Difference between revisions of "Device Tree Reference"
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=== Device-tree Mailing List ===
=== Device-tree Mailing List ===
Revision as of 20:47, 5 October 2021
- 1 Introduction
- 2 Device Tree Usage
- 3 What Is Device Tree
- 4 Request for Documentation Suggestions
- 5 History
- 6 Future
- 7 Device Tree Related Communications
- 8 Presentations, Papers, and Articles
- 9 Device Tree Mysteries
- 10 Linux Specific
- 11 Standards
- 12 Device Tree Source Format
- 13 Bindings
- 14 Bindings Documentation in Linux Kernel Source Tree
- 15 Other Documentation in Linux Kernel Source Tree
- 16 Subsystem specific
- 17 Overlays
- 18 Device Tree Source Validation
- 19 Compiling source into .dtb (FDT binary blob)
- 20 FDT format
- 21 Install
- 22 Boot loaders
- 23 Booting
- 24 Debugging
- 25 Debugging - random hints
- 26 Tools in Linux kernel source tree
- 27 Tools external to Linux kernel source tree
- 28 Validation
- 29 Testing
- 30 Other projects using Device Tree
- 31 Glossary
Device Tree Usage
If Device Tree is new to you, start with Device Tree Usage page. That page describes what Device Tree source looks like. It walks through building the source for a new machine. It describes the basic concepts, shows specific examples, and covers some advanced features.
Device Tree Usage page was previously located at devicetree.org.
What Is Device Tree
The primary purpose of Device Tree in Linux is to provide a way to describe non-discoverable hardware. This information was previously hard coded in source code.
Some more background on what Device Tree is, advantages, and competing solutions, see this page. Most of the contents of this page was previously located at Device_Tree, which now redirects to Device_Tree_Reference.
Request for Documentation Suggestions
If you have any comments or suggestions about the Device Tree documentation on elinux.org, please send them to frowand (dot) list (at) gmail (dot) com
I am currently trying to make the information more organized, more comprehensive, and a more complete index of information available elsewhere. I am looking for comments on what is incorrect, incomplete, or missing. I would appreciate pointers to good documentation, tutorials, etc that I can link to.
- How device tree is changing and where it is headed
- Includes Linux Plumbers conference, Kernel Summit, etc
- Frank's thoughts on what is needed to complete basic overlay support
Device Tree Related Communications
The Device Tree IRC channel is #devicetree on libera.chat.
default server name: irc.libera.chat A list of regional servers is available at: https://libera.chat/guides/connect
You can also connect over the Matrix network with #devicetree:libera.chat
You are likely to find many people connected to the channel, but many of them are not actively monitoring traffic. There may be a delay of several days or weeks before a question or comment is acknowledged.
You should register your nick (your username) so that it can not be hijacked by someone else. Instructions for this can be found at https://libera.chat/guides/registration
Device-tree Mailing List
This list contained all devicetree related discussion until February 2014. At that time, the devicetree.spec and devicetree.compiler lists were created to provide lower volume lists for those specific topic areas.
Additional archive added in 2019:
https://lore.kernel.org/linux-devicetree/ (the git clone seems to have entries beginning July 29, 2008) git clone --mirror https://lore.kernel.org/linux-devicetree/0 linux-devicetree/git/0.git public inbox mirroring also available
After July 2013:
http://vger.kernel.org/vger-lists.html#devicetree archive: http://www.spinics.net/lists/devicetree/ archive: http://dir.gmane.org/gmane.linux.drivers.devicetree
Up through July 2013:
https://lists.ozlabs.org/listinfo/devicetree-discuss archive: https://lists.ozlabs.org/pipermail/devicetree-discuss/ archive: http://news.gmane.org/gmane.linux.drivers.devicetree
Core devicetree binding / Devicetree Specification Mailing List
Created February 2014.
The devicetree.spec list is for "core" binding discussions; anything that
affects entire subsystems or the kinds of things that would make sense
to be added to the
ePAPR Devicetree Specification.
Individual device bindings continue to be posted to firstname.lastname@example.org, but anything affecting subsystems or generic patterns should be posted to this list.
After April 28, 2016:
The creation and updating of the Devicetree Specification is also discussed on this list.
archive: http://dir.gmane.org/gmane.comp.devicetree.spec archive: https://www.spinics.net/lists/devicetree-spec/
Device-tree Compiler and Tools Mailing List
Created February 2014.
The devicetree.compiler list is for discussion related to dtc and other tools.
It is specifically for discussing dt tooling topics (parsing, schema validation, data format, etc).
Presentations, Papers, and Articles
The Device Tree presentations, papers, and articles page is organized by topic area and by date.
Device Tree Mysteries
If you can not find a specific piece of information elsewhere, it might be on the Device_Tree_Mysteries page.
Devicetree Specification Release 0.2, located on the devicetree.org web site has superseded the ePAPR for the Linux kernel. The specification is meant to be OS independent. The Linux kernel devicetree maintainers are actively involved in the specification process.
The Devicetree Specification will continue to be updated.
Power.org Standard for Embedded Power Architecture Platform Requirements (ePAPR) v1.1
Device Tree Source Format
- Devicetree Specification Release 0.2
- chapter 2 "The Devicetree"
- chapter 6 "Devicetree Source Format (version 1)"
- Power.org Standard for Embedded Power Architecture Platform Requirements (ePAPR) v1.1
- superseded by the Devicetree Specification
- chapter 2 "The Device Tree"
- Appendix A "Device Tree Source Format (Version 1)"
- dtc compiler repository documentation
- Documentation/dts-format.txt Device Tree Source Format
- Device_Tree_Source_Undocumented contains some information not documented elsewhere
Bindings are documented in
Bindings Documentation in Linux Kernel Source Tree
Documentation files about device tree is available in the Linux kernel source at Documentation/devicetree
Specific bindings specifications are located in the Linux kernel source at: Documentation/devicetree/bindings
Before submitting a binding patch, read these:
- comments on stable binding and general bindings rules
- DOs and DON'Ts for designing and writing Devicetree bindings
- important details for patch submitters and kernel maintainers
Some especially useful files (in alphabetic order) are:
- description of /sys/firmware/devicetree/* on the target system
- the bindings directory has details about the syntax and expected elements for each device type representable in the dts and used by kernel frameworks and drivers
- how to name properties which have a unit of measure
- *-name properties containing an ordered list of names corresponding to another property'
- vendor prefix registry
Some subsystems have overview bindings descriptions under devicetree/bindings/:
Other Documentation in Linux Kernel Source Tree
- original powerpc document on booting without Open Firmware
- Description of changesets. Apply changes such that full set of changes is applied successfully or none are.
- Implementation of resolver, used when applying overlays.
- How data for device tree unit tests is added to the kernel
- Kernel internal implementation notes on overlays
- Overview of: Linux and the Device Tree
- gpio / pinctrl
Mainline Linux Support
Run time overlay apply and run time overlay remove from user space are not supported in the mainline kernel. There are out of tree patches to implement this feature via an overlay manager. The overlay manager is used successfully by many users for specific overlays on specific boards with specific environments and use cases. However, there are many issues with the Linux kernel overlay implementation due to incomplete and incorrect code. The overlay manager has not been accepted in mainline due to these issues. Once these issues are resolved, it is expected that some method of run time overlay apply and overlay removal from user space will be supported by the Linux kernel.
There is a possibility that overlay apply and overlay remove support could be phased in slowly, feature by feature, as specific issues are resolved.
Boot Loader Support
An alternative to Linux kernel run time overlay apply is boot loader overlay apply. For example, U-Boot supports overlay apply. This method of overlay apply avoids the complications and issues of run time Linux kernel overlay apply. This method is likely to be more robust and less problematic than run time Linux kernel overlay apply and is thus the recommended technique.
Overlay Source Format
In early overlay days, much of the overlay metadata was hand coded in the overlay source file. The current dtc compiler in the Linux kernel source tree eliminates the need for this hand coding. It is expected that the Linux kernel overlay apply code will at some time in the future refuse to apply an overlay compiled from source with hand coded metadata. The metadata includes fragment nodes and nodes with names beginning with an underscore, such as __overlay__, __fixup__, __local_fixup__, and __symbols__.
For examples of transforming overlay source from the hand coded metadata form, see slides 29 - 34 of the ELCE 2018 Devicetree BoF [PDF].
Device Tree Source Validation
See device tree future validation work.
The most basic validation is warnings from the dtc compiler. Noisy checks are disabled by default. Enable the noisy checks by specifying 'W=1' on the make command line.
Compiling source into .dtb (FDT binary blob)
Documentation/manual.txt Device Tree Compiler Manual
via Linux kernel make system
All configured .dtb
A specific .dtb
This is an example of compiling arch/arm/boot/dts/qcom-msm8960-cdp.dts
The make target is the name of the .dts file, with ".dts" changed to ".dtb".
Check that the config option that enables the dtb is set, for example:
arch/arm/boot/dts/Makefile: dtb-$(CONFIG_ARCH_QCOM) += \ ... qcom-msm8960-cdp.dtb \ ... $ grep CONFIG_ARCH_QCOM .config CONFIG_ARCH_QCOM=y
Either specify the correct architecture in the make command (or export the ARCH)
export ARCH="arm" make qcom-msm8960-cdp.dtb
ARCH="arm" make qcom-msm8960-cdp.dtb
via generated script in build directory
After a .dtb has been built by the Linux kernel make system, files containing the precise build commands will remain in the build directory:
Other names for Flattened Device Tree (FDT) are "binary blob" and ".dtb".
The format of version 17 of the FDT is described in chapter 5, "Flat Devicetree Physical Structure" of the Devicetree Specification Release 0.2.
The format of version 17 of the FDT is described in chapter 8, "Flat Device Tree Physical Structure" of the ePAPR v1.1 (superseded by the Devicetree Specification).
Section "II - The DT block format" of booting-without-of is a more expansive description of the format of FDT:
booting-without-of.rst from Linux v5.8 booting-without-of.rst from Linux v5.9 (no longer present after v5.9)
- make dtbs_install
- installs in /boot/dtbs/<kernel version>
- many other ways to install ....
(Also see Device_Tree_presentations_papers_articles#debug.)
- diffing .dts, .dtb, /proc/devicetree/
- in the Linux kernel source tree as of 4.6-rc1
- use cases, advantages, and things to be aware of are described in "Solving Device Tree Issues" (updated), ELCE October 2015 by Frank Rowand (PDF). dtx_diff is referred to as "dtdiff" in this presentation.
- locating source file and line for nodes and properties
- scripts/dtc/dtx_diff --annotate
- in the Linux kernel source tree as of 5.0-rc1
- useful for 'dtx_diff DTx'
- not useful for 'dtx_diff DTx_1 DTx_2'
- scripts/dtc/dtx_diff --annotate
- boot time messages
- device creation
- driver registration
- binding driver to device
- deferred binding
Debugging - random hints
You can set CONFIG_PROC_DEVICETREE to be able to see the device tree information in /proc after booting. Build the kernel with this option set to 'Y', boot the kernel, then 'cd /proc/device-tree'
/proc/device-tree still does not exist. Now what??? Is CONFIG_PROC_FS enabled? Is CONFIG_OF enabled? Does /sys/firmware/devicetree/base exist? (Note that this path is not an ABI, but currently /proc/devicetree is a soft link to this location.) Did the bootloader load a devicetree? (Check the boot console or use dmesg to print the boot messages.)
For newer kernels where the CONFIG_PROC_DEVICETREE option does not exist, /proc/device-tree will be created if CONFIG_PROC_FS is set to 'Y'.
You might also try CONFIG_DEBUG_DRIVER=Y.
Also, often, you can set the line: "#define DEBUG 1" to an individual C file, to produce add debug statements to the routines in that file. This will activate any pr_debug() lines in the source for that file.
Alternatively, you can add the following to drivers/of/Makefile:
CFLAGS_base.o := -DDEBUG CFLAGS_device.o := -DDEBUG CFLAGS_platform.o := -DDEBUG CFLAGS_fdt.o := -DDEBUG
Tools in Linux kernel source tree
dtc (in kernel)
dtc (Device Tree Compiler) - converts between the human editable device tree source "dts" format and the compact device tree blob "dtb" representation usable by the kernel or assembler source. dtc is also a dtb decompiler.
- The linux version of dtc is located in scripts/dtc/ in the kernel source directory. New versions are periodically pulled from the upstream project.
- dtc is built by the Linux kernel build process as a dependency when needed. If you want to explicitly build it in the Linux kernel source tree, use the command: make scripts
- Information about the upstream project is in a nearby section.
- The Maintainers are listed in the file README
dtx_diff has two modes of operation:
- compare two dtX files
- compile a single dtX file (using the normal Linux includes and .config) then decompiles that into a device tree source file.
A dtX file can be a device tree source file, a device tree compiled file (aka .dtb, FDT, or device tree blob), or a file system based subtree (either /proc/device-tree on the target system, or /proc/device-tree can be tarred on the target system and untarred on the system containing dtx_diff).
Examples of how to use dtx_diff can be found in "Solving Device Tree Issues" (updated), ELCE October 2015, Frank Rowand . Note that dtx_diff was named 'dtdiff' in the presentation.
dtx_diff was merged in 4.6-rc1
--annotate was merged in 5.0-rc1
dt_to_config is a tool to provide information about the Linux kernel configuration options needed by a given device tree.
dt_to_config can also report on whether the kernel configuration options are set correctly.
Examples of how to use dt_to_config and some limitations to be aware of can be found in "Solving Device Tree Issues - part 2", LinuxCon Japan, July 2016, Frank Rowand .
dt_to_config was merged in 4.8-rc1.
Tools external to Linux kernel source tree
dtc (upstream project)
The dtc (Device Tree Compiler) upstream project is periodically pulled into the Linux kernel source tree. The upstream project is maintained in
- git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
Info on submitting patches is in section 1.1 of Documentation/manual.txt
How to build dtc, fdtdump, and other tools in the upstream project:
From the github README.mkd: a quick attempt at getting basic highlighting for Device Tree syntax in emacs.
From the announcement: Today I cobbled together a rudimentary devicetree major mode for emacs. At this point it's pretty much limited to rather basic syntax highlighting but works fairly well all things considered. It can be found on Github. Patches are of course quite welcome.
Project location on [github].
I am not an emacs user, so I would appreciate any feedback on how useful this tool is and additional information that could be added to this description (or if the tool is useless and should be removed from this page). Email me at frowand (dot) list (at) gmail (dot) com
- Xilinx EDK device-tree generator - Generates an FDT from Xilinx FPGA design files.
"The device tree generator is a Xilinx EDK tool that plugs into the Automatic BSP Generation features of the tool, XPS"
fdtdump is a tool to convert an FDT (flattened device tree, aka device tree blob) to source.
The dtc compiler is an alternate tool that also has an option to convert an FDT to source (-O dts).
fdtdump differs in some ways from "dtc -O dts":
- fdtdump prints the FDT header as a source comment.
- The format of data differs in appearance (number of hex digits printed) but both formats result in the same FDT when compiled.
- The --scan option of fdtdump will search through a file that embeds an FDT, attempting to find the FDT. The embedded FDT will be converted to dts.
An unmaintained version of the source of fdtdump existed in the Linux kernel source tree. There is no makefile entry to build fdtdump in the Linux kernel source tree. fdtdump was removed from the Linux kernel source tree in 5.11.
fdtdump is located in the dtc project and can easily be built there.
fdtoverlay is a tool created to test libfdt. It applies one or more overlay .dtb files to a base .dtb file. It appears that some people are also using fdtoverlay to apply overlays pre-boot.
fdtoverlay is located in the dtc project and can easily be built there. It is queued up to be added to the Linux kernel source tree in Linux 5.11 or 5.12-rc1.
in kernel documentation
Documentation/devicetree/writing-schema.rst provides in kernel documentation:
- Writing DeviceTree Bindings in json-schema
- how to write the binding schemas that devicetree source is validated against
- how to install the validation tools
- Running Checks
- how to run the validation tools
validation tools project
linux kernel framework
The dtc tests are located in the upstream dtc project (see dtc (upstream project)).
build dtc: make dtc build tests: make tests run tests: make check run tests with valgrind --tool=memcheck: make checkm run tests in verbose mode: make checkv
Each test will report PASS or FAIL.
An example test result summary is:
********** TEST SUMMARY * Total testcases: 1565 * PASS: 1565 * FAIL: 0 * Bad configuration: 0 * Strange test result: 0 **********
Other projects using Device Tree
User manual: Devicetree support
BSD operating system
Git repository effective 2021-02-28: git clone https://source.denx.de/u-boot/u-boot.git (previous location: gitlab.denx.de)
Instead of loading a DTB and parsing it at run time, Zephyr generates source files from the devicetree .dts. The resulting devicetree data file is compiled as part of the Zephyr build.
- .dtb - File name suffix, by convention, for compiled devicetree.
- .dts - File name suffix, by convention, for devicetree source.
- .dtsi - File name suffix, by convention, for devicetree source to be included by a .dts or .dtsi file.
- Binary Blob - File containing a compiled devicetree.
- dtc - Devicetree compiler.
- EDT Expanded Device Tree - A copy of an FDT, but converted to kernel data structures, in the kernel memory space.
- FDT Flattened Device Tree - A copy of a binary blob in the kernel memory space.