1 .. SPDX-License-Identifier: GPL-2.0
3 Writing Devicetree Bindings in json-schema
4 ==========================================
6 Devicetree bindings are written using json-schema vocabulary. Schema files are
7 written in a JSON-compatible subset of YAML. YAML is used instead of JSON as it
8 is considered more human readable and has some advantages such as allowing
9 comments (Prefixed with '#').
11 Also see :ref:`example-schema`.
16 Each schema doc is a structured json-schema which is defined by a set of
17 top-level properties. Generally, there is one binding defined per file. The
18 top-level json-schema properties used are:
21 A json-schema unique identifier string. The string must be a valid
22 URI typically containing the binding's filename and path. For DT schema, it must
23 begin with "http://devicetree.org/schemas/". The URL is used in constructing
24 references to other files specified in schema "$ref" properties. A $ref value
25 with a leading '/' will have the hostname prepended. A $ref value with only a
26 relative path or filename will be prepended with the hostname and path
27 components of the current schema file's '$id' value. A URL is used even for
28 local files, but there may not actually be files present at those locations.
31 Indicates the meta-schema the schema file adheres to.
34 A one-line description on the contents of the binding schema.
37 A DT specific property. Contains a list of email address(es)
38 for maintainers of this binding.
41 Optional. A multi-line text block containing any detailed
42 information about this binding. It should contain things such as what the block
43 or device does, standards the device conforms to, and links to datasheets for
47 Optional. A json-schema used to match nodes for applying the
48 schema. By default, without 'select', nodes are matched against their possible
49 compatible-string values or node name. Most bindings should not need select.
52 Optional. A list of other schemas to include. This is used to
53 include other schemas the binding conforms to. This may be schemas for a
54 particular class of devices such as I2C or SPI controllers.
57 A set of sub-schema defining all the DT properties for the
58 binding. The exact schema syntax depends on whether properties are known,
59 common properties (e.g. 'interrupts') or are binding/vendor-specific
62 A property can also define a child DT node with child properties defined
65 For more details on properties sections, see 'Property Schema' section.
68 Optional. Similar to 'properties', but names are regex.
71 A list of DT properties from the 'properties' section that
72 must always be present.
75 Optional. A list of one or more DTS hunks implementing the
76 binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.
78 Unless noted otherwise, all properties are required.
83 The 'properties' section of the schema contains all the DT properties for a
84 binding. Each property contains a set of constraints using json-schema
85 vocabulary for that property. The properties schemas are what are used for
86 validation of DT files.
88 For common properties, only additional constraints not covered by the common,
89 binding schema need to be defined such as how many values are valid or what
90 possible values are valid.
92 Vendor-specific properties will typically need more detailed schema. With the
93 exception of boolean properties, they should have a reference to a type in
94 schemas/types.yaml. A "description" property is always required.
96 The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
97 dtc. They are simplified to make them more compact and avoid a bunch of
98 boilerplate. The tools process the schema files to produce the final schema for
99 validation. There are currently 2 transformations the tools perform.
101 The default for arrays in json-schema is they are variable-sized and allow more
102 entries than explicitly defined. This can be restricted by defining 'minItems',
103 'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
104 size is desired in most cases, so these properties are added based on the
105 number of entries in an 'items' list.
107 The YAML Devicetree format also makes all string values an array and scalar
108 values a matrix (in order to define groupings) even when only a single value
109 is present. Single entries in schemas are fixed up to match this encoding.
117 The DT schema project must be installed in order to validate the DT schema
118 binding documents and validate DTS files using the DT schema. The DT schema
119 project can be installed with pip::
121 pip3 install git+https://github.com/devicetree-org/dt-schema.git@master
123 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
124 installed. Ensure they are in your PATH (~/.local/bin by default).
126 dtc must also be built with YAML output support enabled. This requires that
127 libyaml and its headers be installed on the host system. For some distributions
128 that involves installing the development package, such as:
132 apt-get install libyaml-dev
136 dnf -y install libyaml-devel
141 The DT schema binding documents must be validated using the meta-schema (the
142 schema for the schema) to ensure they are both valid json-schema and valid
143 binding schema. All of the DT binding documents can be validated using the
144 ``dt_binding_check`` target::
146 make dt_binding_check
148 In order to perform validation of DT source files, use the ``dtbs_check`` target::
152 Note that ``dtbs_check`` will skip any binding schema files with errors. It is
153 necessary to use ``dt_binding_check`` to get all the validation errors in the
154 binding schema files.
156 It is possible to run both in a single command::
158 make dt_binding_check dtbs_check
160 It is also possible to run checks with a single schema file by setting the
161 ``DT_SCHEMA_FILES`` variable to a specific schema file.
165 make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
166 make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
169 json-schema Resources
170 ---------------------
173 `JSON-Schema Specifications <http://json-schema.org/>`_
175 `Using JSON Schema Book <http://usingjsonschema.com/>`_
179 Annotated Example Schema
180 ------------------------
182 Also available as a separate file: :download:`example-schema.yaml`
184 .. literalinclude:: example-schema.yaml