1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 Writing Devicetree Bindings in json-schema 3 Writing Devicetree Bindings in json-schema
4 ========================================== 4 ==========================================
5 5
6 Devicetree bindings are written using json-sch 6 Devicetree bindings are written using json-schema vocabulary. Schema files are
7 written in a JSON-compatible subset of YAML. Y 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 8 is considered more human readable and has some advantages such as allowing
9 comments (Prefixed with '#'). 9 comments (Prefixed with '#').
10 10
11 Also see :ref:`example-schema`. 11 Also see :ref:`example-schema`.
12 12
13 Schema Contents 13 Schema Contents
14 --------------- 14 ---------------
15 15
16 Each schema doc is a structured json-schema wh 16 Each schema doc is a structured json-schema which is defined by a set of
17 top-level properties. Generally, there is one 17 top-level properties. Generally, there is one binding defined per file. The
18 top-level json-schema properties used are: 18 top-level json-schema properties used are:
19 19
20 $id 20 $id
21 A json-schema unique identifier string. The 21 A json-schema unique identifier string. The string must be a valid
22 URI typically containing the binding's filen 22 URI typically containing the binding's filename and path. For DT schema, it must
23 begin with "http://devicetree.org/schemas/". 23 begin with "http://devicetree.org/schemas/". The URL is used in constructing
24 references to other files specified in schem 24 references to other files specified in schema "$ref" properties. A $ref value
25 with a leading '/' will have the hostname pr 25 with a leading '/' will have the hostname prepended. A $ref value with only a
26 relative path or filename will be prepended 26 relative path or filename will be prepended with the hostname and path
27 components of the current schema file's '$id 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 f 28 local files, but there may not actually be files present at those locations.
29 29
30 $schema 30 $schema
31 Indicates the meta-schema the schema file ad 31 Indicates the meta-schema the schema file adheres to.
32 32
33 title 33 title
34 A one-line description of the hardware being !! 34 A one-line description on the contents of the binding schema.
35 35
36 maintainers 36 maintainers
37 A DT specific property. Contains a list of e 37 A DT specific property. Contains a list of email address(es)
38 for maintainers of this binding. 38 for maintainers of this binding.
39 39
40 description 40 description
41 Optional. A multi-line text block containing 41 Optional. A multi-line text block containing any detailed
42 information about this hardware. It should c !! 42 information about this binding. It should contain things such as what the block
43 or device does, standards the device conform 43 or device does, standards the device conforms to, and links to datasheets for
44 more information. 44 more information.
45 45
46 select 46 select
47 Optional. A json-schema used to match nodes 47 Optional. A json-schema used to match nodes for applying the
48 schema. By default, without 'select', nodes 48 schema. By default, without 'select', nodes are matched against their possible
49 compatible-string values or node name. Most 49 compatible-string values or node name. Most bindings should not need select.
50 50
51 allOf 51 allOf
52 Optional. A list of other schemas to include 52 Optional. A list of other schemas to include. This is used to
53 include other schemas the binding conforms t 53 include other schemas the binding conforms to. This may be schemas for a
54 particular class of devices such as I2C or S 54 particular class of devices such as I2C or SPI controllers.
55 55
56 properties 56 properties
57 A set of sub-schema defining all the DT prop 57 A set of sub-schema defining all the DT properties for the
58 binding. The exact schema syntax depends on 58 binding. The exact schema syntax depends on whether properties are known,
59 common properties (e.g. 'interrupts') or are 59 common properties (e.g. 'interrupts') or are binding/vendor-specific
60 properties. 60 properties.
61 61
62 A property can also define a child DT node wit 62 A property can also define a child DT node with child properties defined
63 under it. 63 under it.
64 64
65 For more details on properties sections, see ' 65 For more details on properties sections, see 'Property Schema' section.
66 66
67 patternProperties 67 patternProperties
68 Optional. Similar to 'properties', but names 68 Optional. Similar to 'properties', but names are regex.
69 69
70 required 70 required
71 A list of DT properties from the 'properties 71 A list of DT properties from the 'properties' section that
72 must always be present. 72 must always be present.
73 73
74 additionalProperties / unevaluatedProperties <<
75 Keywords controlling how schema will validat <<
76 schema's 'properties' or 'patternProperties' <<
77 have exactly one of these keywords in top-le <<
78 additionalProperties or unevaluatedPropertie <<
79 being objects, are supposed to have one as w <<
80 <<
81 * additionalProperties: false <<
82 Most common case, where no additional sc <<
83 binding allows subset of properties from <<
84 <<
85 * unevaluatedProperties: false <<
86 Used when this binding references other <<
87 should be allowed. <<
88 <<
89 * additionalProperties: true <<
90 Rare case, used for schemas implementing <<
91 schemas are supposed to be referenced by <<
92 'unevaluatedProperties: false'. Typical <<
93 <<
94 examples 74 examples
95 Optional. A list of one or more DTS hunks im !! 75 Optional. A list of one or more DTS hunks implementing the
96 Example should not contain unrelated device !! 76 binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.
97 provider binding, other nodes referenced by <<
98 Note: YAML doesn't allow leading tabs, so sp <<
99 77
100 Unless noted otherwise, all properties are req 78 Unless noted otherwise, all properties are required.
101 79
102 Property Schema 80 Property Schema
103 --------------- 81 ---------------
104 82
105 The 'properties' section of the schema contain 83 The 'properties' section of the schema contains all the DT properties for a
106 binding. Each property contains a set of const 84 binding. Each property contains a set of constraints using json-schema
107 vocabulary for that property. The properties s 85 vocabulary for that property. The properties schemas are what are used for
108 validation of DT files. 86 validation of DT files.
109 87
110 For common properties, only additional constra 88 For common properties, only additional constraints not covered by the common,
111 binding schema need to be defined such as how 89 binding schema need to be defined such as how many values are valid or what
112 possible values are valid. 90 possible values are valid.
113 91
114 Vendor-specific properties will typically need 92 Vendor-specific properties will typically need more detailed schema. With the
115 exception of boolean properties, they should h 93 exception of boolean properties, they should have a reference to a type in
116 schemas/types.yaml. A "description" property i 94 schemas/types.yaml. A "description" property is always required.
117 95
118 The Devicetree schemas don't exactly match the 96 The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
119 dtc. They are simplified to make them more com 97 dtc. They are simplified to make them more compact and avoid a bunch of
120 boilerplate. The tools process the schema file 98 boilerplate. The tools process the schema files to produce the final schema for
121 validation. There are currently 2 transformati 99 validation. There are currently 2 transformations the tools perform.
122 100
123 The default for arrays in json-schema is they 101 The default for arrays in json-schema is they are variable-sized and allow more
124 entries than explicitly defined. This can be r 102 entries than explicitly defined. This can be restricted by defining 'minItems',
125 'maxItems', and 'additionalItems'. However, fo 103 'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
126 size is desired in most cases, so these proper 104 size is desired in most cases, so these properties are added based on the
127 number of entries in an 'items' list. 105 number of entries in an 'items' list.
128 106
129 The YAML Devicetree format also makes all stri 107 The YAML Devicetree format also makes all string values an array and scalar
130 values a matrix (in order to define groupings) 108 values a matrix (in order to define groupings) even when only a single value
131 is present. Single entries in schemas are fixe 109 is present. Single entries in schemas are fixed up to match this encoding.
132 110
133 Coding style 111 Coding style
134 ------------ 112 ------------
135 113
136 Use YAML coding style (two-space indentation). 114 Use YAML coding style (two-space indentation). For DTS examples in the schema,
137 preferred is four-space indentation. 115 preferred is four-space indentation.
138 116
139 Testing 117 Testing
140 ------- 118 -------
141 119
142 Dependencies 120 Dependencies
143 ~~~~~~~~~~~~ 121 ~~~~~~~~~~~~
144 122
145 The DT schema project must be installed in ord 123 The DT schema project must be installed in order to validate the DT schema
146 binding documents and validate DTS files using 124 binding documents and validate DTS files using the DT schema. The DT schema
147 project can be installed with pip:: 125 project can be installed with pip::
148 126
149 pip3 install dtschema 127 pip3 install dtschema
150 128
151 Note that 'dtschema' installation requires 'sw 129 Note that 'dtschema' installation requires 'swig' and Python development files
152 installed first. On Debian/Ubuntu systems:: 130 installed first. On Debian/Ubuntu systems::
153 131
154 apt install swig python3-dev 132 apt install swig python3-dev
155 133
156 Several executables (dt-doc-validate, dt-mk-sc 134 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
157 installed. Ensure they are in your PATH (~/.lo 135 installed. Ensure they are in your PATH (~/.local/bin by default).
158 136
159 Recommended is also to install yamllint (used 137 Recommended is also to install yamllint (used by dtschema when present).
160 138
161 Running checks 139 Running checks
162 ~~~~~~~~~~~~~~ 140 ~~~~~~~~~~~~~~
163 141
164 The DT schema binding documents must be valida 142 The DT schema binding documents must be validated using the meta-schema (the
165 schema for the schema) to ensure they are both 143 schema for the schema) to ensure they are both valid json-schema and valid
166 binding schema. All of the DT binding document 144 binding schema. All of the DT binding documents can be validated using the
167 ``dt_binding_check`` target:: 145 ``dt_binding_check`` target::
168 146
169 make dt_binding_check 147 make dt_binding_check
170 148
171 In order to perform validation of DT source fi 149 In order to perform validation of DT source files, use the ``dtbs_check`` target::
172 150
173 make dtbs_check 151 make dtbs_check
174 152
175 Note that ``dtbs_check`` will skip any binding 153 Note that ``dtbs_check`` will skip any binding schema files with errors. It is
176 necessary to use ``dt_binding_check`` to get a 154 necessary to use ``dt_binding_check`` to get all the validation errors in the
177 binding schema files. 155 binding schema files.
178 156
179 It is possible to run both in a single command 157 It is possible to run both in a single command::
180 158
181 make dt_binding_check dtbs_check 159 make dt_binding_check dtbs_check
182 160
183 It is also possible to run checks with a subse 161 It is also possible to run checks with a subset of matching schema files by
184 setting the ``DT_SCHEMA_FILES`` variable to 1 !! 162 setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern.
185 patterns (partial match of a fixed string). Ea <<
186 separated by ':'. <<
187 163
188 :: 164 ::
189 165
190 make dt_binding_check DT_SCHEMA_FILES=triv 166 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
191 make dt_binding_check DT_SCHEMA_FILES=triv <<
192 make dt_binding_check DT_SCHEMA_FILES=/gpi 167 make dt_binding_check DT_SCHEMA_FILES=/gpio/
193 make dtbs_check DT_SCHEMA_FILES=trivial-de 168 make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml
194 169
195 170
196 json-schema Resources 171 json-schema Resources
197 --------------------- 172 ---------------------
198 173
199 174
200 `JSON-Schema Specifications <http://json-schem 175 `JSON-Schema Specifications <http://json-schema.org/>`_
201 176
202 `Using JSON Schema Book <http://usingjsonschem 177 `Using JSON Schema Book <http://usingjsonschema.com/>`_
203 178
204 .. _example-schema: 179 .. _example-schema:
205 180
206 Annotated Example Schema 181 Annotated Example Schema
207 ------------------------ 182 ------------------------
208 183
209 Also available as a separate file: :download:` 184 Also available as a separate file: :download:`example-schema.yaml`
210 185
211 .. literalinclude:: example-schema.yaml 186 .. literalinclude:: example-schema.yaml
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