Device Tree

Introduction

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.
      • Flash device node might specify flash partitions. For more details see Flash Partitions
    • 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.

aliases and chosen nodes

Using an alias with a common name for a particular node makes it easier for you to write board-independent source code. Device Tree aliases nodes are used for this purpose, by mapping certain generic, commonly used names to specific hardware resources:

aliases {
   led0 = &led0;
   sw0 = &button0;
   sw1 = &button1;
   uart-0 = &uart0;
   uart-1 = &uart1;
};

Certain software subsystems require a specific hardware resource to bind to in order to function properly. Some of those subsystems are used with many different boards, which makes using the Device Tree chosen nodes very convenient. By doing, so the software subsystem can rely on having the specific hardware peripheral assigned to it. In the following example we bind the shell to uart1 in this board:

chosen {
   zephyr,shell-uart = &uart1;
};

The full set of Zephyr-specific chosen nodes follows:

chosen node name Generated symbol
zephyr,flash CONFIG_FLASH
zephyr,sram CONFIG_SRAM
zephyr,ccm DT_CCM
zephyr,console DT_UART_CONSOLE_ON_DEV_NAME
zephyr,shell-uart DT_UART_SHELL_ON_DEV_NAME
zephyr,bt-uart DT_BT_UART_ON_DEV_NAME
zephyr,uart-pipe DT_UART_PIPE_ON_DEV_NAME
zephyr,bt-mon-uart DT_BT_MONITOR_ON_DEV_NAME
zephyr,uart-mcumgr DT_UART_MCUMGR_ON_DEV_NAME

As chosen properties tend to be related to software configuration, it can be useful for the build system to know if a chosen property was defined. We generate a define for each chosen property, for example:

zephyr,flash will generate: #define DT_CHOSEN_ZEPHYR_FLASH 1

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.

orphan:

Flash Partitions

Device tree can be used to describe a partition layout for any flash device in the system.

Two important uses for this mechanism are:

  1. To force the Zephyr image to be linked into a specific area on Flash.

    This is useful, for example, if the Zephyr image must be linked at some offset from the flash device’s start, to be loaded by a bootloader at runtime.

  2. To generate compile-time definitions for the partition layout, which can be shared by Zephyr subsystems and applications to operate on specific areas in flash.

    This is useful, for example, to create areas for storing file systems or other persistent state. These defines only describe the boundaries of each partition. They don’t, for example, initialize a partition’s flash contents with a file system.

Partitions are generally managed using device tree overlays. Refer to Device Tree Overlays for details on using overlay files.

Defining Partitions

The partition layout for a flash device is described inside the partitions child node of the flash device’s node in the device tree.

You can define partitions for any flash device on the system.

Most Zephyr-supported SoCs with flash support in device tree will define a label flash0. This label refers to the primary on-die flash programmed to run Zephyr. To generate partitions for this device, add the following snippet to a device tree overlay file:

&flash0 {
        partitions {
                compatible = "fixed-partitions";
                #address-cells = <1>;
                #size-cells = <1>;

                /* Define your partitions here; see below */
        };
};

To define partitions for another flash device, modify the above to either use its label or provide a complete path to the flash device node in the device tree.

The content of the partitions node looks like this:

partitions {
        compatible = "fixed-partitions";
        #address-cells = <1>;
        #size-cells = <1>;

        partition1_label: partition@START_OFFSET_1 {
                label = "partition1_name";
                reg = <0xSTART_OFFSET_1 0xSIZE_1>;
        };

        /* ... */

        partitionN_label: partition@START_OFFSET_N {
                label = "partitionN_name";
                reg = <0xSTART_OFFSET_N 0xSIZE_N>;
        };
};

Where:

  • partitionX_label are device tree labels that can be used elsewhere in the device tree to refer to the partition
  • partitionX_name controls how defines generated by the Zephyr build system for this partition will be named
  • START_OFFSET_x is the start offset in hexadecimal notation of the partition from the beginning of the flash device
  • SIZE_x is the hexadecimal size, in bytes, of the flash partition

The partitions do not have to cover the entire flash device. The device tree compiler currently does not check if partitions overlap; you must ensure they do not when defining them.

Example Primary Flash Partition Layout

Here is a complete (but hypothetical) example device tree overlay snippet illustrating these ideas. Notice how the partitions do not overlap, but also do not cover the entire device.

&flash0 {
        partitions {
                compatible = "fixed-partitions";
                #address-cells = <1>;
                #size-cells = <1>;

                code_dts_label: partition@8000 {
                        label = "zephyr-code";
                        reg = <0x00008000 0x34000>;
                };

                data_dts_label: partition@70000 {
                        label = "application-data";
                        reg = <0x00070000 0xD000>;
                };
        };
};

Linking Zephyr Within a Partition

To force the linker to output a Zephyr image within a given flash partition, add this to a device tree overlay:

/ {
        chosen {
                zephyr,code-partition = &slot0_partition;
        };
};

If the chosen node has no zephyr,code-partition property, the application image link uses the entire flash device. If a zephyr,code-partition property is defined, the application link will be restricted to that partition.

Flash Partition Macros

The Zephyr build system generates definitions for each flash device partition. These definitions are available to any files which include <zephyr.h>.

Consider this flash partition:

dts_label: partition@START_OFFSET {
        label = "def-name";
        reg = <0xSTART_OFFSET 0xSIZE>;
};

The build system will generate the following corresponding defines:

#define FLASH_AREA_DEF_NAME_LABEL        "def-name"
#define FLASH_AREA_DEF_NAME_OFFSET_0     0xSTART_OFFSET
#define FLASH_AREA_DEF_NAME_SIZE_0       0xSIZE
#define FLASH_AREA_DEF_NAME_OFFSET       FLASH_AREA_MCUBOOT_OFFSET_0
#define FLASH_AREA_DEF_NAME_SIZE         FLASH_AREA_MCUBOOT_SIZE_0

As you can see, the label property is capitalized when forming the macro names. Other simple conversions to ensure it is a valid C identifier, such as converting “-” to “_”, are also performed. The offsets and sizes are available as well.

MCUboot Partitions

MCUboot is a secure bootloader for 32-bit microcontrollers.

Some Zephyr boards provide definitions for the flash partitions which are required to build MCUboot itself, as well as any applications which must be chain-loaded by MCUboot.

The device tree labels for these partitions are:

boot_partition
This is the partition where the bootloader is expected to be placed. MCUboot’s build system will attempt to link the MCUboot image into this partition.
slot0_partition
MCUboot loads the executable application image from this partition. Any application bootable by MCUboot must be linked to run from this partition.
slot1_partition
This is the partition which stores firmware upgrade images. Zephyr applications which receive firmware updates must ensure the upgrade images are placed in this partition (the Zephyr DFU subsystem can be used for this purpose). MCUboot checks for upgrade images in this partition, and can move them to slot0_partition for execution. The slot0_partition and slot1_partition must be the same size.
scratch_partition
This partition is used as temporary storage while swapping the contents of slot0_partition and slot1_partition.

Important

Upgrade images are only temporarily stored in slot1_partition. They must be linked to execute of out of slot0_partition.

See the MCUboot documentation for more details on these partitions.

File System Partitions

storage_partition
This is the area where e.g. NFFS expects its partition.