173 lines
6.3 KiB
ReStructuredText
173 lines
6.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Writing DeviceTree Bindings in json-schema
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==========================================
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Devicetree bindings are written using json-schema vocabulary. Schema files are
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written in a JSON compatible subset of YAML. YAML is used instead of JSON as it
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is considered more human readable and has some advantages such as allowing
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comments (Prefixed with '#').
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Schema Contents
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---------------
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Each schema doc is a structured json-schema which is defined by a set of
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top-level properties. Generally, there is one binding defined per file. The
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top-level json-schema properties used are:
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$id
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A json-schema unique identifier string. The string must be a valid
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URI typically containing the binding's filename and path. For DT schema, it must
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begin with "http://devicetree.org/schemas/". The URL is used in constructing
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references to other files specified in schema "$ref" properties. A $ref value
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with a leading '/' will have the hostname prepended. A $ref value a relative
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path or filename only will be prepended with the hostname and path components
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of the current schema file's '$id' value. A URL is used even for local files,
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but there may not actually be files present at those locations.
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$schema
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Indicates the meta-schema the schema file adheres to.
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title
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A one line description on the contents of the binding schema.
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maintainers
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A DT specific property. Contains a list of email address(es)
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for maintainers of this binding.
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description
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Optional. A multi-line text block containing any detailed
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information about this binding. It should contain things such as what the block
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or device does, standards the device conforms to, and links to datasheets for
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more information.
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select
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Optional. A json-schema used to match nodes for applying the
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schema. By default without 'select', nodes are matched against their possible
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compatible string values or node name. Most bindings should not need select.
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allOf
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Optional. A list of other schemas to include. This is used to
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include other schemas the binding conforms to. This may be schemas for a
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particular class of devices such as I2C or SPI controllers.
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properties
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A set of sub-schema defining all the DT properties for the
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binding. The exact schema syntax depends on whether properties are known,
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common properties (e.g. 'interrupts') or are binding/vendor specific properties.
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A property can also define a child DT node with child properties defined
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under it.
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For more details on properties sections, see 'Property Schema' section.
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patternProperties
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Optional. Similar to 'properties', but names are regex.
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required
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A list of DT properties from the 'properties' section that
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must always be present.
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examples
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Optional. A list of one or more DTS hunks implementing the
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binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.
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Unless noted otherwise, all properties are required.
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Property Schema
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---------------
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The 'properties' section of the schema contains all the DT properties for a
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binding. Each property contains a set of constraints using json-schema
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vocabulary for that property. The properties schemas are what is used for
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validation of DT files.
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For common properties, only additional constraints not covered by the common
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binding schema need to be defined such as how many values are valid or what
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possible values are valid.
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Vendor specific properties will typically need more detailed schema. With the
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exception of boolean properties, they should have a reference to a type in
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schemas/types.yaml. A "description" property is always required.
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The Devicetree schemas don't exactly match the YAML encoded DT data produced by
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dtc. They are simplified to make them more compact and avoid a bunch of
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boilerplate. The tools process the schema files to produce the final schema for
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validation. There are currently 2 transformations the tools perform.
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The default for arrays in json-schema is they are variable sized and allow more
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entries than explicitly defined. This can be restricted by defining 'minItems',
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'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
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size is desired in most cases, so these properties are added based on the
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number of entries in an 'items' list.
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The YAML Devicetree format also makes all string values an array and scalar
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values a matrix (in order to define groupings) even when only a single value
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is present. Single entries in schemas are fixed up to match this encoding.
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Testing
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-------
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Dependencies
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~~~~~~~~~~~~
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The DT schema project must be installed in order to validate the DT schema
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binding documents and validate DTS files using the DT schema. The DT schema
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project can be installed with pip::
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pip3 install git+https://github.com/devicetree-org/dt-schema.git@master
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Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
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installed. Ensure they are in your PATH (~/.local/bin by default).
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dtc must also be built with YAML output support enabled. This requires that
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libyaml and its headers be installed on the host system. For some distributions
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that involves installing the development package, such as:
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Debian::
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apt-get install libyaml-dev
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Fedora::
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dnf -y install libyaml-devel
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Running checks
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~~~~~~~~~~~~~~
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The DT schema binding documents must be validated using the meta-schema (the
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schema for the schema) to ensure they are both valid json-schema and valid
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binding schema. All of the DT binding documents can be validated using the
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``dt_binding_check`` target::
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make dt_binding_check
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In order to perform validation of DT source files, use the ``dtbs_check`` target::
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make dtbs_check
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Note that ``dtbs_check`` will skip any binding schema files with errors. It is
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necessary to use ``dt_binding_check`` to get all the validation errors in the
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binding schema files.
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It is possible to run both in a single command::
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make dt_binding_check dtbs_check
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It is also possible to run checks with a single schema file by setting the
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``DT_SCHEMA_FILES`` variable to a specific schema file.
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::
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make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
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make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
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json-schema Resources
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---------------------
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`JSON-Schema Specifications <http://json-schema.org/>`_
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`Using JSON Schema Book <http://usingjsonschema.com/>`_
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