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 Annotated 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 $ref value with only a relative path or filename will be prepended with the hostname and path components of the current schema file’s ‘$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 of the hardware being described in 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 hardware. It should contain things such as what the block or device does, standards the device conforms to, and links to datasheets for more information.
The YAML format has several options for defining the formatting of the text block. The options are controlled with indicator characters following the key (e.g. “description: |”). The minimum formatting needed for a block should be used. The formatting controls can not only affect whether the YAML can be parsed correctly, but are important when the text blocks are rendered to another form. The options are as follows.
The default without any indicators is flowed, plain scalar style where single line breaks and leading whitespace are stripped. Paragraphs are delimited by blank lines (i.e. double line break). This style cannot contain “: “ in it as it will be interpretted as a key. Any “ #” sequence will be interpretted as a comment. There’s other restrictions on characters as well. Most restrictions are on what the first character can be.
The second style is folded which is indicated by “>” character. In addition to maintaining line breaks on double line breaks, the folded style also maintains leading whitespace beyond indentation of the first line. The line breaks on indented lines are also maintained.
The third style is literal which is indicated by “|” character. The literal style maintains all line breaks and whitespace (beyond indentation of the first line).
The above is not a complete description of YAML text blocks. More details on multi-line YAML text blocks can be found online:
- 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.
- additionalProperties / unevaluatedProperties
Keywords controlling how schema will validate properties not matched by this schema’s ‘properties’ or ‘patternProperties’. Each schema is supposed to have exactly one of these keywords in top-level part, so either additionalProperties or unevaluatedProperties. Nested nodes, so properties being objects, are supposed to have one as well.
- additionalProperties: false
Most common case, where no additional schema is referenced or if this binding allows subset of properties from other referenced schemas.
- unevaluatedProperties: false
Used when this binding references other schema whose all properties should be allowed.
- additionalProperties: true
Rare case, used for schemas implementing common set of properties. Such schemas are supposed to be referenced by other schemas, which then use ‘unevaluatedProperties: false’. Typically bus or common-part schemas.
- examples
Optional. A list of one or more DTS hunks implementing this binding only. Example should not contain unrelated device nodes, e.g. consumer nodes in a provider binding, other nodes referenced by phandle. 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 are 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.
Coding style¶
Use YAML coding style (two-space indentation). For DTS examples in the schema, preferred is four-space indentation.
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 dtschema
Note that ‘dtschema’ installation requires ‘swig’ and Python development files installed first. On Debian/Ubuntu systems:
apt install swig python3-dev
Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be installed. Ensure they are in your PATH (~/.local/bin by default).
Recommended is also to install yamllint (used by dtschema when present).
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 subset of matching schema files by
setting the DT_SCHEMA_FILES variable to 1 or more specific schema files or
patterns (partial match of a fixed string). Each file or pattern should be
separated by ‘:’.
make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
make dt_binding_check DT_SCHEMA_FILES=/gpio/
make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml
json-schema Resources¶
Annotated Example Schema¶
Also available as a separate file: example-schema.yaml
# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
# Copyright 2018 Linaro Ltd.
%YAML 1.2
---
# All the top-level keys are standard json-schema keywords except for
# 'maintainers' and 'select'
# $id is a unique identifier based on the filename. There may or may not be a
# file present at the URL.
$id: http://devicetree.org/schemas/example-schema.yaml#
# $schema is the meta-schema this schema should be validated with.
$schema: http://devicetree.org/meta-schemas/core.yaml#
title: An Example Device
maintainers:
- Rob Herring <robh@kernel.org>
description: |
A more detailed multi-line description of the binding.
Details about the hardware device and any links to datasheets can go here.
Literal blocks are marked with the '|' at the beginning. The end is marked by
indentation less than the first line of the literal block. Lines also cannot
begin with a tab character.
select: false
# 'select' is a schema applied to a DT node to determine if this binding
# schema should be applied to the node. It is optional and by default the
# possible compatible strings are extracted and used to match.
# In this case, a 'false' schema will never match.
properties:
# A dictionary of DT properties for this binding schema
compatible:
# More complicated schema can use oneOf (XOR), anyOf (OR), or allOf (AND)
# to handle different conditions.
# In this case, it's needed to handle a variable number of values as there
# isn't another way to express a constraint of the last string value.
# The boolean schema must be a list of schemas.
oneOf:
- items:
# items is a list of possible values for the property. The number of
# values is determined by the number of elements in the list.
# Order in lists is significant, order in dicts is not
# Must be one of the 1st enums followed by the 2nd enum
#
# Each element in items should be 'enum' or 'const'
- enum:
- vendor,soc4-ip
- vendor,soc3-ip
- vendor,soc2-ip
- const: vendor,soc1-ip
# additionalItems being false is implied
# minItems/maxItems equal to 2 is implied
- items:
# 'const' is just a special case of an enum with a single possible value
- const: vendor,soc1-ip
reg:
# The core schema already checks that reg values are numbers, so device
# specific schema don't need to do those checks.
# The description of each element defines the order and implicitly defines
# the number of reg entries.
items:
- description: core registers
- description: aux registers
# minItems/maxItems equal to 2 is implied
reg-names:
# The core schema enforces this (*-names) is a string array
items:
- const: core
- const: aux
clocks:
# Cases that have only a single entry just need to express that with maxItems
maxItems: 1
description: bus clock. A description is only needed for a single item if
there's something unique to add.
The items should have a fixed order, so pattern matching names are
discouraged.
clock-names:
# For single-entry lists in clocks, resets etc., the xxx-names often do not
# bring any value, especially if they copy the IP block name. In such case
# just skip the xxx-names.
items:
- const: bus
interrupts:
# Either 1 or 2 interrupts can be present
minItems: 1
items:
- description: tx or combined interrupt
- description: rx interrupt
description:
A variable number of interrupts warrants a description of what conditions
affect the number of interrupts. Otherwise, descriptions on standard
properties are not necessary.
The items should have a fixed order, so pattern matching names are
discouraged.
interrupt-names:
# minItems must be specified here because the default would be 2
minItems: 1
items:
- const: tx irq
- const: rx irq
# Property names starting with '#' must be quoted
'#interrupt-cells':
# A simple case where the value must always be '2'.
# The core schema handles that this must be a single integer.
const: 2
interrupt-controller: true
# The core checks this is a boolean, so just have to list it here to be
# valid for this binding.
clock-frequency:
# The type is set in the core schema. Per-device schema only need to set
# constraints on the possible values.
minimum: 100
maximum: 400000
# The value that should be used if the property is not present
default: 200
foo-gpios:
maxItems: 1
description: A connection of the 'foo' gpio line.
# *-supply is always a single phandle, so nothing more to define.
foo-supply: true
# Vendor-specific properties
#
# Vendor-specific properties have slightly different schema requirements than
# common properties. They must have at least a type definition and
# 'description'.
vendor,int-property:
description: Vendor-specific properties must have a description
$ref: /schemas/types.yaml#/definitions/uint32
enum: [2, 4, 6, 8, 10]
vendor,bool-property:
description: Vendor-specific properties must have a description. Boolean
properties are one case where the json-schema 'type' keyword can be used
directly.
type: boolean
vendor,string-array-property:
description: Vendor-specific properties should reference a type in the
core schema.
$ref: /schemas/types.yaml#/definitions/string-array
items:
- enum: [foo, bar]
- enum: [baz, boo]
vendor,property-in-standard-units-microvolt:
description: Vendor-specific properties having a standard unit suffix
don't need a type.
enum: [ 100, 200, 300 ]
vendor,int-array-variable-length-and-constrained-values:
description: Array might define what type of elements might be used (e.g.
their range).
$ref: /schemas/types.yaml#/definitions/uint32-array
minItems: 2
maxItems: 3
items:
minimum: 0
maximum: 8
child-node:
description: Child nodes are just another property from a json-schema
perspective.
type: object # DT nodes are json objects
# Child nodes also need additionalProperties or unevaluatedProperties
additionalProperties: false
properties:
vendor,a-child-node-property:
description: Child node properties have all the same schema
requirements.
type: boolean
required:
- vendor,a-child-node-property
# Describe the relationship between different properties
dependencies:
# 'vendor,bool-property' is only allowed when 'vendor,string-array-property'
# is present
vendor,bool-property: [ 'vendor,string-array-property' ]
# Expressing 2 properties in both orders means all of the set of properties
# must be present or none of them.
vendor,string-array-property: [ 'vendor,bool-property' ]
required:
- compatible
- reg
- interrupts
- interrupt-controller
# if/then schema can be used to handle conditions on a property affecting
# another property. A typical case is a specific 'compatible' value changes the
# constraints on other properties.
#
# For multiple 'if' schema, group them under an 'allOf'.
#
# If the conditionals become too unweldy, then it may be better to just split
# the binding into separate schema documents.
allOf:
- if:
properties:
compatible:
contains:
const: vendor,soc2-ip
then:
required:
- foo-supply
else:
# If otherwise the property is not allowed:
properties:
foo-supply: false
# Altering schema depending on presence of properties is usually done by
# dependencies (see above), however some adjustments might require if:
- if:
required:
- vendor,bool-property
then:
properties:
vendor,int-property:
enum: [2, 4, 6]
# Ideally, the schema should have this line otherwise any other properties
# present are allowed. There's a few common properties such as 'status' and
# 'pinctrl-*' which are added automatically by the tooling.
#
# This can't be used in cases where another schema is referenced
# (i.e. allOf: [{$ref: ...}]).
# If and only if another schema is referenced and arbitrary children nodes can
# appear, "unevaluatedProperties: false" could be used. A typical example is
# an I2C controller where no name pattern matching for children can be added.
additionalProperties: false
examples:
# Examples are now compiled with dtc and validated against the schemas
#
# Examples have a default #address-cells and #size-cells value of 1. This can
# be overridden or an appropriate parent bus node should be shown (such as on
# i2c buses).
#
# Any includes used have to be explicitly included. Use 4-space indentation.
- |
node@1000 {
compatible = "vendor,soc4-ip", "vendor,soc1-ip";
reg = <0x1000 0x80>,
<0x3000 0x80>;
reg-names = "core", "aux";
interrupts = <10>;
interrupt-controller;
#interrupt-cells = <2>;
};