Documentation/devicetree/bindings/writing-schema.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 Writing Devicetree Bindings in json-schema
   4 ==========================================
   5
   6 Devicetree bindings are written using json-schema vocabulary. Schema files are
   7 written in a JSON-compatible subset of YAML. YAML is used instead of JSON as it
   8 is considered more human readable and has some advantages such as allowing
   9 comments (Prefixed with '#').
  10
  11 Also see :ref:`example-schema`.
  12
  13 Schema Contents
  14 ---------------
  15
  16 Each schema doc is a structured json-schema which is defined by a set of
  17 top-level properties. Generally, there is one binding defined per file. The
  18 top-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
  33 title
  34   A one-line description on the contents of the binding schema.
  35
  36 maintainers
  37   A DT specific property. Contains a list of email address(es)
  38   for maintainers of this binding.
  39
  40 description
  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
  46 select
  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
  51 allOf
  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
  56 properties
  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
  62 A property can also define a child DT node with child properties defined
  63 under it.
  64
  65 For more details on properties sections, see 'Property Schema' section.
  66
  67 patternProperties
  68   Optional. Similar to 'properties', but names are regex.
  69
  70 required
  71   A list of DT properties from the 'properties' section that
  72   must always be present.
  73
  74 examples
  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
  78 Unless noted otherwise, all properties are required.
  79
  80 Property Schema
  81 ---------------
  82
  83 The 'properties' section of the schema contains all the DT properties for a
  84 binding. Each property contains a set of constraints using json-schema
  85 vocabulary for that property. The properties schemas are what are used for
  86 validation of DT files.
  87
  88 For common properties, only additional constraints not covered by the common,
  89 binding schema need to be defined such as how many values are valid or what
  90 possible values are valid.
  91
  92 Vendor-specific properties will typically need more detailed schema. With the
  93 exception of boolean properties, they should have a reference to a type in
  94 schemas/types.yaml. A "description" property is always required.
  95
  96 The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
  97 dtc. They are simplified to make them more compact and avoid a bunch of
  98 boilerplate. The tools process the schema files to produce the final schema for
  99 validation. There are currently 2 transformations the tools perform.
 100
 101 The default for arrays in json-schema is they are variable-sized and allow more
 102 entries than explicitly defined. This can be restricted by defining 'minItems',
 103 'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
 104 size is desired in most cases, so these properties are added based on the
 105 number of entries in an 'items' list.
 106
 107 The YAML Devicetree format also makes all string values an array and scalar
 108 values a matrix (in order to define groupings) even when only a single value
 109 is present. Single entries in schemas are fixed up to match this encoding.
 110
 111 Coding style
 112 ------------
 113
 114 Use YAML coding style (two-space indentation). For DTS examples in the schema,
 115 preferred is four-space indentation.
 116
 117 Testing
 118 -------
 119
 120 Dependencies
 121 ~~~~~~~~~~~~
 122
 123 The DT schema project must be installed in order to validate the DT schema
 124 binding documents and validate DTS files using the DT schema. The DT schema
 125 project can be installed with pip::
 126
 127     pip3 install dtschema
 128
 129 Note that 'dtschema' installation requires 'swig' and Python development files
 130 installed first. On Debian/Ubuntu systems::
 131
 132     apt install swig python3-dev
 133
 134 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
 135 installed. Ensure they are in your PATH (~/.local/bin by default).
 136
 137 Recommended is also to install yamllint (used by dtschema when present).
 138
 139 Running checks
 140 ~~~~~~~~~~~~~~
 141
 142 The DT schema binding documents must be validated using the meta-schema (the
 143 schema for the schema) to ensure they are both valid json-schema and valid
 144 binding schema. All of the DT binding documents can be validated using the
 145 ``dt_binding_check`` target::
 146
 147     make dt_binding_check
 148
 149 In order to perform validation of DT source files, use the ``dtbs_check`` target::
 150
 151     make dtbs_check
 152
 153 Note that ``dtbs_check`` will skip any binding schema files with errors. It is
 154 necessary to use ``dt_binding_check`` to get all the validation errors in the
 155 binding schema files.
 156
 157 It is possible to run both in a single command::
 158
 159     make dt_binding_check dtbs_check
 160
 161 It is also possible to run checks with a subset of matching schema files by
 162 setting 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
 171 json-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
 181 Annotated Example Schema
 182 ------------------------
 183
 184 Also available as a separate file: :download:`example-schema.yaml`
 185
 186 .. literalinclude:: example-schema.yaml