xref: /linux/Documentation/devicetree/bindings/writing-schema.rst (revision 42d37fc0c819b81f6f6afd108b55d04ba9d32d0f)
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 of the hardware being described in 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 hardware. 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
74additionalProperties / unevaluatedProperties
75  Keywords controlling how schema will validate properties not matched by this
76  schema's 'properties' or 'patternProperties'. Each schema is supposed to
77  have exactly one of these keywords in top-level part, so either
78  additionalProperties or unevaluatedProperties. Nested nodes, so properties
79  being objects, are supposed to have one as well.
80
81  * additionalProperties: false
82      Most common case, where no additional schema is referenced or if this
83      binding allows subset of properties from other referenced schemas.
84
85  * unevaluatedProperties: false
86      Used when this binding references other schema whose all properties
87      should be allowed.
88
89  * additionalProperties: true
90      Rare case, used for schemas implementing common set of properties. Such
91      schemas are supposed to be referenced by other schemas, which then use
92      'unevaluatedProperties: false'.  Typically bus or common-part schemas.
93
94examples
95  Optional. A list of one or more DTS hunks implementing this binding only.
96  Example should not contain unrelated device nodes, e.g. consumer nodes in a
97  provider binding, other nodes referenced by phandle.
98  Note: YAML doesn't allow leading tabs, so spaces must be used instead.
99
100Unless noted otherwise, all properties are required.
101
102Property Schema
103---------------
104
105The 'properties' section of the schema contains all the DT properties for a
106binding. Each property contains a set of constraints using json-schema
107vocabulary for that property. The properties schemas are what are used for
108validation of DT files.
109
110For common properties, only additional constraints not covered by the common,
111binding schema need to be defined such as how many values are valid or what
112possible values are valid.
113
114Vendor-specific properties will typically need more detailed schema. With the
115exception of boolean properties, they should have a reference to a type in
116schemas/types.yaml. A "description" property is always required.
117
118The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
119dtc. They are simplified to make them more compact and avoid a bunch of
120boilerplate. The tools process the schema files to produce the final schema for
121validation. There are currently 2 transformations the tools perform.
122
123The default for arrays in json-schema is they are variable-sized and allow more
124entries than explicitly defined. This can be restricted by defining 'minItems',
125'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
126size is desired in most cases, so these properties are added based on the
127number of entries in an 'items' list.
128
129The YAML Devicetree format also makes all string values an array and scalar
130values a matrix (in order to define groupings) even when only a single value
131is present. Single entries in schemas are fixed up to match this encoding.
132
133Coding style
134------------
135
136Use YAML coding style (two-space indentation). For DTS examples in the schema,
137preferred is four-space indentation.
138
139Testing
140-------
141
142Dependencies
143~~~~~~~~~~~~
144
145The DT schema project must be installed in order to validate the DT schema
146binding documents and validate DTS files using the DT schema. The DT schema
147project can be installed with pip::
148
149    pip3 install dtschema
150
151Note that 'dtschema' installation requires 'swig' and Python development files
152installed first. On Debian/Ubuntu systems::
153
154    apt install swig python3-dev
155
156Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
157installed. Ensure they are in your PATH (~/.local/bin by default).
158
159Recommended is also to install yamllint (used by dtschema when present).
160
161Running checks
162~~~~~~~~~~~~~~
163
164The DT schema binding documents must be validated using the meta-schema (the
165schema for the schema) to ensure they are both valid json-schema and valid
166binding schema. All of the DT binding documents can be validated using the
167``dt_binding_check`` target::
168
169    make dt_binding_check
170
171In order to perform validation of DT source files, use the ``dtbs_check`` target::
172
173    make dtbs_check
174
175Note that ``dtbs_check`` will skip any binding schema files with errors. It is
176necessary to use ``dt_binding_check`` to get all the validation errors in the
177binding schema files.
178
179It is possible to run both in a single command::
180
181    make dt_binding_check dtbs_check
182
183It is also possible to run checks with a subset of matching schema files by
184setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or
185patterns (partial match of a fixed string). Each file or pattern should be
186separated by ':'.
187
188::
189
190    make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
191    make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
192    make dt_binding_check DT_SCHEMA_FILES=/gpio/
193    make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml
194
195
196json-schema Resources
197---------------------
198
199
200`JSON-Schema Specifications <http://json-schema.org/>`_
201
202`Using JSON Schema Book <http://usingjsonschema.com/>`_
203
204.. _example-schema:
205
206Annotated Example Schema
207------------------------
208
209Also available as a separate file: :download:`example-schema.yaml`
210
211.. literalinclude:: example-schema.yaml
212