1 .. SPDX-License-Identifier: GPL-2.0 2 3 Writing Devicetree Bindings in json-schema 4 ========================================== 5 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 '#'). 10 11 Also see :ref:`example-schema`. 12 13 Schema Contents 14 --------------- 15 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: 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 33 title 34 A one-line description of the hardware being described in the binding schema. 35 36 maintainers 37 A DT specific property. Contains a list of email address(es) 38 for maintainers of this binding. 39 40 description 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 46 select 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 51 allOf 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 56 properties 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 62 A property can also define a child DT node with child properties defined 63 under it. 64 65 For more details on properties sections, see 'Property Schema' section. 66 67 patternProperties 68 Optional. Similar to 'properties', but names are regex. 69 70 required 71 A list of DT properties from the 'properties' section that 72 must always be present. 73 74 additionalProperties / 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 94 examples 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. 99 100 Unless noted otherwise, all properties are required. 101 102 Property Schema 103 --------------- 104 105 The 'properties' section of the schema contains all the DT properties for a 106 binding. Each property contains a set of constraints using json-schema 107 vocabulary for that property. The properties schemas are what are used for 108 validation of DT files. 109 110 For common properties, only additional constraints not covered by the common, 111 binding schema need to be defined such as how many values are valid or what 112 possible values are valid. 113 114 Vendor-specific properties will typically need more detailed schema. With the 115 exception of boolean properties, they should have a reference to a type in 116 schemas/types.yaml. A "description" property is always required. 117 118 The Devicetree schemas don't exactly match the YAML-encoded DT data produced by 119 dtc. They are simplified to make them more compact and avoid a bunch of 120 boilerplate. The tools process the schema files to produce the final schema for 121 validation. There are currently 2 transformations the tools perform. 122 123 The default for arrays in json-schema is they are variable-sized and allow more 124 entries than explicitly defined. This can be restricted by defining 'minItems', 125 'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 126 size is desired in most cases, so these properties are added based on the 127 number of entries in an 'items' list. 128 129 The YAML Devicetree format also makes all string values an array and scalar 130 values a matrix (in order to define groupings) even when only a single value 131 is present. Single entries in schemas are fixed up to match this encoding. 132 133 Coding style 134 ------------ 135 136 Use YAML coding style (two-space indentation). For DTS examples in the schema, 137 preferred is four-space indentation. 138 139 Testing 140 ------- 141 142 Dependencies 143 ~~~~~~~~~~~~ 144 145 The DT schema project must be installed in order to validate the DT schema 146 binding documents and validate DTS files using the DT schema. The DT schema 147 project can be installed with pip:: 148 149 pip3 install dtschema 150 151 Note that 'dtschema' installation requires 'swig' and Python development files 152 installed first. On Debian/Ubuntu systems:: 153 154 apt install swig python3-dev 155 156 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 157 installed. Ensure they are in your PATH (~/.local/bin by default). 158 159 Recommended is also to install yamllint (used by dtschema when present). 160 161 Running checks 162 ~~~~~~~~~~~~~~ 163 164 The DT schema binding documents must be validated using the meta-schema (the 165 schema for the schema) to ensure they are both valid json-schema and valid 166 binding schema. All of the DT binding documents can be validated using the 167 ``dt_binding_check`` target:: 168 169 make dt_binding_check 170 171 In order to perform validation of DT source files, use the ``dtbs_check`` target:: 172 173 make dtbs_check 174 175 Note that ``dtbs_check`` will skip any binding schema files with errors. It is 176 necessary to use ``dt_binding_check`` to get all the validation errors in the 177 binding schema files. 178 179 It is possible to run both in a single command:: 180 181 make dt_binding_check dtbs_check 182 183 It is also possible to run checks with a subset of matching schema files by 184 setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or 185 patterns (partial match of a fixed string). Each file or pattern should be 186 separated 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 194 195 196 json-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: 205 206 Annotated Example Schema 207 ------------------------ 208 209 Also available as a separate file: :download:`example-schema.yaml` 210 211 .. literalinclude:: example-schema.yaml
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