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Introduction to Device Tree¶

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Device tree is a way of describing hardware and configuration informationfor boards. Device tree was adopted for use in the Linux kernel for thePowerPC architecture. However, it is now in use for ARM and otherarchitectures.

The device tree is a data structure for dynamically describing hardwareusing a Device Tree Source (DTS) data structure language, and compiledinto a compact Device Tree Blob (DTB) using a Device Tree Compiler (DTC).Rather than hard coding every detail of a board’s hardware into theoperating system, the hardware-describing DTB is passed to the operatingsystem at boot time. This allows the same compiled Linux kernel to supportdifferent hardware configurations within an architecture family (e.g., ARM,x86, PowerPC) and moves a significant part of the hardware description out ofthe kernel binary itself.

Traditional usage of device tree involves storing of the Device Tree Blob.The DTB is then used during runtime for configuration of device drivers. InZephyr, the DTS information will be used only during compile time.Information about the system is extracted from the compiled DTS and used tocreate the application image.

Device tree uses a specific format to describe the device nodes in a system.This format is described in the Device Tree Specification.

More device tree information can be found at the device tree repository.

System build requirements¶

The Zephyr device tree feature requires a device tree compiler (DTC) and PythonYAML packages. Refer to the installation guide for your specific host OS:

Zephyr and Device Tree¶

Zephyr

In Zephyr, device tree is used to not only describe hardware, but also todescribe Zephyr-specific configuration information. The Zephyr-specificinformation is intended to augment the device tree descriptions. The mainreason for this is to leverage existing device tree files that a SoC vendor mayalready have defined for a given platform.

Device Tree provides a unified description of a hardware system used in anapplication. It is used in Zephyr to describe hardware and provide a boot-timeconfiguration of this hardware.

The device tree files are compiled using the device tree compiler. The compilerruns the .dts file through the C preprocessor to resolve any macro or #definesutilized in the file. The output of the compile is another dts formatted file.

After compilation, a python script extracts information from the compiled devicetree file using a set of rules specified in YAML files. The extractedinformation is placed in a header file that is used by the rest of the code asthe project is compiled.

A temporary fixup file is required for device tree support on most devices.This .fixup file by default resides in the board directory and is nameddts.fixup. This fixup file maps the generated include information to thecurrent driver/source usage.

Device Tree vs Kconfig¶

As mentioned above there are several tools used to configure the build withZephyr.The two main ones, Device Tree and Kconfig, might seem to overlap in purpose,but in fact they do not. This section serves as a reference to help you decidewhether to place configuration items in Device Tree or Kconfig.

The scope of each configuration tool can be summarized as follows:

  • Device Tree is used to describe the hardware and its boot-timeconfiguration.
  • Kconfig is used to describe which software features will be built intothe final image, and their configuration.

Hence Device Tree deals mostly with hardware and Kconfig with software.A couple of noteworthy exceptions are:

  • Device Tree’s chosen keyword, which allows the user to select aparticular instance of a hardware device to be used for a concrete purposeby the software. An example of this is selecting a particular UART for use asthe system’s console.
  • Device Tree’s status keyword, which allows the user to enable or disablea particular instance of a hardware device in the Device Tree file itself.This takes precedence over Kconfig.

To further clarify this separation, let’s use a particular, well-knownexample to illustrate this: serial ports a.k.a. UARTs. Let’s consider aboard containing a SoC with 2 UART instances:

  • The fact that the target hardware contains 2 UARTs is described with DeviceTree. This includes the UART type, its driver compatibility and certainimmutable (i.e. not software-configurable) settings such as the base addressof the hardware peripheral in memory or its interrupt line.
  • Additionally, hardware boot-time configuration is also described with DeviceTree. This includes things such as the IRQ priority and boot-time UARTbaudrate. These may also be modifiable at runtime later, but their boot-timedefault configuration is described in Device Tree.
  • The fact that the user intends to include software support for UART in thebuild is described in Kconfig. Through the use of Kconfig, users can opt notto include support for this particular hardware item if they don’t require it.

Another example is a device with a 2.4GHz, multi-protocol radio supportingboth the Bluetooth Low Energy and 802.15.4 wireless technologies. In this case:

  • Device Tree describes the presence of a radio peripheral compatible with acertain radio driver.
  • Additional hardware boot-time configuration settings may also be presentin the Device Tree files. In this particular case it could be adefault TX power in dBm if the hardware does have a simple, cross-wirelesstechnology register for that.
  • Kconfig will describe which protocol stack is to be used with that radio.The user may decide to select BLE or 802.15.4, which will both depend onthe presence of a compatible radio in the Device Tree files.

Device tree file formats¶

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Hardware and software is described inside of device tree files in clear text format.These files have the file suffix of .dtsi or .dts. The .dtsi files are meant tobe included by other files. Typically for a given board you have some number of.dtsi include files that pull in common device descriptions that are used acrossa given SoC family.

Example: FRDM K64F Board and Hexiwear K64¶

Both of these boards are based on the same NXP Kinetis SoC family, the K6X. Thefollowing shows the include hierarchy for both boards.

boards/arm/frdm_k64f/frdm_k64f.dts includes the following files:

boards/arm/hexiwear_k64/hexiwear_k64.dts includes the same files:

The board-specific .dts files enable nodes, define the Zephyr-specific items,and also adds board-specific changes like gpio/pinmux assignments. These typesof things will vary based on the board layout and application use.

Currently supported boards¶

Device tree is currently supported on all ARM targets. Support for all otherarchitectures is to be completed by release 1.11.

Adding support for a board¶

Adding device tree support for a given board requires adding a number of files.These files will contain the DTS information that describes a platform, theYAML descriptions that define the contents of a given Device Tree peripheralnode, and also any fixup files required to support the platform.

When writing Device Tree Source files, it is good to separate out commonperipheral information that could be used across multiple SoC families orboards. DTS files are identified by their file suffix. A .dtsi suffix denotesa DTS file that is used as an include in another DTS file. A .dts suffixdenotes the primary DTS file for a given board.

The primary DTS file will contain at a minimum a version line, optionalincludes, and the root node definition. The root node will contain a model andcompatible that denotes the unique board described by the .dts file.

Device Tree Source File Template¶

One suggestion for starting from scratch on a platform/board is to examine otherboards and their device tree source files.

The following is a more precise list of required files:

  • Base architecture support
    • Add architecture-specific DTS directory, if not already present.Example: dts/arm for ARM.
    • Add target specific device tree files for base SoC. These should be.dtsi files to be included in the board-specific device tree files.
    • Add target specific YAML files in the dts/bindings/ directory.Create the yaml directory if not present.
  • SoC family support
    • Add one or more SoC family .dtsi files that describe the hardwarefor a set of devices. The file should contain all the relevantnodes and base configuration that would be applicable to all boardsutilizing that SoC family.
    • Add SoC family YAML files that describe the nodes present in the .dtsi file.
  • Board specific support
    • Add a board level .dts file that includes the SoC family .dtsi filesand enables the nodes required for that specific board.
    • Board .dts file should specify the SRAM and FLASH devices, if present.
      • Flash device node might specify flash partitions. For more details seeDevice Tree Flash Partitions
    • Add board-specific YAML files, if required. This would occur if theboard has additional hardware that is not covered by the SoC family.dtsi/.yaml files.
  • Fixup files
    • Fixup files contain mappings from existing Kconfig options to the actualunderlying DTS derived configuration #defines. Fixup files are temporaryartifacts until additional DTS changes are made to make them unnecessary.
  • Overlay Files (optional)
    • Overlay files contain tweaks or changes to the SoC and Board support filesdescribed above. They can be used to modify Device Tree configurationswithout having to change the SoC and Board files. SeeDevice Tree Overlays for more information on overlay files and the Zephyrbuild system.

Adding support for device tree in drivers¶

As drivers and other source code is converted over to make use of device treegenerated information, these drivers may require changes to match the generated#define information.

Source Tree Hierarchy¶

The device tree files are located in a couple of different directories. Thedirectory split is done based on architecture, and there is also a commondirectory where architecture agnostic device tree and yaml files are located.

Assuming the current working directory is the ZEPHYR_BASE, the directoryhierarchy looks like the following:

The common directories contain a skeleton.dtsi include file that defines theaddress and size cells. The yaml subdirectory contains common yaml files forZephyr-specific nodes/properties and generic device properties common acrossarchitectures.

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Example: Subset of DTS/YAML files for NXP FRDM K64F (Subject to Change):

YAML definitions for device nodes¶

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Device tree can describe hardware and configuration, but it doesn’t tell thesystem which pieces of information are useful, or how to generate configurationdata from the device tree nodes. For this, we rely on YAML files to describethe contents or definition of a device tree node.

A YAML description must be provided for every device node that is to be a sourceof information for the system. This YAML description can be used for more thanone purpose. It can be used in conjunction with the device tree to generateinclude information. It can also be used to validate the device tree filesthemselves. A device tree file can successfully compile and still not containthe necessary pieces required to build the rest of the software. YAML providesa means to detect that issue.

YAML files reside in a subdirectory inside the common and architecture-specificdevice tree directories. A YAML template file is provided to show the requiredformat. This file is located at:

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YAML files must end in a .yaml suffix. YAML files are scanned during theinformation extraction phase and are matched to device tree nodes via thecompatible property.