dt-bindings: Add a guide of do's and don't's for writing bindings

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.txt

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.

  See property-units.txt.

- DO define properties in terms of constraints. How many entries? What are

  possible values? What is the order?

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.