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