Devicetree versus Kconfig

Along with devicetree, Zephyr also uses the Kconfig language to configure the source code. Whether to use devicetree or Kconfig for a particular purpose can sometimes be confusing. This section should help you decide which one to use.

In short:

  • Use devicetree to describe hardware and its boot-time configuration. Examples include peripherals on a board, boot-time clock frequencies, interrupt lines, etc.

  • Use Kconfig to configure software support to build into the final image. Examples include whether to add networking support, which drivers are needed by the application, etc.

In other words, devicetree mainly deals with hardware, and Kconfig with software.

For example, consider a board containing a SoC with 2 UART, or serial port, instances.

  • The fact that the board has this UART hardware is described with two UART nodes in the devicetree. These provide the UART type (via the compatible property) and certain settings such as the address range of the hardware peripheral registers in memory (via the reg property).

  • Additionally, the UART boot-time configuration is also described with devicetree. This could include configuration such as the RX IRQ line’s priority and the UART baud rate. These may be modifiable at runtime, but their boot-time configuration is described in devicetree.

  • Whether or not to include software support for UART in the build is controlled via Kconfig. Applications which do not need to use the UARTs can remove the driver source code from the build using Kconfig, even though the board’s devicetree still includes UART nodes.

As another example, consider a device with a 2.4GHz, multi-protocol radio supporting both the Bluetooth Low Energy and 802.15.4 wireless technologies.

  • Devicetree should be used to describe the presence of the radio hardware, what driver or drivers it’s compatible with, etc.

  • Boot-time configuration for the radio, such as TX power in dBm, should also be specified using devicetree.

  • Kconfig should determine which software features should be built for the radio, such as selecting a BLE or 802.15.4 protocol stack.

As another example, Kconfig options that formerly enabled a particular instance of a driver (that is itself enabled by Kconfig) have been removed. The devices are selected individually using devicetree’s status keyword on the corresponding hardware instance.

There are exceptions to these rules:

  • Because Kconfig is unable to flexibly control some instance-specific driver configuration parameters, such as the size of an internal buffer, these options may be defined in devicetree. However, to make clear that they are specific to Zephyr drivers and not hardware description or configuration these properties should be prefixed with zephyr,, e.g. zephyr,random-mac-address in the common Ethernet devicetree properties.

  • Devicetree’s chosen keyword, which allows the user to select a specific instance of a hardware device to be used for a particular purpose. An example of this is selecting a particular UART for use as the system’s console.