xref: /openbmc/u-boot/doc/README.fdt-control (revision b1e6c4c3)
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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
145If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
146startup. This is only useful for sandbox. Use the -d flag to U-Boot to
147specify the file to read.
148
149You cannot use more than one of these options at the same time.
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).
158
159To use this, put something like this in your board header file:
160
161#define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
162
163Build:
164
165After board configuration is done, fdt supported u-boot can be build in two ways:
1661)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
167    $ make
1682)  build the user specified dts file
169    $ make DEVICE_TREE=<dts-file-name>
170
171
172Limitations
173-----------
174
175U-Boot is designed to build with a single architecture type and CPU
176type. So for example it is not possible to build a single ARM binary
177which runs on your AT91 and OMAP boards, relying on an fdt to configure
178the various features. This is because you must select one of
179the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
180time. Similarly you cannot build for multiple cpu types or
181architectures.
182
183That said the complexity reduction by using fdt to support variants of
184boards which use the same SOC / CPU can be substantial.
185
186It is important to understand that the fdt only selects options
187available in the platform / drivers. It cannot add new drivers (yet). So
188you must still have the CONFIG option to enable the driver. For example,
189you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
190but can use the fdt to specific the UART clock, peripheral address, etc.
191In very broad terms, the CONFIG options in general control *what* driver
192files are pulled in, and the fdt controls *how* those files work.
193
194--
195Simon Glass <sjg@chromium.org>
1961-Sep-11
197