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doc: Update devicedocs including how to add tweaks

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This file is about 10 years old and the updates have not covered
everything that has changed, particularly in the last few years. Update
the information and add mention of the u-boot.dtsi files.

Signed-off-by: Simon Glass <sjg@chromium.org>

Fix typos.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
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Simon Glass 2021-08-01 18:57:11 -06:00 committed by Heinrich Schuchardt
parent 3e9fddfc4f
commit 5edf62c300
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doc/develop/devicetree/control.rst
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@ -1,29 +1,29 @@
.. SPDX-License-Identifier: GPL-2.0+ .. SPDX-License-Identifier: GPL-2.0+
.. sectionauthor:: Copyright 2011 The Chromium OS Authors .. sectionauthor:: Copyright 2011 The Chromium OS Authors
Device Tree Control in U-Boot Devicetree Control in U-Boot
============================= ============================
This feature provides for run-time configuration of U-Boot via a flat This feature provides for run-time configuration of U-Boot via a flattened
device tree (fdt). U-Boot configuration has traditionally been done devicetree (fdt).
using CONFIG options in the board config file. This feature aims to
make it possible for a single U-Boot binary to support multiple boards, This feature aims to make it possible for a single U-Boot binary to support
with the exact configuration of each board controlled by a flat device multiple boards, with the exact configuration of each board controlled by
tree (fdt). This is the approach recently taken by the ARM Linux kernel a flattened devicetree (fdt). This is the approach taken by Linux kernel for
and has been used by PowerPC for some time. ARM and RISC-V and has been used by PowerPC for some time.
The fdt is a convenient vehicle for implementing run-time configuration The fdt is a convenient vehicle for implementing run-time configuration
for three reasons. Firstly it is easy to use, being a simple text file. for three reasons:
It is extensible since it consists of nodes and properties in a nice
hierarchical format.
Finally, there is already excellent infrastructure for the fdt: a - There is already excellent infrastructure for the fdt: a compiler checks
compiler checks the text file and converts it to a compact binary the text file and converts it to a compact binary format, and a library
format, and a library is already available in U-Boot (libfdt) for is already available in U-Boot (libfdt) for handling this format
handling this format. - It is extensible since it consists of nodes and properties in a nice
hierarchical format
- It is fairly efficient to read incrementally
The dts directory contains a Makefile for building the device tree blob The arch/<arch>/dts directories contains a Makefile for building the devicetree
and embedding it in your U-Boot image. This is useful since it allows blob and embedding it in the U-Boot image. This is useful since it allows
U-Boot to configure itself according to what it finds there. If you have U-Boot to configure itself according to what it finds there. If you have
a number of similar boards with different peripherals, you can describe a number of similar boards with different peripherals, you can describe
the features of each board in the devicetree file, and have a single the features of each board in the devicetree file, and have a single
@ -32,62 +32,41 @@ generic source base.
To enable this feature, add CONFIG_OF_CONTROL to your board config file. To enable this feature, add CONFIG_OF_CONTROL to your board config file.
What is a Flat Device Tree? What is a Flattened Devicetree?
--------------------------- -------------------------------
An fdt can be specified in source format as a text file. To read about An fdt can be specified in source format as a text file. To read about
the fdt syntax, take a look at the specification (dtspec_). the fdt syntax, take a look at the specification (dtspec_).
You also might find this section of the Linux kernel documentation There is also a mailing list (dtlist_) for the compiler and associated
useful: (access this in the Linux kernel source code) tools.
Documentation/devicetree/booting-without-of.txt In case you are wondering, OF stands for Open Firmware. This follows the
convention used in Linux.
There is also a mailing list:
http://lists.ozlabs.org/listinfo/devicetree-discuss
In case you are wondering, OF stands for Open Firmware.
Tools Tools
----- -----
To use this feature you will need to get the device tree compiler. This is To create flattened device trees the device tree compiler is used. This is
provided by U-Boot automatically. If you have a system version of dtc provided by U-Boot automatically. If you have a system version of dtc
(typically in the 'device-tree-compiler' package), it is currently not used. (typically in the 'device-tree-compiler' package), that system version is
currently not used.
If you want to build your own dtc, it is kept here:: If you want to build your own dtc, it is kept here::
git://git.kernel.org/pub/scm/utils/dtc/dtc.git git://git.kernel.org/pub/scm/utils/dtc/dtc.git
For example:: You can decode a binary file with::
$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git dtc -I dtb -O dts <filename.dtb>
$ cd dtc
$ make
$ sudo make install
Then run the compiler (your version will vary):: That repo also includes `fdtget`/`fdtput` for reading and writing properties in
a binary file. U-Boot adds its own `fdtgrep` for creating subsets of the file.
$ dtc -v
Version: DTC 1.2.0-g2cb4b51f
$ make tests
$ cd tests
$ ./run_tests.sh
********** TEST SUMMARY
* Total testcases: 1371
* PASS: 1371
* FAIL: 0
* Bad configuration: 0
* Strange test result: 0
You will also find a useful fdtdump utility for decoding a binary file, as
well as fdtget/fdtput for reading and writing properties in a binary file.
Where do I get an fdt file for my board? Where do I get a devicetree file for my board?
---------------------------------------- ----------------------------------------------
You may find that the Linux kernel has a suitable file. Look in the You may find that the Linux kernel has a suitable file. Look in the
kernel source in arch/<arch>/boot/dts. kernel source in arch/<arch>/boot/dts.
@ -109,25 +88,20 @@ Use::
to set the filename of the devicetree source. Then put your devicetree to set the filename of the devicetree source. Then put your devicetree
file into:: file into::
board/<vendor>/dts/<name>.dts arch/<arch>/dts/<name>.dts
This should include your CPU or SOC's devicetree file, placed in This should include your CPU or SOC's devicetree file, placed in
arch/<arch>/dts, and then make any adjustments required. `arch/<arch>/dts`, and then make any adjustments required using a u-boot-dtsi
file for your board.
If CONFIG_OF_EMBED is defined, then it will be picked up and built into If CONFIG_OF_EMBED is defined, then it will be picked up and built into
the U-Boot image (including u-boot.bin). This is suitable for debugging the U-Boot image (including u-boot.bin). This is suitable for debugging
and development only and is not recommended for production devices. and development only and is not recommended for production devices.
If CONFIG_OF_SEPARATE is defined, then it will be built and placed in If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed
join the two:: in u-boot.bin so you can still just flash u-boot,bin onto your board. If you are
using CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
cat u-boot-nodtb.bin u-boot.dtb >image.bin
and then flash image.bin onto your board. Note that U-Boot creates
u-boot-dtb.bin which does the above step for you also. Resulting
u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
tree binary. tree binary.
If CONFIG_OF_BOARD is defined, a board-specific routine will provide the If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
@ -136,7 +110,8 @@ it and passes it to U-Boot.
If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
startup. This is only useful for sandbox. Use the -d flag to U-Boot to startup. This is only useful for sandbox. Use the -d flag to U-Boot to
specify the file to read. specify the file to read, -D for the default and -T for the test devicetree,
used to run sandbox unit tests.
You cannot use more than one of these options at the same time. You cannot use more than one of these options at the same time.
@ -165,7 +140,7 @@ To use this, put something like this in your board header file::
Build: Build:
After board configuration is done, fdt supported u-boot can be build in two After the board configuration is done, fdt supported u-boot can be built in two
ways: ways:
# build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE:: # build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
@ -177,6 +152,35 @@ ways:
$ make DEVICE_TREE=<dts-file-name> $ make DEVICE_TREE=<dts-file-name>
Adding tweaks for U-Boot
------------------------
It is strongly recommended that devicetree files in U-Boot are an exact copy of
those in Linux, so that it is easy to sync them up from time to time.
U-Boot is of course a very different project from Linux, e.g. it operates under
much more restrictive memory and code-size constraints. Where Linux may use a
full clock driver with Common Clock Format (CCF) to find the input clock to the
UART, U-Boot typically wants to output a banner as early as possible before too
much code has run.
A second difference is that U-Boot includes different phases. For SPL,
constraints are even more extreme and the devicetree is shrunk to remove
unwanted nodes, or even turned into C code to avoid access overhead.
U-Boot automatically looks for and includes a file with updates to the standard
devicetree for your board, searching for them in the same directory as the
main file, in this order::
<orig_filename>-u-boot.dtsi
<CONFIG_SYS_SOC>-u-boot.dtsi
<CONFIG_SYS_CPU>-u-boot.dtsi
<CONFIG_SYS_VENDOR>-u-boot.dtsi
u-boot.dtsi
Only one of these is selected but of course you can #include another one within
that file, to create a hierarchy of shared files.
Relocation, SPL and TPL Relocation, SPL and TPL
----------------------- -----------------------
@ -201,24 +205,24 @@ If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM
containing the board ID for example), it possible to start with a generic DTB containing the board ID for example), it possible to start with a generic DTB
and then switch over to the right DTB after the detection. For this purpose, and then switch over to the right DTB after the detection. For this purpose,
the platform code must call fdtdec_resetup(). Based on the returned flag, the the platform code must call fdtdec_resetup(). Based on the returned flag, the
platform may have to re-initiliaze the DM subusystem using dm_uninit() and platform may have to re-initialise the DM subsystem using dm_uninit() and
dm_init_and_scan(). dm_init_and_scan().
Limitations Limitations
----------- -----------
U-Boot is designed to build with a single architecture type and CPU Devicetrees can help reduce the complexity of supporting variants of boards
which use the same SOC / CPU.
However U-Boot is designed to build for a single architecture type and CPU
type. So for example it is not possible to build a single ARM binary type. So for example it is not possible to build a single ARM binary
which runs on your AT91 and OMAP boards, relying on an fdt to configure which runs on your AT91 and OMAP boards, relying on an fdt to configure
the various features. This is because you must select one of the various features. This is because you must select one of
the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
time. Similarly you cannot build for multiple cpu types or time. Similarly U-Boot cannot be built for multiple cpu types or
architectures. architectures.
That said the complexity reduction by using fdt to support variants of
boards which use the same SOC / CPU can be substantial.
It is important to understand that the fdt only selects options It is important to understand that the fdt only selects options
available in the platform / drivers. It cannot add new drivers (yet). So available in the platform / drivers. It cannot add new drivers (yet). So
you must still have the CONFIG option to enable the driver. For example, you must still have the CONFIG option to enable the driver. For example,
@ -227,4 +231,19 @@ but can use the fdt to specific the UART clock, peripheral address, etc.
In very broad terms, the CONFIG options in general control *what* driver In very broad terms, the CONFIG options in general control *what* driver
files are pulled in, and the fdt controls *how* those files work. files are pulled in, and the fdt controls *how* those files work.
.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf History
-------
U-Boot configuration was previous done using CONFIG options in the board
config file. This eventually got out of hand with nearly 10,000 options.
U-Boot adopted devicetrees around the same time as Linux and early boards
used it before Linux (e.g. snow). The two projects developed in parallel
and there are still some differences in the bindings for certain boards.
While there has been discussion of having a separate repository for devicetree
files, in practice the Linux kernel Git repository has become the place where
these are stored, with U-Boot taking copies and adding tweaks with u-boot.dtsi
files.
.. _dtspec: https://www.devicetree.org/specifications/
.. _dtlist: https://www.spinics.net/lists/devicetree-compiler/