# Writing DeviceTree Bindings in json-schema

Devicetree bindings are written using json-schema vocabulary. Schema files are
written in a JSON compatible subset of YAML. YAML is used instead of JSON as it
considered more human readable and has some advantages such as allowing
comments (Prefixed with '#').

## Schema Contents

Each schema doc is a structured json-schema which is defined by a set of
top-level properties. Generally, there is one binding defined per file. The
top-level json-schema properties used are:

- __$id__ - A json-schema unique identifier string. The string must be a valid
URI typically containing the binding's filename and path. For DT schema, it must
begin with "http://devicetree.org/schemas/". The URL is used in constructing
references to other files specified in schema "$ref" properties. A $ref values
with a leading '/' will have the hostname prepended. A $ref value a relative
path or filename only will be prepended with the hostname and path components
of the current schema file's '$id' value. A URL is used even for local files,
but there may not actually be files present at those locations.

- __$schema__ - Indicates the meta-schema the schema file adheres to.

- __title__ - A one line description on the contents of the binding schema.

- __maintainers__ - A DT specific property. Contains a list of email address(es)
for maintainers of this binding.

- __description__ - Optional. A multi-line text block containing any detailed
information about this binding. It should contain things such as what the block
or device does, standards the device conforms to, and links to datasheets for
more information.

- __select__ - Optional. A json-schema used to match nodes for applying the
schema. By default without 'select', nodes are matched against their possible
compatible string values or node name. Most bindings should not need select.

- __allOf__ - Optional. A list of other schemas to include. This is used to
include other schemas the binding conforms to. This may be schemas for a
particular class of devices such as I2C or SPI controllers.

- __properties__ - A set of sub-schema defining all the DT properties for the
binding. The exact schema syntax depends on whether properties are known,
common properties (e.g. 'interrupts') or are binding/vendor specific properties.

  A property can also define a child DT node with child properties defined
under it.

  For more details on properties sections, see 'Property Schema' section.

- __patternProperties__ - Optional. Similar to 'properties', but names are regex.

- __required__ - A list of DT properties from the 'properties' section that
must always be present.

- __examples__ - Optional. A list of one or more DTS hunks implementing the
binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.

Unless noted otherwise, all properties are required.

## Property Schema

The 'properties' section of the schema contains all the DT properties for a
binding. Each property contains a set of constraints using json-schema
vocabulary for that property. The properties schemas are what is used for
validation of DT files.

For common properties, only additional constraints not covered by the common
binding schema need to be defined such as how many values are valid or what
possible values are valid.

Vendor specific properties will typically need more detailed schema. With the
exception of boolean properties, they should have a reference to a type in
schemas/types.yaml. A "description" property is always required.

The Devicetree schemas don't exactly match the YAML encoded DT data produced by
dtc. They are simplified to make them more compact and avoid a bunch of
boilerplate. The tools process the schema files to produce the final schema for
validation. There are currently 2 transformations the tools perform.

The default for arrays in json-schema is they are variable sized and allow more
entries than explicitly defined. This can be restricted by defining 'minItems',
'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
size is desired in most cases, so these properties are added based on the
number of entries in an 'items' list.

The YAML Devicetree format also makes all string values an array and scalar
values a matrix (in order to define groupings) even when only a single value
is present. Single entries in schemas are fixed up to match this encoding.

## Testing

### Dependencies

The DT schema project must be installed in order to validate the DT schema
binding documents and validate DTS files using the DT schema. The DT schema
project can be installed with pip:

`pip3 install git+https://github.com/robherring/yaml-bindings.git@master`

dtc must also be built with YAML output support enabled. This requires that
libyaml and its headers be installed on the host system.

### Running checks

The DT schema binding documents must be validated using the meta-schema (the
schema for the schema) to ensure they are both valid json-schema and valid
binding schema. All of the DT binding documents can be validated using the
`dt_binding_check` target:

`make dt_binding_check`

In order to perform validation of DT source files, use the `dtbs_check` target:

`make dtbs_check`

This will first run the `dt_binding_check` which generates the processed schema.

It is also possible to run checks with a single schema file by setting the
'DT_SCHEMA_FILES' variable to a specific schema file.

`make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml`


## json-schema Resources

[JSON-Schema Specifications](http://json-schema.org/)

[Using JSON Schema Book](http://usingjsonschema.com/)