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 interpretted as a key. Any " #" sequence will be interpretted 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 Rare case, used for schemas implementing common set of properties. Such 121 schemas are supposed to be referenced by other schemas, which then use 122 'unevaluatedProperties: false'. Typically bus or common-part schemas. 123 124examples 125 Optional. A list of one or more DTS hunks implementing this binding only. 126 Example should not contain unrelated device nodes, e.g. consumer nodes in a 127 provider binding, other nodes referenced by phandle. 128 Note: YAML doesn't allow leading tabs, so spaces must be used instead. 129 130Unless noted otherwise, all properties are required. 131 132Property Schema 133--------------- 134 135The 'properties' section of the schema contains all the DT properties for a 136binding. Each property contains a set of constraints using json-schema 137vocabulary for that property. The properties schemas are what are used for 138validation of DT files. 139 140For common properties, only additional constraints not covered by the common, 141binding schema need to be defined such as how many values are valid or what 142possible values are valid. 143 144Vendor-specific properties will typically need more detailed schema. With the 145exception of boolean properties, they should have a reference to a type in 146schemas/types.yaml. A "description" property is always required. 147 148The Devicetree schemas don't exactly match the YAML-encoded DT data produced by 149dtc. They are simplified to make them more compact and avoid a bunch of 150boilerplate. The tools process the schema files to produce the final schema for 151validation. There are currently 2 transformations the tools perform. 152 153The default for arrays in json-schema is they are variable-sized and allow more 154entries than explicitly defined. This can be restricted by defining 'minItems', 155'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 156size is desired in most cases, so these properties are added based on the 157number of entries in an 'items' list. 158 159The YAML Devicetree format also makes all string values an array and scalar 160values a matrix (in order to define groupings) even when only a single value 161is present. Single entries in schemas are fixed up to match this encoding. 162 163Coding style 164------------ 165 166Use YAML coding style (two-space indentation). For DTS examples in the schema, 167preferred is four-space indentation. 168 169Testing 170------- 171 172Dependencies 173~~~~~~~~~~~~ 174 175The DT schema project must be installed in order to validate the DT schema 176binding documents and validate DTS files using the DT schema. The DT schema 177project can be installed with pip:: 178 179 pip3 install dtschema 180 181Note that 'dtschema' installation requires 'swig' and Python development files 182installed first. On Debian/Ubuntu systems:: 183 184 apt install swig python3-dev 185 186Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 187installed. Ensure they are in your PATH (~/.local/bin by default). 188 189Recommended is also to install yamllint (used by dtschema when present). 190 191Running checks 192~~~~~~~~~~~~~~ 193 194The DT schema binding documents must be validated using the meta-schema (the 195schema for the schema) to ensure they are both valid json-schema and valid 196binding schema. All of the DT binding documents can be validated using the 197``dt_binding_check`` target:: 198 199 make dt_binding_check 200 201In order to perform validation of DT source files, use the ``dtbs_check`` target:: 202 203 make dtbs_check 204 205Note that ``dtbs_check`` will skip any binding schema files with errors. It is 206necessary to use ``dt_binding_check`` to get all the validation errors in the 207binding schema files. 208 209It is possible to run both in a single command:: 210 211 make dt_binding_check dtbs_check 212 213It is also possible to run checks with a subset of matching schema files by 214setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or 215patterns (partial match of a fixed string). Each file or pattern should be 216separated by ':'. 217 218:: 219 220 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml 221 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml 222 make dt_binding_check DT_SCHEMA_FILES=/gpio/ 223 make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml 224 225 226json-schema Resources 227--------------------- 228 229 230`JSON-Schema Specifications <http://json-schema.org/>`_ 231 232`Using JSON Schema Book <http://usingjsonschema.com/>`_ 233 234.. _example-schema: 235 236Annotated Example Schema 237------------------------ 238 239Also available as a separate file: :download:`example-schema.yaml` 240 241.. literalinclude:: example-schema.yaml 242