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 46 The YAML format has several options for defining the formatting of the text 47 block. The options are controlled with indicator characters following the key 48 (e.g. "description: \|"). The minimum formatting needed for a block should be 49 used. The formatting controls can not only affect whether the YAML can be 50 parsed correctly, but are important when the text blocks are rendered to 51 another form. The options are as follows. 52 53 The default without any indicators is flowed, plain scalar style where single 54 line breaks and leading whitespace are stripped. Paragraphs are delimited by 55 blank lines (i.e. double line break). This style cannot contain ": " in it as 56 it will be interpreted as a key. Any " #" sequence will be interpreted as 57 a comment. There's other restrictions on characters as well. Most 58 restrictions are on what the first character can be. 59 60 The second style is folded which is indicated by ">" character. In addition 61 to maintaining line breaks on double line breaks, the folded style also 62 maintains leading whitespace beyond indentation of the first line. The line 63 breaks on indented lines are also maintained. 64 65 The third style is literal which is indicated by "\|" character. The literal 66 style maintains all line breaks and whitespace (beyond indentation of the 67 first line). 68 69 The above is not a complete description of YAML text blocks. More details on 70 multi-line YAML text blocks can be found online: 71 72 https://yaml-multiline.info/ 73 74 https://www.yaml.info/learn/quote.html 75 76select 77 Optional. A json-schema used to match nodes for applying the 78 schema. By default, without 'select', nodes are matched against their possible 79 compatible-string values or node name. Most bindings should not need select. 80 81allOf 82 Optional. A list of other schemas to include. This is used to 83 include other schemas the binding conforms to. This may be schemas for a 84 particular class of devices such as I2C or SPI controllers. 85 86properties 87 A set of sub-schema defining all the DT properties for the 88 binding. The exact schema syntax depends on whether properties are known, 89 common properties (e.g. 'interrupts') or are binding/vendor-specific 90 properties. 91 92A property can also define a child DT node with child properties defined 93under it. 94 95For more details on properties sections, see 'Property Schema' section. 96 97patternProperties 98 Optional. Similar to 'properties', but names are regex. 99 100required 101 A list of DT properties from the 'properties' section that 102 must always be present. 103 104additionalProperties / unevaluatedProperties 105 Keywords controlling how schema will validate properties not matched by this 106 schema's 'properties' or 'patternProperties'. Each schema is supposed to 107 have exactly one of these keywords in top-level part, so either 108 additionalProperties or unevaluatedProperties. Nested nodes, so properties 109 being objects, are supposed to have one as well. 110 111 * additionalProperties: false 112 Most common case, where no additional schema is referenced or if this 113 binding allows subset of properties from other referenced schemas. 114 115 * unevaluatedProperties: false 116 Used when this binding references other schema whose all properties 117 should be allowed. 118 119 * additionalProperties: true 120 - Top-level part: 121 Rare case, used for schemas implementing common set of properties. Such 122 schemas are supposed to be referenced by other schemas, which then use 123 'unevaluatedProperties: false'. Typically bus or common-part schemas. 124 - Nested node: 125 When listing only the expected compatible of the nested node and there 126 is an another schema matching that compatible which ends with one of 127 two above cases ('false'). 128 129examples 130 Optional. A list of one or more DTS hunks implementing this binding only. 131 Example should not contain unrelated device nodes, e.g. consumer nodes in a 132 provider binding, other nodes referenced by phandle. 133 Note: YAML doesn't allow leading tabs, so spaces must be used instead. 134 135Unless noted otherwise, all properties are required. 136 137Property Schema 138--------------- 139 140The 'properties' section of the schema contains all the DT properties for a 141binding. Each property contains a set of constraints using json-schema 142vocabulary for that property. The properties schemas are what are used for 143validation of DT files. 144 145For common properties, only additional constraints not covered by the common, 146binding schema need to be defined such as how many values are valid or what 147possible values are valid. 148 149Vendor-specific properties will typically need more detailed schema. With the 150exception of boolean properties, they should have a reference to a type in 151schemas/types.yaml. A "description" property is always required. 152 153The Devicetree schemas don't exactly match the YAML-encoded DT data produced by 154dtc. They are simplified to make them more compact and avoid a bunch of 155boilerplate. The tools process the schema files to produce the final schema for 156validation. There are currently 2 transformations the tools perform. 157 158The default for arrays in json-schema is they are variable-sized and allow more 159entries than explicitly defined. This can be restricted by defining 'minItems', 160'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 161size is desired in most cases, so these properties are added based on the 162number of entries in an 'items' list. 163 164The YAML Devicetree format also makes all string values an array and scalar 165values a matrix (in order to define groupings) even when only a single value 166is present. Single entries in schemas are fixed up to match this encoding. 167 168When bindings cover multiple similar devices that differ in some properties, 169those properties should be constrained for each device. This usually means: 170 171 * In top level 'properties' define the property with the broadest constraints. 172 * In 'if:then:' blocks, further narrow the constraints for those properties. 173 * Do not define the properties within an 'if:then:' block (note that 174 'additionalItems' also won't allow that). 175 176Coding style 177------------ 178 179Use YAML coding style (two-space indentation). For DTS examples in the schema, 180preferred is four-space indentation. 181 182Place entries in 'properties' and 'required' sections in the same order, using 183style from Documentation/devicetree/bindings/dts-coding-style.rst. 184 185Testing 186------- 187 188Dependencies 189~~~~~~~~~~~~ 190 191The DT schema project must be installed in order to validate the DT schema 192binding documents and validate DTS files using the DT schema. The DT schema 193project can be installed with pip:: 194 195 pip3 install dtschema 196 197Note that 'dtschema' installation requires 'swig' and Python development files 198installed first. On Debian/Ubuntu systems:: 199 200 apt install swig python3-dev 201 202Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 203installed. Ensure they are in your PATH (~/.local/bin by default). 204 205Recommended is also to install yamllint (used by dtschema when present). 206 207Running checks 208~~~~~~~~~~~~~~ 209 210The DT schema binding documents must be validated using the meta-schema (the 211schema for the schema) to ensure they are both valid json-schema and valid 212binding schema. All of the DT binding documents can be validated using the 213``dt_binding_check`` target:: 214 215 make dt_binding_check 216 217In order to perform validation of DT source files, use the ``dtbs_check`` target:: 218 219 make dtbs_check 220 221Note that ``dtbs_check`` will skip any binding schema files with errors. It is 222necessary to use ``dt_binding_check`` to get all the validation errors in the 223binding schema files. 224 225It is possible to run both in a single command:: 226 227 make dt_binding_check dtbs_check 228 229It is also possible to run checks with a subset of matching schema files by 230setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or 231patterns (partial match of a fixed string). Each file or pattern should be 232separated by ':'. 233 234:: 235 236 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml 237 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml 238 make dt_binding_check DT_SCHEMA_FILES=/gpio/ 239 make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml 240 241 242json-schema Resources 243--------------------- 244 245 246`JSON-Schema Specifications <http://json-schema.org/>`_ 247 248`Using JSON Schema Book <http://usingjsonschema.com/>`_ 249 250.. _example-schema: 251 252Annotated Example Schema 253------------------------ 254 255Also available as a separate file: :download:`example-schema.yaml` 256 257.. literalinclude:: example-schema.yaml 258