linux-kernel - Re: [RFC PATCH] dt-bindings: add a jsonschema binding example

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Date:   Fri, 20 Apr 2018 14:47:22 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     devicetree@...r.kernel.org, devicetree-spec@...r.kernel.org,
        linux-kernel@...r.kernel.org, grant.likely@....com,
        frowand.list@...il.com, mark.rutland@....com,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Mark Brown <broonie@...nel.org>, shawnguo@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Stephen Boyd <sboyd@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>
Subject: Re: [RFC PATCH] dt-bindings: add a jsonschema binding example

On Wed 18 Apr 15:29 PDT 2018, Rob Herring wrote:

> The current DT binding documentation format of freeform text is painful
> to write, review, validate and maintain.
> 
> This is just an example of what a binding in the schema format looks
> like. It's using jsonschema vocabulary in a YAML encoded document. Using
> jsonschema gives us access to existing tooling. A YAML encoding gives us
> something easy to edit.
> 
> This example is just the tip of the iceberg, but it the part most
> developers writing bindings will interact with. Backing all this up
> are meta-schema (to validate the binding schemas), some DT core schema,
> YAML encoded DT output with dtc, and a small number of python scripts to
> run validation. The gory details including how to run end-to-end
> validation can be found here:
> 
> https://www.spinics.net/lists/devicetree-spec/msg00649.html
> 
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
> Cc list,
> You all review and/or write lots of binding documents. I'd like some feedback
> on the format.
> 

I really like the idea of formalizing the binding document format and
the ability of validating a dtb is really nice.

> Thanks,
> Rob
> 
>  .../devicetree/bindings/example-schema.yaml        | 149 +++++++++++++++++++++
>  1 file changed, 149 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/example-schema.yaml
> 
> diff --git a/Documentation/devicetree/bindings/example-schema.yaml b/Documentation/devicetree/bindings/example-schema.yaml
[..]
> +  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

I assume that a reg with variable number of entries would have
"description" for all of them and then a minItems that matches the
required ones and maxItems matching all of them?

> +
> +  reg-names:
> +    # The core schema enforces this is a string array
> +    items:
> +      - const: core
> +      - const: aux

I presume validation based on this should check that there's equal
number of entries in reg-names as there where in reg. Should this
relationship be described in the schema?

[..]
> +  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.

For validation purposes this could be interrupts with interrupt-parents
or a interrupts-extend, a fact that we probably don't want to duplicate
in every definition?

Perhaps we should just do like you did here and define the "interrupts"
and then in the validation tool - where we need to encode the logic
behind this anyways - we support the different variants.

> +
> +  interrupt-names:
> +    # minItems must be specified here because the default would be 2
> +    minItems: 1

As with reg-names, it would be good to have the validator warn if this
is not the same number of items as entries in "interrupts".

> +    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

If this is specified then interrupt-controller should also be given, or
vise versa. How would we describe that?

> +
> +  interrupt-controller: {}
> +    # The core checks this is a boolean, so just have to list it here to be
> +    # valid for this binding.
> +

Regards,
Bjorn