xref: /freebsd-src/sys/contrib/device-tree/Bindings/writing-bindings.rst (revision b97ee269eae3cbaf35c18f51a459aea581c2a7dc)

.. SPDX-License-Identifier: GPL-2.0

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DOs and DON'Ts for designing and writing Devicetree bindings
============================================================

This is a list of common review feedback items focused on binding design. With
every rule, there are exceptions and bindings have many gray areas.

For guidelines related to patches, see
Documentation/devicetree/bindings/submitting-patches.rst


Overall design
==============

- DO attempt to make bindings complete even if a driver doesn't support some
  features. For example, if a device has an interrupt, then include the
  'interrupts' property even if the driver is only polled mode.

- DON'T refer to Linux or "device driver" in bindings. Bindings should be
  based on what the hardware has, not what an OS and driver currently support.

- DO use node names matching the class of the device. Many standard names are
  defined in the DT Spec. If there isn't one, consider adding it.

- DO check that the example matches the documentation especially after making
  review changes.

- DON'T create nodes just for the sake of instantiating drivers. Multi-function
  devices only need child nodes when the child nodes have their own DT
  resources. A single node can be multiple providers (e.g. clocks and resets).

- DON'T use 'syscon' alone without a specific compatible string. A 'syscon'
  hardware block should have a compatible string unique enough to infer the
  register layout of the entire block (at a minimum).


Properties
==========

- DO make 'compatible' properties specific. DON'T use wildcards in compatible
  strings. DO use fallback compatibles when devices are the same as or a subset
  of prior implementations. DO add new compatibles in case there are new
  features or bugs.

- DO use a vendor prefix on device-specific property names. Consider if
  properties could be common among devices of the same class. Check other
  existing bindings for similar devices.

- DON'T redefine common properties. Just reference the definition and define
  constraints specific to the device.

- DO use common property unit suffixes for properties with scientific units.
  Recommended suffixes are listed at
  https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/property-units.yaml

- DO define properties in terms of constraints. How many entries? What are
  possible values? What is the order?

Typical cases and caveats
=========================

- Phandle entries, like clocks/dmas/interrupts/resets, should always be
  explicitly ordered. Include the {clock,dma,interrupt,reset}-names if there is
  more than one phandle. When used, both of these fields need the same
  constraints (e.g.  list of items).

- For names used in {clock,dma,interrupt,reset}-names, do not add any suffix,
  e.g.: "tx" instead of "txirq" (for interrupt).

- Properties without schema types (e.g. without standard suffix or not defined
  by schema) need the type, even if this is an enum.

- If schema includes other schema (e.g. /schemas/i2c/i2c-controller.yaml) use
  "unevaluatedProperties:false". In other cases, usually use
  "additionalProperties:false".

- For sub-blocks/components of bigger device (e.g. SoC blocks) use rather
  device-based compatible (e.g. SoC-based compatible), instead of custom
  versioning of that component.
  For example use "vendor,soc1234-i2c" instead of "vendor,i2c-v2".

- "syscon" is not a generic property. Use vendor and type, e.g.
  "vendor,power-manager-syscon".

Board/SoC .dts Files
====================

- DO put all MMIO devices under a bus node and not at the top-level.

- DO use non-empty 'ranges' to limit the size of child buses/devices. 64-bit
  platforms don't need all devices to have 64-bit address and size.