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 11Schema Contents 12--------------- 13 14Each schema doc is a structured json-schema which is defined by a set of 15top-level properties. Generally, there is one binding defined per file. The 16top-level json-schema properties used are: 17 18$id 19 A json-schema unique identifier string. The string must be a valid 20 URI typically containing the binding's filename and path. For DT schema, it must 21 begin with "http://devicetree.org/schemas/". The URL is used in constructing 22 references to other files specified in schema "$ref" properties. A $ref value 23 with a leading '/' will have the hostname prepended. A $ref value a relative 24 path or filename only will be prepended with the hostname and path components 25 of the current schema file's '$id' value. A URL is used even for local files, 26 but there may not actually be files present at those locations. 27 28$schema 29 Indicates the meta-schema the schema file adheres to. 30 31title 32 A one line description on the contents of the binding schema. 33 34maintainers 35 A DT specific property. Contains a list of email address(es) 36 for maintainers of this binding. 37 38description 39 Optional. A multi-line text block containing any detailed 40 information about this binding. It should contain things such as what the block 41 or device does, standards the device conforms to, and links to datasheets for 42 more information. 43 44select 45 Optional. A json-schema used to match nodes for applying the 46 schema. By default without 'select', nodes are matched against their possible 47 compatible string values or node name. Most bindings should not need select. 48 49 allOf 50 Optional. A list of other schemas to include. This is used to 51 include other schemas the binding conforms to. This may be schemas for a 52 particular class of devices such as I2C or SPI controllers. 53 54 properties 55 A set of sub-schema defining all the DT properties for the 56 binding. The exact schema syntax depends on whether properties are known, 57 common properties (e.g. 'interrupts') or are binding/vendor specific properties. 58 59A property can also define a child DT node with child properties defined 60under it. 61 62For more details on properties sections, see 'Property Schema' section. 63 64patternProperties 65 Optional. Similar to 'properties', but names are regex. 66 67required 68 A list of DT properties from the 'properties' section that 69 must always be present. 70 71examples 72 Optional. A list of one or more DTS hunks implementing the 73 binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead. 74 75Unless noted otherwise, all properties are required. 76 77Property Schema 78--------------- 79 80The 'properties' section of the schema contains all the DT properties for a 81binding. Each property contains a set of constraints using json-schema 82vocabulary for that property. The properties schemas are what is used for 83validation of DT files. 84 85For common properties, only additional constraints not covered by the common 86binding schema need to be defined such as how many values are valid or what 87possible values are valid. 88 89Vendor specific properties will typically need more detailed schema. With the 90exception of boolean properties, they should have a reference to a type in 91schemas/types.yaml. A "description" property is always required. 92 93The Devicetree schemas don't exactly match the YAML encoded DT data produced by 94dtc. They are simplified to make them more compact and avoid a bunch of 95boilerplate. The tools process the schema files to produce the final schema for 96validation. There are currently 2 transformations the tools perform. 97 98The default for arrays in json-schema is they are variable sized and allow more 99entries than explicitly defined. This can be restricted by defining 'minItems', 100'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 101size is desired in most cases, so these properties are added based on the 102number of entries in an 'items' list. 103 104The YAML Devicetree format also makes all string values an array and scalar 105values a matrix (in order to define groupings) even when only a single value 106is present. Single entries in schemas are fixed up to match this encoding. 107 108Testing 109------- 110 111Dependencies 112~~~~~~~~~~~~ 113 114The DT schema project must be installed in order to validate the DT schema 115binding documents and validate DTS files using the DT schema. The DT schema 116project can be installed with pip:: 117 118 pip3 install git+https://github.com/devicetree-org/dt-schema.git@master 119 120Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 121installed. Ensure they are in your PATH (~/.local/bin by default). 122 123dtc must also be built with YAML output support enabled. This requires that 124libyaml and its headers be installed on the host system. For some distributions 125that involves installing the development package, such as: 126 127Debian:: 128 129 apt-get install libyaml-dev 130 131Fedora:: 132 133 dnf -y install libyaml-devel 134 135Running checks 136~~~~~~~~~~~~~~ 137 138The DT schema binding documents must be validated using the meta-schema (the 139schema for the schema) to ensure they are both valid json-schema and valid 140binding schema. All of the DT binding documents can be validated using the 141``dt_binding_check`` target:: 142 143 make dt_binding_check 144 145In order to perform validation of DT source files, use the ``dtbs_check`` target:: 146 147 make dtbs_check 148 149Note that ``dtbs_check`` will skip any binding schema files with errors. It is 150necessary to use ``dt_binding_check`` to get all the validation errors in the 151binding schema files. 152 153It is possible to run both in a single command:: 154 155 make dt_binding_check dtbs_check 156 157It is also possible to run checks with a single schema file by setting the 158``DT_SCHEMA_FILES`` variable to a specific schema file. 159 160:: 161 162 make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml 163 make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml 164 165 166json-schema Resources 167--------------------- 168 169 170`JSON-Schema Specifications <http://json-schema.org/>`_ 171 172`Using JSON Schema Book <http://usingjsonschema.com/>`_ 173
