[08/36] dt-bindings: Add a writing DT schemas how-to and annotated example

diff --git a/Documentation/devicetree/bindings/example-schema.yaml b/Documentation/devicetree/bindings/example-schema.yaml
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+# SPDX-License-Identifier: 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'
+
+# $id is a unique idenifier based on the filename
+$id: "http://devicetree.org/schemas/example-schema.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: An example schema annotated with jsonschema details
+
+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.
+
+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.
+
+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
+          - enum:
+              - 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 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 is a string array
+    items:
+      - const: core
+      - const: aux
+
+  clocks:
+    maxItems: 1
+    # Only a single entry, so just need to set the max number of items.
+    description: hello
+
+  clock-names:
+    items:
+      - const: bus
+
+  interrupts:
+    # Either 1 or 2 interrupts can be present
+    minItems: 1
+    maxItems: 2
+    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.
+
+  interrupt-names:
+    # minItems must be specified here because the default would be 2
+    minItems: 1
+    maxItems: 2
+    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.
+
+  vendor,int-property:
+    description: Vendor specific properties must have a description
+    allOf:
+      - $ref: "/schemas/types.yaml#/definitions/uint32"
+      - enum: [2, 4, 6, 8, 10]
+
+  vendor,bool-property:
+    description: Vendor specific properties must have a description
+    type: boolean
+
+  vendor,string-array-property:
+    description: Vendor specific properties may refer to core schema
+    allOf:
+      - $ref: '/schemas/types.yaml#/definitions/stringarray'
+      - maxItems: 2
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - interrupt-controller
+
+examples:
+  - |
+    node@1000 {
+          compatible = "vendor,soc4-ip", "vendor,soc1-ip";
+          reg = <0x1000 0x80>,
+                <0x3000 0x80>;
+          reg-names = "core", "aux";
+          interrupts = <10>;
+          interrupt-controller;
+    };
diff --git a/Documentation/devicetree/writing-schema.md b/Documentation/devicetree/writing-schema.md
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+# Writing DeviceTree Bindings in json-schema
+
+Devicetree bindings are written using json-schema vocabulary. Schema files are
+written in a JSON compatible subset of YAML. YAML is used instead of JSON as it
+considered more human readable and has some advantages such as allowing
+comments (Prefixed with '#').
+
+## Schema Contents
+
+Each schema doc is a structured json-schema which is defined by a set of
+top-level properties:
+
+- __$id__ - A json-schema unique identifier string. The string must be a valid
+URI typically containing the binding's filename and path. DT schema it must
+begin with "http://devicetree.org/schema/". The URL is used for non file local
+"$ref" lookups. A URL is used even for local files, and there may not actually
+be files present at those locations.
+
+- __$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 to match nodes for applying the schema.
+By default, nodes are matched against their possible compatible string values
+or node name and select is not necessary.
+
+- __properties__ - A set of sub-schema defining all the DT properties for the
+binding. The exact schema depends on the type of property. A property can also
+define a child DT node with child properties defined under it.
+
+- __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.
+
+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/robherring/yaml-bindings.git@master --process-dependency-links`
+
+dtc must also be built with YAML output support enabled. This requires that
+libyaml and its headers be installed on the host system.
+
+### 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`
+
+This will first run the `dt_binding_check` which generates the processed schema.
+
+## json-schema Resources
+
+[JSON-Schema Specifications](http://json-schema.org/)
+
+[Using JSON Schema Book](http://usingjsonschema.com/)