Rules for upstream bindings

This section includes general rules for writing bindings that you want to submit to the upstream Zephyr Project. (You don’t need to follow these rules for bindings you don’t intend to contribute to the Zephyr Project, but it’s a good idea.)

Decisions made by the Zephyr devicetree maintainer override the contents of this section. If that happens, though, please let them know so they can update this page, or you can send a patch yourself.

Always check for existing bindings 

Zephyr aims for devicetree Source compatibility with other operating systems. Therefore, if there is an existing binding for your device in an authoritative location, you should try to replicate its properties when writing a Zephyr binding, and you must justify any Zephyr-specific divergences.

In particular, this rule applies if:

There is an existing binding in the mainline Linux kernel. See Documentation/devicetree/bindings in Linus’s tree for existing bindings and the Linux devicetree documentation for more information.
Your hardware vendor provides an official binding outside of the Linux kernel.

General rules 

Wherever possible, when writing Devicetree bindings for Zephyr, try to follow the same design guidelines laid out by Linux.

File names 

Bindings which match a compatible must have file names based on the compatible.

For example, a binding for compatible vnd,foo must be named vnd,foo.yaml.
If the binding is bus-specific, you can append the bus to the file name; for example, if the binding YAML has on-bus: bar, you may name the file vnd,foo-bar.yaml.

Recommendations are requirements 

All recommendations in default are requirements when submitting the binding.

In particular, if you use the default: feature, you must justify the value in the property’s description.

Descriptions 

There are only two acceptable ways to write property description: strings.

If your description is short, it’s fine to use this style:

description: my short string

If your description is long or spans multiple lines, you must use this style:

description: |
  My very long string
  goes here.
  Look at all these lines!

This | style prevents YAML parsers from removing the newlines in multi-line descriptions. This in turn makes these long strings display properly in the Bindings index.

Naming conventions 

Do not use uppercase letters (A through Z) or underscores (_) in property names. Use lowercase letters (a through z) instead of uppercase. Use dashes (-) instead of underscores. (The one exception to this rule is if you are replicating a well-established binding from somewhere like Linux.)

Rules for vendor prefixes 

The following general rules apply to vendor prefixes in compatible properties.

If your device is manufactured by a specific vendor, then its compatible should have a vendor prefix.

If your binding describes hardware with a well known vendor from the list in dts/bindings/vendor-prefixes.txt, you must use that vendor prefix.
If your device is not manufactured by a specific hardware vendor, do not invent a vendor prefix. Vendor prefixes are not mandatory parts of compatible properties, and compatibles should not include them unless they refer to an actual vendor. There are some exceptions to this rule, but the practice is strongly discouraged.
Do not submit additions to Zephyr’s dts/bindings/vendor-prefixes.txt file unless you also include users of the new prefix. This means at least a binding and a devicetree using the vendor prefix, and should ideally include a device driver handling that compatible.

For custom bindings, you can add a custom dts/bindings/vendor-prefixes.txt file to any directory in your DTS_ROOT. The devicetree tooling will respect these prefixes, and will not generate warnings or errors if you use them in your own bindings or devicetrees.
We sometimes synchronize Zephyr’s vendor-prefixes.txt file with the Linux kernel’s equivalent file; this process is exempt from the previous rule.
If your binding is describing an abstract class of hardware with Zephyr specific drivers handling the nodes, it’s usually best to use zephyr as the vendor prefix. See Zephyr-specific binding (zephyr) for examples.

Rules for default values 

In any case where default: is used in a devicetree binding, the description: for that property must explain why the value was selected and any conditions that would make it necessary to provide a different value. Additionally, if changing one property would require changing another to create a consistent configuration, then those properties should be made required.

There is no need to document the default value itself; this is already present in the Bindings index output.

There is a risk in using default: when the value in the binding may be incorrect for a particular board or hardware configuration. For example, defaulting the capacity of the connected power cell in a charging IC binding is likely to be incorrect. For such properties it’s better to make the property required: true, forcing the user to make an explicit choice.

Driver developers should use their best judgment as to whether a value can be safely defaulted. Candidates for default values include:

delays that would be different only under unusual conditions (such as intervening hardware)
configuration for devices that have a standard initial configuration (such as a USB audio headset)
defaults which match the vendor-specified power-on reset value (as long as they are independent from other properties)

Examples of how to write descriptions according to these rules:

properties:
  cs-interval:
    type: int
    default: 0
    description: |
      Minimum interval between chip select deassertion and assertion.
      The default corresponds to the reset value of the register field.
  hold-time-ms:
    type: int
    default: 20
    description: |
      Amount of time to hold the power enable GPIO asserted before
      initiating communication. The default was recommended in the
      manufacturer datasheet, and would only change under very
      cold temperatures.

Some examples of what not to do, and why:

properties:
  # Description doesn't mention anything about the default
  foo:
    type: int
    default: 1
    description: number of foos

  # Description mentions the default value instead of why it
  # was chosen
  bar:
    type: int
    default: 2
    description: bar size; default is 2

  # Explanation of the default value is in a comment instead
  # of the description. This won't be shown in the bindings index.
  baz:
    type: int
    # This is the recommended value chosen by the manufacturer.
    default: 2
    description: baz time in milliseconds

The `zephyr,` prefix 

You must add this prefix to property names in the following cases:

Zephyr-specific extensions to bindings we share with upstream Linux. One example is the zephyr,vref-mv ADC channel property which is common to ADC controllers defined in dts/bindings/adc/adc-controller.yaml. This channel binding is partially shared with an analogous Linux binding, and Zephyr-specific extensions are marked as such with the prefix.
Configuration values that are specific to a Zephyr device driver. One example is the zephyr,lazy-load property in the ti,bq274xx binding. Though devicetree in general is a hardware description and configuration language, it is Zephyr’s only mechanism for configuring driver behavior for an individual struct device. Therefore, as a compromise, we do allow some software configuration in Zephyr’s devicetree bindings, as long as they use this prefix to show that they are Zephyr specific.

You may use the zephyr, prefix when naming a devicetree compatible that is specific to Zephyr. One example is zephyr,ipc-openamp-static-vrings. In this case, it’s permitted but not required to add the zephyr, prefix to properties defined in the binding.