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 on the contents of 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 binding. 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 46select 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 51allOf 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 56properties 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 62A property can also define a child DT node with child properties defined 63under it. 64 65For more details on properties sections, see 'Property Schema' section. 66 67patternProperties 68 Optional. Similar to 'properties', but names are regex. 69 70required 71 A list of DT properties from the 'properties' section that 72 must always be present. 73 74examples 75 Optional. A list of one or more DTS hunks implementing the 76 binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead. 77 78Unless noted otherwise, all properties are required. 79 80Property Schema 81--------------- 82 83The 'properties' section of the schema contains all the DT properties for a 84binding. Each property contains a set of constraints using json-schema 85vocabulary for that property. The properties schemas are what are used for 86validation of DT files. 87 88For common properties, only additional constraints not covered by the common, 89binding schema need to be defined such as how many values are valid or what 90possible values are valid. 91 92Vendor-specific properties will typically need more detailed schema. With the 93exception of boolean properties, they should have a reference to a type in 94schemas/types.yaml. A "description" property is always required. 95 96The Devicetree schemas don't exactly match the YAML-encoded DT data produced by 97dtc. They are simplified to make them more compact and avoid a bunch of 98boilerplate. The tools process the schema files to produce the final schema for 99validation. There are currently 2 transformations the tools perform. 100 101The default for arrays in json-schema is they are variable-sized and allow more 102entries than explicitly defined. This can be restricted by defining 'minItems', 103'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 104size is desired in most cases, so these properties are added based on the 105number of entries in an 'items' list. 106 107The YAML Devicetree format also makes all string values an array and scalar 108values a matrix (in order to define groupings) even when only a single value 109is present. Single entries in schemas are fixed up to match this encoding. 110 111Coding style 112------------ 113 114Use YAML coding style (two-space indentation). For DTS examples in the schema, 115preferred is four-space indentation. 116 117Testing 118------- 119 120Dependencies 121~~~~~~~~~~~~ 122 123The DT schema project must be installed in order to validate the DT schema 124binding documents and validate DTS files using the DT schema. The DT schema 125project can be installed with pip:: 126 127 pip3 install dtschema 128 129Note that 'dtschema' installation requires 'swig' and Python development files 130installed first. On Debian/Ubuntu systems:: 131 132 apt install swig python3-dev 133 134Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 135installed. Ensure they are in your PATH (~/.local/bin by default). 136 137Recommended is also to install yamllint (used by dtschema when present). 138 139Running checks 140~~~~~~~~~~~~~~ 141 142The DT schema binding documents must be validated using the meta-schema (the 143schema for the schema) to ensure they are both valid json-schema and valid 144binding schema. All of the DT binding documents can be validated using the 145``dt_binding_check`` target:: 146 147 make dt_binding_check 148 149In order to perform validation of DT source files, use the ``dtbs_check`` target:: 150 151 make dtbs_check 152 153Note that ``dtbs_check`` will skip any binding schema files with errors. It is 154necessary to use ``dt_binding_check`` to get all the validation errors in the 155binding schema files. 156 157It is possible to run both in a single command:: 158 159 make dt_binding_check dtbs_check 160 161It is also possible to run checks with a subset of matching schema files by 162setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern. 163 164:: 165 166 make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml 167 make dt_binding_check DT_SCHEMA_FILES=/gpio/ 168 make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml 169 170 171json-schema Resources 172--------------------- 173 174 175`JSON-Schema Specifications <http://json-schema.org/>`_ 176 177`Using JSON Schema Book <http://usingjsonschema.com/>`_ 178 179.. _example-schema: 180 181Annotated Example Schema 182------------------------ 183 184Also available as a separate file: :download:`example-schema.yaml` 185 186.. literalinclude:: example-schema.yaml 187