Documentation/devicetree/bindings/writing-bindings.rst

.. 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 treat device node names as a stable ABI, but instead use phandles or
  compatibles to find sibling devices. Exception: sub-nodes of given device
  could be treated as ABI, if explicitly documented in the bindings.

- 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).

- DON'T use 'simple-mfd' compatible for non-trivial devices, where children
  depend on some resources from the parent. Similarly, 'simple-bus' should not
  be used for complex buses and even 'regs' property means device is not
  a simple bus.


Properties
==========

- DO make 'compatible' properties specific.

   - DON'T use wildcards or device-family names in compatible strings.

   - DO use fallback compatibles when devices are the same as or a superset of
     prior implementations. Fallback compatibles are applicable especially
     when sharing a programming interface or when able to discover the
     variants.

   - DON'T add fake fallback compatibles when software cannot use such to match
     and bind to a device, and still operate correctly.

   - DO use the commit message to explain why devices that may appear
     compatible in a diff (e.g. no differences in property use, same handling
     by the software) but are not made compatible in the binding, are not
     compatible.

   - DO add new compatibles in case there are new features or bugs.

   - DO use a SoC-specific compatible for all SoC devices, followed by a
     fallback if appropriate. SoC-specific compatibles are also preferred for
     the fallbacks.

   - DON'T use bus suffixes to encode the type of interface device is using.
     The parent bus node already implies that interface.  DON'T add the type of
     device, if the device cannot be anything else.

- 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.

- DON'T add properties to avoid a specific compatible. DON'T add properties if
  they are implied by (deducible from) the compatible.

- 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? All these constraints represent the ABI
  as well.

- DON'T make changes that break the ABI without explicit and detailed rationale
  for why the changes have to be made and their impact. ABI impact goes beyond
  the Linux kernel, because it also covers other open-source upstream projects.


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".

- Do not add instance index (IDs) properties or custom OF aliases.  If the
  devices have different programming model, they might need different
  compatibles.  If such devices use some other device in a different way, e.g.
  they program the phy differently, use cell/phandle arguments.

- Bindings files should be named like compatible: vendor,device.yaml. In case
  of multiple compatibles in the binding, use one of the fallbacks or a more
  generic name, yet still matching compatible style.

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.