[RFC v5 5/5] dt-bindings: firmware: arm,scmi: Add support for pinctrl protocol

[Souvik Chakravarty]

Hi Linus,

On 13/11/2023 13:32, Linus Walleij wrote:
> Hi Souvik,
> 
> thanks for looking into this!
> 
> On Mon, Nov 13, 2023 at 1:56 PM Souvik Chakravarty
> <souvik.chakravarty at arm.com> wrote:
> 
>> The initial assumption always was that GPIOs can be considered as a
>> specific function. Note that the spec does not define the types of
>> function and leaves it to the DT binding (or driver) to figure out the
>> function descriptions/names.
> 
> Does this mean that each system using pinctrl-SCMI will need
> to specify the available pins, groups and functions in a device tree
> binding? For e.g. DT validation using schema?

Sorry seems I made a typo above ("descriptions/names" should have been 
"description from names") which resulted in turning things on its head.

I really meant that the driver has to figure out the exact type or 
meaning of what the function does from its name. SCMI still continues to 
provide the list of pins/groups/functions and their names.

Regards,
Souvik

> 
> This creates the problem of where to put it since
> Documentation/devicetree/bindings/firmware/arm,scmi.yaml
> is all we have, and for schemas to be applicable the implicit
> assumption is that this is done per-compatible.
> 
> If we want to use device tree validation of the strings put into
> the pinctrl node we need to allow for a per-soc compatible
> under the pinctrl node like:
> 
> protocol at 19 {
>      compatible = "vendor,soc-scmi-pinctrl";
> (...)
> 
> Then a DT schema can be made to match that and check it.
> 
> I'm uncertain about that because the SCMI binding has nothing
> like this at the moment, all the protocol nodes are pretty
> self-describing and don't seem to need any further configuration
> to be used, but pin control may be the first instance where we
> have to add some per-soc configuration into the protocol nodes :/
> 
> It's easy to do:
> 
> +  protocol at 19:
> +    type: object
> +    allOf:
> +      - $ref: "#/$defs/protocol-node"
> +      - $ref: "../pinctrl/pinctrl.yaml"
> +    unevaluatedProperties: false
> +
> +    properties:
> 
>          compatible:
>              items:
>                - enum:
>                     - vendor1,soc1-scmi-pinctrl
>                     - vendor2,soc2-scmi-pinctrl
>                     - vendor3,soc3-scmi-pinctrl
> 
> This should be enough for just establishing the different
> pin control configurations we can have in the device tree.
> 
> We are then able to put a more detailed schema for the
> specific SoC pin control, such as a list of valid groups and
> functions etc under the ordinary pinctrl bindings such as
> Documentation/devicetree/bindings/pinctrl/vendor1,soc1-scmi-pinctrl.yaml
> etc.
> 
> We should preferably put some pattern like this in place from
> day 1 so developers know what is expected here. A mock
> SoC is fine for the time being (we can delete it later when there
> are some serious ones).
> 
> I'm uncertain because it feels like a first thing, but I can't really
> think how it would work otherwise, part of me don't want to
> pollute the SCMI binding with any per-soc compatibles, but
> yet since these group and function strings will be per-soc I don't
> see any other way, if they are supposed to be validated
> with schema.
> 
> Yours,
> Linus Walleij