your version will vary):
73
74	$ dtc -v
75	Version: DTC 1.2.0-g2cb4b51f
76	$ make tests
77	$ cd tests
78	$ ./run_tests.sh
79	********** TEST SUMMARY
80	*     Total testcases:	1371
81	*                PASS:	1371
82	*                FAIL:	0
83	*   Bad configuration:	0
84	* Strange test result:	0
85
86You will also find a useful fdtdump utility for decoding a binary file, as
87well as fdtget/fdtput for reading and writing properties in a binary file.
88
89
90Where do I get an fdt file for my board?
91----------------------------------------
92
93You may find that the Linux kernel has a suitable file. Look in the
94kernel source in arch/<arch>/boot/dts.
95
96If not you might find other boards with suitable files that you can
97modify to your needs. Look in the board directories for files with a
98.dts extension.
99
100Failing that, you could write one from scratch yourself!
101
102
103Configuration
104-------------
105
106Use:
107
108#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
109
110to set the filename of the device tree source. Then put your device tree
111file into
112
113	board/<vendor>/dts/<name>.dts
114
115This should include your CPU or SOC's device tree file, placed in
116arch/<arch>/dts, and then make any adjustments required.
117
118If CONFIG_OF_EMBED is defined, then it will be picked up and built into
119the U-Boot image (including u-boot.bin). This is suitable for debugging
120and development only and is not recommended for production devices.
121
122If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
123a u-boot.dtb file alongside u-boot.bin. A common approach is then to
124join the two:
125
126	cat u-boot.bin u-boot.dtb >image.bin
127
128and then flash image.bin onto your board. Note that U-Boot creates
129u-boot-dtb.bin which does the above step for you also. If you are using
130CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
131tree binary.
132
133If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
134device tree at runtime, for example if an earlier bootloader stage creates
135it and passes it to U-Boot.
136
137If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
138startup. This is only useful for sandbox. Use the -d flag to U-Boot to
139specify the file to read.
140
141You cannot use more than one of these options at the same time.
142
143To use a device tree file that you have compiled yourself, pass
144EXT_DTB=<filename> to 'make', as in:
145
146	make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
147
148Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
149if used, and u-boot-dtb.bin.
150
151If you wish to put the fdt at a different address in memory, you can
152define the "fdtcontroladdr" environment variable. This is the hex
153address of the fdt binary blob, and will override either of the options.
154Be aware that this environment variable is checked prior to relocation,
155when only the compiled-in environment is available. Therefore it is not
156possible to define this variable in the saved SPI/NAND flash
157environment, for example (it will be ignored). After relocation, this
158variable will be set to the address of the newly relocated fdt blob.
159It is read-only and cannot be changed. It can optionally be used to
160control the boot process of Linux with bootm/bootz commands.
161
162To use this, put something like this in your board header file:
163
164#define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
165
166Build:
167
168After board configuration is done, fdt supported u-boot can be build in two ways:
1691)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
170    $ make
1712)  build the user specified dts file
172    $ make DEVICE_TREE=<dts-file-name>
173
174
175Limitations
176-----------
177
178U-Boot is designed to build with a single architecture type and CPU
179type. So for example it is not possible to build a single ARM binary
180which runs on your AT91 and OMAP boards, relying on an fdt to configure
181the various features. This is because you must select one of
182the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
183time. Similarly you cannot build for multiple cpu types or
184architectures.
185
186That said the complexity reduction by using fdt to support variants of
187boards which use the same SOC / CPU can be substantial.
188
189It is important to understand that the fdt only selects options
190available in the platform / drivers. It cannot add new drivers (yet). So
191you must still have the CONFIG option to enable the driver. For example,
192you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
193but can use the fdt to specific the UART clock, peripheral address, etc.
194In very broad terms, the CONFIG options in general control *what* driver
195files are pulled in, and the fdt controls *how* those files work.
196
197--
198Simon Glass <sjg@chromium.org>
1991-Sep-11
200