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Simon Glass38d6b8d2011-10-15 05:48:21 +00003
Simon Glassb4fbaaf2021-08-01 18:57:11 -06004Devicetree Control in U-Boot
5============================
Simon Glass38d6b8d2011-10-15 05:48:21 +00006
Simon Glassb4fbaaf2021-08-01 18:57:11 -06007This feature provides for run-time configuration of U-Boot via a flattened
8devicetree (fdt).
9
10This feature aims to make it possible for a single U-Boot binary to support
11multiple boards, with the exact configuration of each board controlled by
12a flattened devicetree (fdt). This is the approach taken by Linux kernel for
13ARM and RISC-V and has been used by PowerPC for some time.
Simon Glass38d6b8d2011-10-15 05:48:21 +000014
15The fdt is a convenient vehicle for implementing run-time configuration
Simon Glassb4fbaaf2021-08-01 18:57:11 -060016for three reasons:
Simon Glass38d6b8d2011-10-15 05:48:21 +000017
Simon Glassb4fbaaf2021-08-01 18:57:11 -060018- There is already excellent infrastructure for the fdt: a compiler checks
19 the text file and converts it to a compact binary format, and a library
20 is already available in U-Boot (libfdt) for handling this format
21- It is extensible since it consists of nodes and properties in a nice
22 hierarchical format
23- It is fairly efficient to read incrementally
Simon Glass38d6b8d2011-10-15 05:48:21 +000024
Simon Glassb4fbaaf2021-08-01 18:57:11 -060025The arch/<arch>/dts directories contains a Makefile for building the devicetree
26blob and embedding it in the U-Boot image. This is useful since it allows
Simon Glass38d6b8d2011-10-15 05:48:21 +000027U-Boot to configure itself according to what it finds there. If you have
28a number of similar boards with different peripherals, you can describe
Simon Glassb4fbaaf2021-08-01 18:57:11 -060029the features of each board in the devicetree file, and have a single
Simon Glass38d6b8d2011-10-15 05:48:21 +000030generic source base.
31
32To enable this feature, add CONFIG_OF_CONTROL to your board config file.
33
34
Simon Glassb4fbaaf2021-08-01 18:57:11 -060035What is a Flattened Devicetree?
36-------------------------------
Simon Glass38d6b8d2011-10-15 05:48:21 +000037
38An fdt can be specified in source format as a text file. To read about
Simon Glassac35c2f2021-08-01 18:57:10 -060039the fdt syntax, take a look at the specification (dtspec_).
Simon Glass38d6b8d2011-10-15 05:48:21 +000040
Simon Glassb4fbaaf2021-08-01 18:57:11 -060041There is also a mailing list (dtlist_) for the compiler and associated
42tools.
Simon Glass38d6b8d2011-10-15 05:48:21 +000043
Simon Glassb4fbaaf2021-08-01 18:57:11 -060044In case you are wondering, OF stands for Open Firmware. This follows the
45convention used in Linux.
Simon Glass38d6b8d2011-10-15 05:48:21 +000046
Simon Glass38d6b8d2011-10-15 05:48:21 +000047
48Tools
49-----
50
Simon Glassb4fbaaf2021-08-01 18:57:11 -060051To create flattened device trees the device tree compiler is used. This is
Simon Glass9665f192018-10-01 12:22:16 -060052provided by U-Boot automatically. If you have a system version of dtc
Simon Glassb4fbaaf2021-08-01 18:57:11 -060053(typically in the 'device-tree-compiler' package), that system version is
54currently not used.
Simon Glass9665f192018-10-01 12:22:16 -060055
Simon Glassac35c2f2021-08-01 18:57:10 -060056If you want to build your own dtc, it is kept here::
Simon Glass38d6b8d2011-10-15 05:48:21 +000057
Simon Glassb4fbaaf2021-08-01 18:57:11 -060058 git://git.kernel.org/pub/scm/utils/dtc/dtc.git
Simon Glass38d6b8d2011-10-15 05:48:21 +000059
Simon Glassb4fbaaf2021-08-01 18:57:11 -060060You can decode a binary file with::
Simon Glass38d6b8d2011-10-15 05:48:21 +000061
Simon Glassb4fbaaf2021-08-01 18:57:11 -060062 dtc -I dtb -O dts <filename.dtb>
Simon Glass38d6b8d2011-10-15 05:48:21 +000063
Simon Glassb4fbaaf2021-08-01 18:57:11 -060064That repo also includes `fdtget`/`fdtput` for reading and writing properties in
65a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.
Simon Glass38d6b8d2011-10-15 05:48:21 +000066
Simon Glass38d6b8d2011-10-15 05:48:21 +000067
Simon Glassb4fbaaf2021-08-01 18:57:11 -060068Where do I get a devicetree file for my board?
69----------------------------------------------
Simon Glass38d6b8d2011-10-15 05:48:21 +000070
71You may find that the Linux kernel has a suitable file. Look in the
72kernel source in arch/<arch>/boot/dts.
73
74If not you might find other boards with suitable files that you can
75modify to your needs. Look in the board directories for files with a
76.dts extension.
77
78Failing that, you could write one from scratch yourself!
79
80
81Configuration
82-------------
83
Simon Glassac35c2f2021-08-01 18:57:10 -060084Use::
Simon Glass38d6b8d2011-10-15 05:48:21 +000085
Simon Glassac35c2f2021-08-01 18:57:10 -060086 #define CONFIG_DEFAULT_DEVICE_TREE "<name>"
Simon Glass38d6b8d2011-10-15 05:48:21 +000087
Simon Glassb4fbaaf2021-08-01 18:57:11 -060088to set the filename of the devicetree source. Then put your devicetree
Simon Glassac35c2f2021-08-01 18:57:10 -060089file into::
Simon Glass38d6b8d2011-10-15 05:48:21 +000090
Simon Glassb4fbaaf2021-08-01 18:57:11 -060091 arch/<arch>/dts/<name>.dts
Simon Glass38d6b8d2011-10-15 05:48:21 +000092
Simon Glassb4fbaaf2021-08-01 18:57:11 -060093This should include your CPU or SOC's devicetree file, placed in
94`arch/<arch>/dts`, and then make any adjustments required using a u-boot-dtsi
95file for your board.
Simon Glass38d6b8d2011-10-15 05:48:21 +000096
97If CONFIG_OF_EMBED is defined, then it will be picked up and built into
Simon Glass67bce6b2014-06-02 22:04:50 -060098the U-Boot image (including u-boot.bin). This is suitable for debugging
99and development only and is not recommended for production devices.
Simon Glass38d6b8d2011-10-15 05:48:21 +0000100
101If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600102a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
103in u-boot.bin so you can still just flash u-boot,bin onto your board. If you are
104using CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
Simon Glass67bce6b2014-06-02 22:04:50 -0600105tree binary.
Simon Glass38d6b8d2011-10-15 05:48:21 +0000106
Alex Deymo5b661ec2017-04-02 01:25:20 -0700107If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600108devicetree at runtime, for example if an earlier bootloader stage creates
Alex Deymo5b661ec2017-04-02 01:25:20 -0700109it and passes it to U-Boot.
110
Simon Glass15393432013-04-20 08:42:41 +0000111If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
112startup. This is only useful for sandbox. Use the -d flag to U-Boot to
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600113specify the file to read, -D for the default and -T for the test devicetree,
114used to run sandbox unit tests.
Simon Glass15393432013-04-20 08:42:41 +0000115
116You cannot use more than one of these options at the same time.
Simon Glass38d6b8d2011-10-15 05:48:21 +0000117
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600118To use a devicetree file that you have compiled yourself, pass
Simon Glassac35c2f2021-08-01 18:57:10 -0600119EXT_DTB=<filename> to 'make', as in::
Simon Glass67bce6b2014-06-02 22:04:50 -0600120
Simon Glassac35c2f2021-08-01 18:57:10 -0600121 make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
Simon Glass67bce6b2014-06-02 22:04:50 -0600122
123Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
124if used, and u-boot-dtb.bin.
125
Simon Glassdc6fa642011-10-24 19:15:34 +0000126If you wish to put the fdt at a different address in memory, you can
127define the "fdtcontroladdr" environment variable. This is the hex
128address of the fdt binary blob, and will override either of the options.
129Be aware that this environment variable is checked prior to relocation,
130when only the compiled-in environment is available. Therefore it is not
131possible to define this variable in the saved SPI/NAND flash
Thomas Chou4fda2812015-10-16 08:44:51 +0800132environment, for example (it will be ignored). After relocation, this
133variable will be set to the address of the newly relocated fdt blob.
134It is read-only and cannot be changed. It can optionally be used to
135control the boot process of Linux with bootm/bootz commands.
Simon Glassdc6fa642011-10-24 19:15:34 +0000136
Simon Glassac35c2f2021-08-01 18:57:10 -0600137To use this, put something like this in your board header file::
Simon Glassdc6fa642011-10-24 19:15:34 +0000138
Simon Glassac35c2f2021-08-01 18:57:10 -0600139 #define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
Simon Glassdc6fa642011-10-24 19:15:34 +0000140
Jagannadha Sutradharudu Teki79e63a42013-02-28 10:20:18 +0000141Build:
142
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600143After the board configuration is done, fdt supported u-boot can be built in two
Simon Glassac35c2f2021-08-01 18:57:10 -0600144ways:
145
146# build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
147
Jagannadha Sutradharudu Teki79e63a42013-02-28 10:20:18 +0000148 $ make
Simon Glassac35c2f2021-08-01 18:57:10 -0600149
150# build the user specified dts file::
151
Jagannadha Sutradharudu Teki79e63a42013-02-28 10:20:18 +0000152 $ make DEVICE_TREE=<dts-file-name>
153
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600154
155Adding tweaks for U-Boot
156------------------------
157
158It is strongly recommended that devicetree files in U-Boot are an exact copy of
159those in Linux, so that it is easy to sync them up from time to time.
160
161U-Boot is of course a very different project from Linux, e.g. it operates under
162much more restrictive memory and code-size constraints. Where Linux may use a
163full clock driver with Common Clock Format (CCF) to find the input clock to the
164UART, U-Boot typically wants to output a banner as early as possible before too
165much code has run.
166
167A second difference is that U-Boot includes different phases. For SPL,
168constraints are even more extreme and the devicetree is shrunk to remove
169unwanted nodes, or even turned into C code to avoid access overhead.
170
171U-Boot automatically looks for and includes a file with updates to the standard
172devicetree for your board, searching for them in the same directory as the
173main file, in this order::
174
175 <orig_filename>-u-boot.dtsi
176 <CONFIG_SYS_SOC>-u-boot.dtsi
177 <CONFIG_SYS_CPU>-u-boot.dtsi
178 <CONFIG_SYS_VENDOR>-u-boot.dtsi
179 u-boot.dtsi
180
181Only one of these is selected but of course you can #include another one within
182that file, to create a hierarchy of shared files.
Simon Glass38d6b8d2011-10-15 05:48:21 +0000183
Simon Glassa31dc3d2018-10-01 12:22:17 -0600184Relocation, SPL and TPL
185-----------------------
186
187U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
188
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600189The full devicetree is available to U-Boot proper, but normally only a subset
Simon Glassa31dc3d2018-10-01 12:22:17 -0600190(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
Heinrich Schuchardtc79f03c2020-02-25 21:35:39 +0100191'SPL Support' in doc/driver-model/design.rst for more details.
Simon Glassa31dc3d2018-10-01 12:22:17 -0600192
193
Jean-Jacques Hiblot7c530e32018-12-07 14:50:52 +0100194Using several DTBs in the SPL (CONFIG_SPL_MULTI_DTB)
195----------------------------------------------------
196In some rare cases it is desirable to let SPL be able to select one DTB among
197many. This usually not very useful as the DTB for the SPL is small and usually
198fits several platforms. However the DTB sometimes include information that do
199work on several platforms (like IO tuning parameters).
200In this case it is possible to use CONFIG_SPL_MULTI_DTB. This option appends to
201the SPL a FIT image containing several DTBs listed in SPL_OF_LIST.
202board_fit_config_name_match() is called to select the right DTB.
203
204If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
205containing the board ID for example), it possible to start with a generic DTB
206and then switch over to the right DTB after the detection. For this purpose,
207the platform code must call fdtdec_resetup(). Based on the returned flag, the
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600208platform may have to re-initialise the DM subsystem using dm_uninit() and
Jean-Jacques Hiblot7c530e32018-12-07 14:50:52 +0100209dm_init_and_scan().
210
211
Simon Glass38d6b8d2011-10-15 05:48:21 +0000212Limitations
213-----------
214
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600215Devicetrees can help reduce the complexity of supporting variants of boards
216which use the same SOC / CPU.
217
218However U-Boot is designed to build for a single architecture type and CPU
Simon Glass38d6b8d2011-10-15 05:48:21 +0000219type. So for example it is not possible to build a single ARM binary
220which runs on your AT91 and OMAP boards, relying on an fdt to configure
221the various features. This is because you must select one of
222the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600223time. Similarly U-Boot cannot be built for multiple cpu types or
Simon Glass38d6b8d2011-10-15 05:48:21 +0000224architectures.
225
Simon Glass38d6b8d2011-10-15 05:48:21 +0000226It is important to understand that the fdt only selects options
227available in the platform / drivers. It cannot add new drivers (yet). So
228you must still have the CONFIG option to enable the driver. For example,
229you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
230but can use the fdt to specific the UART clock, peripheral address, etc.
231In very broad terms, the CONFIG options in general control *what* driver
232files are pulled in, and the fdt controls *how* those files work.
233
Simon Glassb4fbaaf2021-08-01 18:57:11 -0600234History
235-------
236
237U-Boot configuration was previous done using CONFIG options in the board
238config file. This eventually got out of hand with nearly 10,000 options.
239
240U-Boot adopted devicetrees around the same time as Linux and early boards
241used it before Linux (e.g. snow). The two projects developed in parallel
242and there are still some differences in the bindings for certain boards.
243While there has been discussion of having a separate repository for devicetree
244files, in practice the Linux kernel Git repository has become the place where
245these are stored, with U-Boot taking copies and adding tweaks with u-boot.dtsi
246files.
247
248.. _dtspec: https://www.devicetree.org/specifications/
249.. _dtlist: https://www.spinics.net/lists/devicetree-compiler/