linux-kernel - Re: [RFC 4/4] dt-bindings: gpio: Add bindings for SCMI pinctrl based gpio

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Date:   Mon, 2 Oct 2023 15:58:27 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     Rob Herring <robh@...nel.org>
Cc:     AKASHI Takahiro <takahiro.akashi@...aro.org>, sudeep.holla@....com,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        linus.walleij@...aro.org, Oleksii_Moisieiev@...m.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [RFC 4/4] dt-bindings: gpio: Add bindings for SCMI pinctrl based
 gpio

On Mon, Oct 02, 2023 at 09:41:55AM -0500, Rob Herring wrote:
> On Mon, Oct 02, 2023 at 11:16:02AM +0900, AKASHI Takahiro wrote:
> > A dt binding for SCMI pinctrl based gpio driver is defined in this
> > commit. It basically conforms to generic pinctrl-gpio mapping framework.

[ snip]

> > +    additionalProperties: false
> > +
> > +required:
> > +  - compatible
> > +  - gpio-controller
> > +  - "#gpio-cells"
> > +  - gpio-ranges
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > +  - |
> > +    #include <dt-bindings/gpio/gpio.h>
> > +
> > +    scmi_gpio_0: scmi_gpio@0 {
> 
> gpio {
> 
> But doesn't SCMI have protocol numbers?
> 

My understanding is that this RFC GPIO driver from Akashi is built
completely on Pinctrl facilities (as he says in the cover), it is not
indeed a typical pure SCMI driver, it just happen to trigger the use
of SCMI if the underlying backend pinctrl driver is pinctrl-scmi;
but this driver does not really call directly into any SCMI API by
itself, i.e. it does not get and call any SCMI protocol ops.
(but it could indeed trigger the backend Pinctrl SCMI driver to issue
 such call on its behalf AFAIU...)

I wonder why it has even a dependency on PINCTRL_SCMI at this point;
is not that it could work (generically) even if the backend Pinctrl
driver is NOT SCMI ?
What makes it usable only against an SCMI Pinctrl backend ?
Cannot be a generic GPIO driver based on top of Pinctrl, no matter which
Pinctrl backend driver has been configured ?

...I maybe missing something here about Pinctrl AND GPIO frameworks :P

Thanks,
Cristian