.. _device-tree:

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 `EPAPR document`_.

.. _EPAPR document: http://www.devicetree.org/specifications-pdf

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

.. _device tree repository: https://git.kernel.org/pub/scm/utils/dtc/dtc.git


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:

* :ref:`installing_zephyr_win`
* :ref:`installation_linux`
* :ref:`installing_zephyr_mac`


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.

Today, configuration in Zephyr comes from a number of different places.  It can
come from Kconfig files, CMSIS header files, vendor header files, prj.conf
files, and other miscellaneous sources.  The intent of using device tree is to
replace or curtail the use of Kconfig files throughout the system, and instead
use device tree files to describe the configuration of device nodes.  CMSIS and
vendor header files can be used in conjunction with the device tree to fully
describe hardware.  Device tree is not intended to replace CMSIS or vendor
include files.

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 resides in the dts architecture directory and has the same
name as the master .dts file.  The only difference is the suffix is .fixup.
This fixup file maps the generated include information to the current
driver/source usage.

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.

dts/arm/frdm_k64.dts includes the following two files::

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

dts/arm/hexiwear_k64.dts includes the same two 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 1.9.

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
================================

.. code-block:: yaml

  /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 to dts/<ARCH>/Makefile or create Makefile if not present
  * 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/<ARCH>/yaml 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.

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/common/yaml
  dts/<ARCH>/
  dts/<ARCH>/yaml

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: DTS/YAML files for NXP FRDM K64F::

  dts/arm/armv7-m.dtsi
  dts/arm/k6x/nxp_k6x.dtsi
  dts/arm/frdm_k64f.dts
  dts/arm/yaml/arm,v7m-nvic.yaml
  dts/arm/yaml/k64gpio.yaml
  dts/arm/yaml/k64pinmux.yaml
  dts/arm/yaml/k64uart.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/common/yaml/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.