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diff --git a/Documentation/devicetree/bindings/writing-schema.rst b/Documentation/devicetree/bindings/writing-schema.rst new file mode 100644 index 000000000..4a381d20f --- /dev/null +++ b/Documentation/devicetree/bindings/writing-schema.rst @@ -0,0 +1,186 @@ +.. SPDX-License-Identifier: GPL-2.0 + +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 +is considered more human readable and has some advantages such as allowing +comments (Prefixed with '#'). + +Also see :ref:`example-schema`. + +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 value + with a leading '/' will have the hostname prepended. A $ref value with only a + relative path or filename will be prepended with the hostname and path + components of the current schema file's '$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 are 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. + +Coding style +------------ + +Use YAML coding style (two-space indentation). For DTS examples in the schema, +preferred is four-space indentation. + +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 dtschema + +Note that 'dtschema' installation requires 'swig' and Python development files +installed first. On Debian/Ubuntu systems:: + + apt install swig python3-dev + +Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be +installed. Ensure they are in your PATH (~/.local/bin by default). + +Recommended is also to install yamllint (used by dtschema when present). + +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 + +Note that ``dtbs_check`` will skip any binding schema files with errors. It is +necessary to use ``dt_binding_check`` to get all the validation errors in the +binding schema files. + +It is possible to run both in a single command:: + + make dt_binding_check dtbs_check + +It is also possible to run checks with a subset of matching schema files by +setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern. + +:: + + make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml + make dt_binding_check DT_SCHEMA_FILES=/gpio/ + make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml + + +json-schema Resources +--------------------- + + +`JSON-Schema Specifications <http://json-schema.org/>`_ + +`Using JSON Schema Book <http://usingjsonschema.com/>`_ + +.. _example-schema: + +Annotated Example Schema +------------------------ + +Also available as a separate file: :download:`example-schema.yaml` + +.. literalinclude:: example-schema.yaml |