Device Tree Reference

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.

History

 * How device tree got into Linux and how it has evolved

Future

 * How device tree is changing and where it is headed
 * Frank's thoughts on what is needed to complete basic overlay support

Device-Tree irc
The Device Tree irc channel is #devicetree on freenode.net.

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.

As of 08/28/18: "Due to the persistent ongoing spam, all new connections are being set +R (block messages from unidentified users)..." This means that the "/join" command will not succeed until you register your nick with freenode. For instructions on how to register, see https://freenode.net/kb/answer/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.

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 devicetree@vger.kernel.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.

http://vger.kernel.org/vger-lists.html#devicetree-spec

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).

http://vger.kernel.org/vger-lists.html#devicetree-compiler

archive: https://www.spinics.net/lists/devicetree-compiler/

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.

Linux Specific
Device Tree Linux

Standards
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.

[[Media:LoPAPR_DRAFT_v11_24March2016.pdf | Open Power Foundation Linux on Power Architecture Platform Reference (LoPAPR), v1.1, 24 march 2016]] Source: https://members.openpowerfoundation.org/document/dl/469

[[Media:Power_ePAPR_APPROVED_v1.1.pdf | Power.org Standard for Embedded Power Architecture Platform Requirements (ePAPR) v1.1]] Source: https://www.power.org/documentation/power-org-standard-for-embedded-power-architecture-platform-requirements-epapr-v1-1-2/

Device Tree Source Format

 * Devicetree Specification Release 0.2
 * chapter 2 "The Devicetree"
 * chapter 6 "Devicetree Source Format (version 1)"


 * [[Media:Power_ePAPR_APPROVED_v1.1.pdf | 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


 * differences between ePAPR and the Linux dtc compiler


 * presentations
 * "Device Tree for Dummies", ELC 2014 by Thomas Petazzoni
 * [[Media:petazzoni-device-tree-dummies_0.pdf | PDF]]
 * YouTube video


 * Undocumented
 * Device_Tree_Source_Undocumented contains some information not documented elsewhere

Bindings
Bindings are documented in
 * Standards documents
 * Linux kernel source tree

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

Some especially useful files (in alphabetic order) are:


 * ABI/testing/sysfs-firmware-ofw
 * description of /sys/firmware/devicetree/* on the target system


 * devicetree/bindings
 * 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


 * devicetree/bindings/ABI.txt
 * comments on stable binding and general bindings rules


 * devicetree/bindings/property-units.txt
 * how to name properties which have a unit of measure


 * devicetree/bindings/resource-names
 * *-name properties containing an ordered list of names corresponding to another property


 * devicetree/bindings/submitting-patches.txt
 * important details for patch submitters and kernel maintainers


 * devicetree/bindings/vendor-prefixes.txt
 * vendor prefix registry

Some subsystems have overview bindings descriptions under devicetree/bindings/:


 * bus/mvebu.bus
 * bus/simple-pm-bus.txt
 * clock/clock-bindings.txt
 * display/mipi-dsi-bus.txt
 * dma/dma.txt
 * gpio/gpio.txt
 * hwlock/hwlock.txt
 * iio/iio.txt
 * interrupt-controller/interrupts.txt
 * iommu/iommu.txt
 * lpddr2/lpddr2.txt
 * mailbox/mailbox.txt
 * memory-controllers/mvebu-devbus.txt
 * mfd/mfd.txt
 * mips/cavium/bootbus.txt
 * mmc/mmc.txt


 * pci/pci.txt
 * phy/phy-bindings.txt
 * pinctrl/pinctrl-bindings.txt
 * power_supply/power_supply.txt
 * pwm/pwm.txt
 * regmap/regmap.txt
 * regulator/regulator.txt
 * reserved-memory/reserved-memory.txt
 * reset/reset.txt
 * spi/spi-bus.txt
 * spmi/spmi.txt
 * thermal/thermal.txt
 * xillybus/xillybus.txt

Other Documentation in Linux Kernel Source Tree

 * devicetree/booting-without-of.txt
 * original powerpc document on booting without Open Firmware


 * devicetree/changesets.txt
 * Description of changesets. Apply changes such that full set of changes is applied successfully or none are.


 * devicetree/dynamic-resolution-notes.txt
 * Implementation of resolver, used when applying overlays.


 * devicetree/of_unittest.txt
 * How data for device tree unit tests is added to the kernel


 * devicetree/overlay-notes.txt
 * Kernel internal implementation notes on overlays


 * devicetree/usage-model.txt
 * Overview of: Linux and the Device Tree

Subsystem specific

 * gpio / pinctrl


 * interrupts


 * timers


 * etc

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.
 * Frank's thoughts on what is needed to complete basic overlay support

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 [[Media:Elce 2018 dt bof.pdf | 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

All configured .dtb
make dtbs

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:


 * arch/arm/boot/dts/.*.dtb.cmd
 * arch/${ARCH}/boot/dts/.*.dtb.cmd

FDT format
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 0.1.

The format of version 17 of the FDT is described in chapter 8, "Flat Device Tree Physical Structure" of the [[Media:Power_ePAPR_APPROVED_v1.1.pdf | ePAPR v1.1]] (superseded by the Devicetree Specification).

Section "II - The DT block format" of booting-without-of.txt is a more expansive description of the format of FDT.

Install

 * make dtbs_install
 * installs in /boot/dtbs/


 * many other ways to install ....

Debugging
(Also see Device_Tree_presentations_papers_articles.)


 * tutorials
 * "Solving Device Tree Issues" (updated), ELCE October 2015 by Frank Rowand
 * [[Media:Dt_debugging_elce_2015_151006_0421.pdf | PDF slides]]
 * Supporting Resources (patches, etc)


 * diffing .dts, .dtb, /proc/devicetree/
 * scripts/dtc/dtx_diff
 * in the Linux kernel source tree as of 4.6-rc1
 * use cases, advantages, and things to be aware of are described in [[Media:Dt_debugging_elce_2015_151006_0421.pdf | "Solving Device Tree Issues"]] (updated), ELCE October 2015 by Frank Rowand (PDF). dtx_diff is referred to as "dtdiff" in this presentation.


 * locating source location for properties


 * 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

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.
 * Documentation/manual.txt Device Tree Compiler Manual
 * Documentation/dts-format.txt Device Tree Source Format
 * The Maintainers are listed in the file README

dtx_diff
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 [[Media:Dt_debugging_elce_2015_151006_0421.pdf | "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.

dt_to_config
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 [[Media:Dt_debugging_part_2.pdf | "Solving Device Tree Issues - part 2", LinuxCon Japan, July 2016, Frank Rowand ]].

dt_to_config was merged in 4.8-rc1.

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
 * https://git.kernel.org/cgit/utils/dtc/dtc.git
 * 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

Building dtc, fdtdump, and other tools in the upstream project:
 * make

dts-mode
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[1]. 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

EDK
"The device tree generator is a Xilinx EDK tool that plugs into the Automatic BSP Generation features of the tool, XPS"
 * Xilinx EDK device-tree generator - Generates an FDT from Xilinx FPGA design files.
 * http://xilinx.wikidot.com/device-tree-generator

fdtdump
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 exists in the Linux kernel source tree. There is no makefile entry to build fdtdump in the Linux kernel source tree. fdtdump may be removed from the Linux kernel source tree in the future.

fdtdump is located in the dtc project and can easily be built there.

linux kernel framework
Enable unit tests through config option CONFIG_OF_UNITTEST


 * Device Drivers ---> Device Tree and Open Firmware support ---> Device Tree runtime unit tests

The unit tests will be performed near the end of the boot sequence. The start of the tests is indicated by:


 * 1) dt-test ### start of unittest - you will see error messages

An example test result summary is:


 * 1) dt-test ### end of unittest - 148 passed, 0 failed

The unit tests are located in
 * drivers/of/unittest.c
 * drivers/of/unittest-data/

dtc
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 **********

BSD
FreeBSD FlattenedDeviceTree wiki

FreeBSDArmBoards wiki

U-Boot
http://www.denx.de/wiki/U-Boot/UBootFdtInfo

http://www.crashcourse.ca/wiki/index.php/U-Boot_and_device_trees

Zephyr
presentations on elinux.org

Glossary

 * .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.