184 lines
6.5 KiB
ReStructuredText
184 lines
6.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Writing Devicetree Bindings in json-schema
|
|
==========================================
|
|
|
|
Devicetree bindings are written using json-schema vocabulary. Schema files are
|
|
written in a JSON compatible subset of YAML. YAML is used instead of JSON as it
|
|
is considered more human readable and has some advantages such as allowing
|
|
comments (Prefixed with '#').
|
|
|
|
Also see :ref:`example-schema`.
|
|
|
|
Schema Contents
|
|
---------------
|
|
|
|
Each schema doc is a structured json-schema which is defined by a set of
|
|
top-level properties. Generally, there is one binding defined per file. The
|
|
top-level json-schema properties used are:
|
|
|
|
$id
|
|
A json-schema unique identifier string. The string must be a valid
|
|
URI typically containing the binding's filename and path. For DT schema, it must
|
|
begin with "http://devicetree.org/schemas/". The URL is used in constructing
|
|
references to other files specified in schema "$ref" properties. A $ref value
|
|
with a leading '/' will have the hostname prepended. A $ref value a relative
|
|
path or filename only will be prepended with the hostname and path components
|
|
of the current schema file's '$id' value. A URL is used even for local files,
|
|
but there may not actually be files present at those locations.
|
|
|
|
$schema
|
|
Indicates the meta-schema the schema file adheres to.
|
|
|
|
title
|
|
A one line description on the contents of the binding schema.
|
|
|
|
maintainers
|
|
A DT specific property. Contains a list of email address(es)
|
|
for maintainers of this binding.
|
|
|
|
description
|
|
Optional. A multi-line text block containing any detailed
|
|
information about this binding. It should contain things such as what the block
|
|
or device does, standards the device conforms to, and links to datasheets for
|
|
more information.
|
|
|
|
select
|
|
Optional. A json-schema used to match nodes for applying the
|
|
schema. By default without 'select', nodes are matched against their possible
|
|
compatible string values or node name. Most bindings should not need select.
|
|
|
|
allOf
|
|
Optional. A list of other schemas to include. This is used to
|
|
include other schemas the binding conforms to. This may be schemas for a
|
|
particular class of devices such as I2C or SPI controllers.
|
|
|
|
properties
|
|
A set of sub-schema defining all the DT properties for the
|
|
binding. The exact schema syntax depends on whether properties are known,
|
|
common properties (e.g. 'interrupts') or are binding/vendor specific properties.
|
|
|
|
A property can also define a child DT node with child properties defined
|
|
under it.
|
|
|
|
For more details on properties sections, see 'Property Schema' section.
|
|
|
|
patternProperties
|
|
Optional. Similar to 'properties', but names are regex.
|
|
|
|
required
|
|
A list of DT properties from the 'properties' section that
|
|
must always be present.
|
|
|
|
examples
|
|
Optional. A list of one or more DTS hunks implementing the
|
|
binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.
|
|
|
|
Unless noted otherwise, all properties are required.
|
|
|
|
Property Schema
|
|
---------------
|
|
|
|
The 'properties' section of the schema contains all the DT properties for a
|
|
binding. Each property contains a set of constraints using json-schema
|
|
vocabulary for that property. The properties schemas are what is used for
|
|
validation of DT files.
|
|
|
|
For common properties, only additional constraints not covered by the common
|
|
binding schema need to be defined such as how many values are valid or what
|
|
possible values are valid.
|
|
|
|
Vendor specific properties will typically need more detailed schema. With the
|
|
exception of boolean properties, they should have a reference to a type in
|
|
schemas/types.yaml. A "description" property is always required.
|
|
|
|
The Devicetree schemas don't exactly match the YAML encoded DT data produced by
|
|
dtc. They are simplified to make them more compact and avoid a bunch of
|
|
boilerplate. The tools process the schema files to produce the final schema for
|
|
validation. There are currently 2 transformations the tools perform.
|
|
|
|
The default for arrays in json-schema is they are variable sized and allow more
|
|
entries than explicitly defined. This can be restricted by defining 'minItems',
|
|
'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
|
|
size is desired in most cases, so these properties are added based on the
|
|
number of entries in an 'items' list.
|
|
|
|
The YAML Devicetree format also makes all string values an array and scalar
|
|
values a matrix (in order to define groupings) even when only a single value
|
|
is present. Single entries in schemas are fixed up to match this encoding.
|
|
|
|
Testing
|
|
-------
|
|
|
|
Dependencies
|
|
~~~~~~~~~~~~
|
|
|
|
The DT schema project must be installed in order to validate the DT schema
|
|
binding documents and validate DTS files using the DT schema. The DT schema
|
|
project can be installed with pip::
|
|
|
|
pip3 install git+https://github.com/devicetree-org/dt-schema.git@master
|
|
|
|
Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
|
|
installed. Ensure they are in your PATH (~/.local/bin by default).
|
|
|
|
dtc must also be built with YAML output support enabled. This requires that
|
|
libyaml and its headers be installed on the host system. For some distributions
|
|
that involves installing the development package, such as:
|
|
|
|
Debian::
|
|
|
|
apt-get install libyaml-dev
|
|
|
|
Fedora::
|
|
|
|
dnf -y install libyaml-devel
|
|
|
|
Running checks
|
|
~~~~~~~~~~~~~~
|
|
|
|
The DT schema binding documents must be validated using the meta-schema (the
|
|
schema for the schema) to ensure they are both valid json-schema and valid
|
|
binding schema. All of the DT binding documents can be validated using the
|
|
``dt_binding_check`` target::
|
|
|
|
make dt_binding_check
|
|
|
|
In order to perform validation of DT source files, use the ``dtbs_check`` target::
|
|
|
|
make dtbs_check
|
|
|
|
Note that ``dtbs_check`` will skip any binding schema files with errors. It is
|
|
necessary to use ``dt_binding_check`` to get all the validation errors in the
|
|
binding schema files.
|
|
|
|
It is possible to run both in a single command::
|
|
|
|
make dt_binding_check dtbs_check
|
|
|
|
It is also possible to run checks with a single schema file by setting the
|
|
``DT_SCHEMA_FILES`` variable to a specific schema file.
|
|
|
|
::
|
|
|
|
make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
|
|
make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
|
|
|
|
|
|
json-schema Resources
|
|
---------------------
|
|
|
|
|
|
`JSON-Schema Specifications <http://json-schema.org/>`_
|
|
|
|
`Using JSON Schema Book <http://usingjsonschema.com/>`_
|
|
|
|
.. _example-schema:
|
|
|
|
Annotated Example Schema
|
|
------------------------
|
|
|
|
Also available as a separate file: :download:`example-schema.yaml`
|
|
|
|
.. literalinclude:: example-schema.yaml
|