xref: /openbmc/u-boot/doc/README.fdt-control (revision e35171e9)
1# SPDX-License-Identifier: GPL-2.0+
2#
3# Copyright (c) 2011 The Chromium OS Authors.
4
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
59To 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:
64
65	git://git.kernel.org/pub/scm/utils/dtc/dtc.git
66
67For example:
68
69	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
70	$ 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
88You 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.
90
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
118arch/<arch>/dts, and then make any adjustments required.
119
120If CONFIG_OF_EMBED is defined, then it will be picked up and built into
121the U-Boot image (including u-boot.bin). This is suitable for debugging
122and development only and is not recommended for production devices.
123
124If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
125a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
126join the two:
127
128	cat u-boot-nodtb.bin u-boot.dtb >image.bin
129
130and then flash image.bin onto your board. Note that U-Boot creates
131u-boot-dtb.bin which does the above step for you also. Resulting
132u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
133CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
134tree binary.
135
136If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
137device tree at runtime, for example if an earlier bootloader stage creates
138it and passes it to U-Boot.
139
140If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
141startup. This is only useful for sandbox. Use the -d flag to U-Boot to
142specify the file to read.
143
144You cannot use more than one of these options at the same time.
145
146To use a device tree file that you have compiled yourself, pass
147EXT_DTB=<filename> to 'make', as in:
148
149	make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
150
151Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
152if used, and u-boot-dtb.bin.
153
154If you wish to put the fdt at a different address in memory, you can
155define the "fdtcontroladdr" environment variable. This is the hex
156address of the fdt binary blob, and will override either of the options.
157Be aware that this environment variable is checked prior to relocation,
158when only the compiled-in environment is available. Therefore it is not
159possible to define this variable in the saved SPI/NAND flash
160environment, for example (it will be ignored). After relocation, this
161variable will be set to the address of the newly relocated fdt blob.
162It is read-only and cannot be changed. It can optionally be used to
163control the boot process of Linux with bootm/bootz commands.
164
165To use this, put something like this in your board header file:
166
167#define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
168
169Build:
170
171After board configuration is done, fdt supported u-boot can be build in two ways:
1721)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
173    $ make
1742)  build the user specified dts file
175    $ make DEVICE_TREE=<dts-file-name>
176
177
178Relocation, SPL and TPL
179-----------------------
180
181U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
182
183The full device tree is available to U-Boot proper, but normally only a subset
184(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
185'SPL Support' in doc/driver-model/README.txt for more details.
186
187
188Using several DTBs in the SPL (CONFIG_SPL_MULTI_DTB)
189----------------------------------------------------
190In some rare cases it is desirable to let SPL be able to select one DTB among
191many. This usually not very useful as the DTB for the SPL is small and usually
192fits several platforms. However the DTB sometimes include information that do
193work on several platforms (like IO tuning parameters).
194In this case it is possible to use CONFIG_SPL_MULTI_DTB. This option appends to
195the SPL a FIT image containing several DTBs listed in SPL_OF_LIST.
196board_fit_config_name_match() is called to select the right DTB.
197
198If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
199containing the board ID for example), it possible to start with a generic DTB
200and then switch over to the right DTB after the detection. For this purpose,
201the platform code must call fdtdec_resetup(). Based on the returned flag, the
202platform may have to re-initiliaze the DM subusystem using dm_uninit() and
203dm_init_and_scan().
204
205
206Limitations
207-----------
208
209U-Boot is designed to build with a single architecture type and CPU
210type. So for example it is not possible to build a single ARM binary
211which runs on your AT91 and OMAP boards, relying on an fdt to configure
212the various features. This is because you must select one of
213the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
214time. Similarly you cannot build for multiple cpu types or
215architectures.
216
217That said the complexity reduction by using fdt to support variants of
218boards which use the same SOC / CPU can be substantial.
219
220It is important to understand that the fdt only selects options
221available in the platform / drivers. It cannot add new drivers (yet). So
222you must still have the CONFIG option to enable the driver. For example,
223you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
224but can use the fdt to specific the UART clock, peripheral address, etc.
225In very broad terms, the CONFIG options in general control *what* driver
226files are pulled in, and the fdt controls *how* those files work.
227
228--
229Simon Glass <sjg@chromium.org>
2301-Sep-11
231