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 $ref value a relative 26 path or filename only will be prepended with the hostname and path components 27 of the current schema file's '$id' value. A URL is used even for local files, 28 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 properties. 60 61A property can also define a child DT node with child properties defined 62under it. 63 64For more details on properties sections, see 'Property Schema' section. 65 66patternProperties 67 Optional. Similar to 'properties', but names are regex. 68 69required 70 A list of DT properties from the 'properties' section that 71 must always be present. 72 73examples 74 Optional. A list of one or more DTS hunks implementing the 75 binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead. 76 77Unless noted otherwise, all properties are required. 78 79Property Schema 80--------------- 81 82The 'properties' section of the schema contains all the DT properties for a 83binding. Each property contains a set of constraints using json-schema 84vocabulary for that property. The properties schemas are what is used for 85validation of DT files. 86 87For common properties, only additional constraints not covered by the common 88binding schema need to be defined such as how many values are valid or what 89possible values are valid. 90 91Vendor specific properties will typically need more detailed schema. With the 92exception of boolean properties, they should have a reference to a type in 93schemas/types.yaml. A "description" property is always required. 94 95The Devicetree schemas don't exactly match the YAML encoded DT data produced by 96dtc. They are simplified to make them more compact and avoid a bunch of 97boilerplate. The tools process the schema files to produce the final schema for 98validation. There are currently 2 transformations the tools perform. 99 100The default for arrays in json-schema is they are variable sized and allow more 101entries than explicitly defined. This can be restricted by defining 'minItems', 102'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed 103size is desired in most cases, so these properties are added based on the 104number of entries in an 'items' list. 105 106The YAML Devicetree format also makes all string values an array and scalar 107values a matrix (in order to define groupings) even when only a single value 108is present. Single entries in schemas are fixed up to match this encoding. 109 110Testing 111------- 112 113Dependencies 114~~~~~~~~~~~~ 115 116The DT schema project must be installed in order to validate the DT schema 117binding documents and validate DTS files using the DT schema. The DT schema 118project can be installed with pip:: 119 120 pip3 install git+https://github.com/devicetree-org/dt-schema.git@master 121 122Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be 123installed. Ensure they are in your PATH (~/.local/bin by default). 124 125dtc must also be built with YAML output support enabled. This requires that 126libyaml and its headers be installed on the host system. For some distributions 127that involves installing the development package, such as: 128 129Debian:: 130 131 apt-get install libyaml-dev 132 133Fedora:: 134 135 dnf -y install libyaml-devel 136 137Running checks 138~~~~~~~~~~~~~~ 139 140The DT schema binding documents must be validated using the meta-schema (the 141schema for the schema) to ensure they are both valid json-schema and valid 142binding schema. All of the DT binding documents can be validated using the 143``dt_binding_check`` target:: 144 145 make dt_binding_check 146 147In order to perform validation of DT source files, use the ``dtbs_check`` target:: 148 149 make dtbs_check 150 151Note that ``dtbs_check`` will skip any binding schema files with errors. It is 152necessary to use ``dt_binding_check`` to get all the validation errors in the 153binding schema files. 154 155It is possible to run both in a single command:: 156 157 make dt_binding_check dtbs_check 158 159It is also possible to run checks with a single schema file by setting the 160``DT_SCHEMA_FILES`` variable to a specific schema file. 161 162:: 163 164 make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml 165 make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml 166 167 168json-schema Resources 169--------------------- 170 171 172`JSON-Schema Specifications <http://json-schema.org/>`_ 173 174`Using JSON Schema Book <http://usingjsonschema.com/>`_ 175 176.. _example-schema: 177 178Annotated Example Schema 179------------------------ 180 181Also available as a separate file: :download:`example-schema.yaml` 182 183.. literalinclude:: example-schema.yaml 184