1.. SPDX-License-Identifier: GPL-2.0
2
3Writing Devicetree Bindings in json-schema
4==========================================
5
6Devicetree bindings are written using json-schema vocabulary. Schema files are
7written in a JSON-compatible subset of YAML. YAML is used instead of JSON as it
8is considered more human readable and has some advantages such as allowing
9comments (Prefixed with '#').
10
11Also see :ref:`example-schema`.
12
13Schema Contents
14---------------
15
16Each schema doc is a structured json-schema which is defined by a set of
17top-level properties. Generally, there is one binding defined per file. The
18top-level json-schema properties used are:
19
20$id
21  A json-schema unique identifier string. The string must be a valid
22  URI typically containing the binding's filename and path. For DT schema, it must
23  begin with "http://devicetree.org/schemas/". The URL is used in constructing
24  references to other files specified in schema "$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.
29
30$schema
31  Indicates the meta-schema the schema file adheres to.
32
33title
34  A one-line description on the contents of the binding schema.
35
36maintainers
37  A DT specific property. Contains a list of email address(es)
38  for maintainers of this binding.
39
40description
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
44  more information.
45
46select
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.
50
51allOf
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.
55
56properties
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
60  properties.
61
62A property can also define a child DT node with child properties defined
63under it.
64
65For more details on properties sections, see 'Property Schema' section.
66
67patternProperties
68  Optional. Similar to 'properties', but names are regex.
69
70required
71  A list of DT properties from the 'properties' section that
72  must always be present.
73
74examples
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.
77
78Unless noted otherwise, all properties are required.
79
80Property Schema
81---------------
82
83The 'properties' section of the schema contains all the DT properties for a
84binding. Each property contains a set of constraints using json-schema
85vocabulary for that property. The properties schemas are what are used for
86validation of DT files.
87
88For common properties, only additional constraints not covered by the common,
89binding schema need to be defined such as how many values are valid or what
90possible values are valid.
91
92Vendor-specific properties will typically need more detailed schema. With the
93exception of boolean properties, they should have a reference to a type in
94schemas/types.yaml. A "description" property is always required.
95
96The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
97dtc. They are simplified to make them more compact and avoid a bunch of
98boilerplate. The tools process the schema files to produce the final schema for
99validation. There are currently 2 transformations the tools perform.
100
101The default for arrays in json-schema is they are variable-sized and allow more
102entries than explicitly defined. This can be restricted by defining 'minItems',
103'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
104size is desired in most cases, so these properties are added based on the
105number of entries in an 'items' list.
106
107The YAML Devicetree format also makes all string values an array and scalar
108values a matrix (in order to define groupings) even when only a single value
109is present. Single entries in schemas are fixed up to match this encoding.
110
111Testing
112-------
113
114Dependencies
115~~~~~~~~~~~~
116
117The DT schema project must be installed in order to validate the DT schema
118binding documents and validate DTS files using the DT schema. The DT schema
119project can be installed with pip::
120
121    pip3 install git+https://github.com/devicetree-org/dt-schema.git@master
122
123Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
124installed. Ensure they are in your PATH (~/.local/bin by default).
125
126dtc must also be built with YAML output support enabled. This requires that
127libyaml and its headers be installed on the host system. For some distributions
128that involves installing the development package, such as:
129
130Debian::
131
132  apt-get install libyaml-dev
133
134Fedora::
135
136  dnf -y install libyaml-devel
137
138Running checks
139~~~~~~~~~~~~~~
140
141The DT schema binding documents must be validated using the meta-schema (the
142schema for the schema) to ensure they are both valid json-schema and valid
143binding schema. All of the DT binding documents can be validated using the
144``dt_binding_check`` target::
145
146    make dt_binding_check
147
148In order to perform validation of DT source files, use the ``dtbs_check`` target::
149
150    make dtbs_check
151
152Note that ``dtbs_check`` will skip any binding schema files with errors. It is
153necessary to use ``dt_binding_check`` to get all the validation errors in the
154binding schema files.
155
156It is possible to run both in a single command::
157
158    make dt_binding_check dtbs_check
159
160It is also possible to run checks with a single schema file by setting the
161``DT_SCHEMA_FILES`` variable to a specific schema file.
162
163::
164
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
167
168
169json-schema Resources
170---------------------
171
172
173`JSON-Schema Specifications <http://json-schema.org/>`_
174
175`Using JSON Schema Book <http://usingjsonschema.com/>`_
176
177.. _example-schema:
178
179Annotated Example Schema
180------------------------
181
182Also available as a separate file: :download:`example-schema.yaml`
183
184.. literalinclude:: example-schema.yaml
185