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Design goals

Zephyr’s use of devicetree has evolved significantly over time, and further changes are expected. The following are the general design goals, along with specific examples about how they impact Zephyr’s source code, and areas where more work remains to be done.

Single source for all hardware information

Zephyr shall obtain its hardware descriptions exclusively from devicetree.

Examples

  • New device drivers shall use devicetree APIs to determine which devices to create if possible.

  • In-tree sample applications shall use aliases to determine which of multiple possible generic devices of a given type will be used in the current build. For example, the Blinky uses this to determine the LED to blink.

Example remaining work

  • Zephyr’s Sanity Tests currently use board.yaml files to determine the hardware supported by a board. This should be obtained from devicetree instead.

  • Various device drivers currently use Kconfig to determine which instances of a particular compatible are enabled. This can and should be done with devicetree overlays instead.

  • Board-level documentation still contains tables of hardware support which are generated and maintained by hand. This can and should be obtained from the board level devicetree instead.

  • Runtime determination of struct device relationships should be done using information obtained from devicetree, e.g. for device power management.

  • Pin muxing and pin control, at least at boot time, should be accomplished via devicetree.

Source compatibility with other operating systems

Zephyr’s devicetree tooling is based on a generic layer which is interoperable with other devicetree users, such as the Linux kernel.

Zephyr’s binding language semantics can support Zephyr-specific attributes, but shall not express Zephyr-specific relationships.

Examples

  • Zephyr’s devicetree source parser, dtlib.py, is source-compatible with other tools like dtc in both directions: dtlib.py can parse dtc output, and dtc can parse dtlib.py output.

  • Zephyr’s “extended dtlib” library, edtlib.py, shall not include Zephyr-specific features. Its purpose is to provide a higher-level view of the devicetree for common elements like interrupts and buses.

    Only the high-level gen_defines.py script, which is built on top of edtlib.py, contains Zephyr-specific knowledge and features.

Example remaining work

  • Zephyr has a custom Devicetree bindings language syntax. While Linux’s dtschema does not yet meet Zephyr’s needs, we should try to follow what it is capable of representing in Zephyr’s own bindings.

  • Due to inflexibility in the bindings language, Zephyr cannot support the full set of bindings supported by Linux.

  • Devicetree source sharing between Zephyr and Linux is not done.

To be determined

The scope and amount of code generation for device instantiation has been the subject of extended discussion and prototyping without reaching a firm conclusion.