Device Tree in Zephyr

Introduction to Device Tree

Device tree is a way of describing hardware and configuration information for boards. Device tree was adopted for use in the Linux kernel for the PowerPC architecture. However, it is now in use for ARM and other architectures.

The device tree is a data structure for dynamically describing hardware using a Device Tree Source (DTS) data structure language, and compiled into a compact Device Tree Blob (DTB) using a Device Tree Compiler (DTC). Rather than hard coding every detail of a board’s hardware into the operating system, the hardware-describing DTB is passed to the operating system at boot time. This allows the same compiled Linux kernel to support different hardware configurations within an architecture family (e.g., ARM, x86, PowerPC) and moves a significant part of the hardware description out of the 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. In Zephyr, the DTS information will be used only during compile time. Information about the system is extracted from the compiled DTS and used to create 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 Python YAML packages. Refer to the installation guide for your specific host OS:

Zephyr and Device Tree

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

Device Tree provides a unified description of a hardware system used in an application. It is used in Zephyr to describe hardware and provide a boot-time configuration of this hardware.

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

After compilation, a python script extracts information from the compiled device tree file using a set of rules specified in YAML files. The extracted information is placed in a header file that is used by the rest of the code as the 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 named dts_fixup.h. This fixup file maps the generated include information to the current driver/source usage.

Device Tree vs Kconfig

As mentioned above there are several tools used to configure the build with Zephyr. 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 decide whether 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-time configuration.
  • Kconfig is used to describe which software features will be built into the 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 a particular instance of a hardware device to be used for a concrete purpose by the software. An example of this is selecting a particular UART for use as the system’s console.
  • Device Tree’s status keyword, which allows the user to enable or disable a 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-known example to illustrate this: serial ports a.k.a. UARTs. Let’s consider a board containing a SoC with 2 UART instances:

  • The fact that the target hardware contains 2 UARTs is described with Device Tree. This includes the UART type, its driver compatibility and certain immutable (i.e. not software-configurable) settings such as the base address of the hardware peripheral in memory or its interrupt line.
  • Additionally, hardware boot-time configuration is also described with Device Tree. This includes things such as the IRQ priority and boot-time UART baudrate. These may also be modifiable at runtime later, but their boot-time default configuration is described in Device Tree.
  • The fact that the user intends to include software support for UART in the build is described in Kconfig. Through the use of Kconfig, users can opt not to 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 supporting both the Bluetooth Low Energy and 802.15.4 wireless technologies. In this case:

  • Device Tree describes the presence of a radio peripheral compatible with a certain radio driver.
  • Additional hardware boot-time configuration settings may also be present in the Device Tree files. In this particular case it could be a default TX power in dBm if the hardware does have a simple, cross-wireless technology 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 on the presence of a compatible radio in the Device Tree files.

Device tree file formats

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 to be 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 across a 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. The following shows the include hierarchy for both boards.

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

dts/arm/nxp/nxp_k6x.dtsi
dts/arm/armv7-m.dtsi

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

dts/arm/nxp/nxp_k6x.dtsi
dts/arm/armv7-m.dtsi

The board-specific .dts files enable nodes, define the Zephyr-specific items, and also adds board-specific changes like gpio/pinmux assignments. These types of 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 other architectures 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, the YAML descriptions that define the contents of a given Device Tree peripheral node, and also any fixup files required to support the platform.

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

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

Device Tree Source File Template

/dts-v1/
/ {
        model = "Model name for your board";
        compatible = "compatible for your board";
        /* rest of file */
};

One suggestion for starting from scratch on a platform/board is to examine other boards 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 hardware for a set of devices. The file should contain all the relevant nodes and base configuration that would be applicable to all boards utilizing 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 files and enables the nodes required for that specific board.
    • Board .dts file should specify the SRAM and FLASH devices, if present.
    • Add board-specific YAML files, if required. This would occur if the board 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 actual underlying DTS derived configuration #defines. Fixup files are temporary artifacts 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 files described above. They can be used to modify Device Tree configurations without having to change the SoC and Board files. See Device Tree Overlays for more information on overlay files and the Zephyr build system.

Adding support for device tree in drivers

As drivers and other source code is converted over to make use of device tree generated 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. The directory split is done based on architecture, and there is also a common directory where architecture agnostic device tree and yaml files are located.

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

dts/common/
dts/<ARCH>/
dts/bindings/
boards/<ARCH>/<BOARD>/

The common directories contain a skeleton.dtsi include file that defines the address and size cells. The yaml subdirectory contains common yaml files for Zephyr-specific nodes/properties and generic device properties common across architectures.

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

dts/arm/armv7-m.dtsi
dts/arm/k6x/nxp_k6x.dtsi
boards/arm/frdm_k64f/frdm_k64f.dts
dts/bindings/interrupt-controller/arm,v7m-nvic.yaml
dts/bindings/gpio/nxp,kinetis-gpio.yaml
dts/bindings/pinctrl/nxp,kinetis-pinmux.yaml
dts/bindings/serial/nxp,kinetis-uart.yaml

YAML definitions for device nodes

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

A YAML description must be provided for every device node that is to be a source of information for the system. This YAML description can be used for more than one purpose. It can be used in conjunction with the device tree to generate include information. It can also be used to validate the device tree files themselves. A device tree file can successfully compile and still not contain the necessary pieces required to build the rest of the software. YAML provides a means to detect that issue.

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

dts/bindings/device_node.yaml.template

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