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+# 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/)