linux-kernel - Re: [RFC v5 0/4] firmware: arm_scmi: Add SCMI v3.2 pincontrol protocol basic support

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Message-ID: <CACRpkdZ4GborirSpa3GK_PwMgCvY0ePEmZO+CwnLcP6nAdieow@mail.gmail.com>
Date:   Sun, 5 Nov 2023 22:50:31 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Oleksii Moisieiev <Oleksii_Moisieiev@...m.com>
Cc:     "sudeep.holla@....com" <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Takahiro Akashi <takahiro.akashi@...aro.org>
Subject: Re: [RFC v5 0/4] firmware: arm_scmi: Add SCMI v3.2 pincontrol
 protocol basic support

Hi Oleksii,

thanks for this patch, which still looks very good to me.

A question that was raised in discussion with Takahiro Akashi was how we
identify pins that can be used for GPIO and if the spec or implementation
has given any thought to that.

I can think of a few, such that:

1. Pins that can be used for GPIO all belong to some group - possibly even
  one group per pin such as "gpioA1", "gpioA2", "gpioA3" etc - that can be
  assigned a function named "gpio" or similar.

2. GPIO is seen as something external or "third usecase" that is handled
  by pin config, not by pin mux.

If it is 1 - which I find likely - it would be good to standardize the name of
the function to be "gpio" and somehow make it clear that all pins that are
desired to be used for GPIO need to have a (group, function) tuple pair
such as ("gpio001", "gpio") that will put the pin into GPIO mode.

If the assumption is anything goes, i.e. a vendor could say something
like ("io-group-99", "generic-io") to put a certain pin into GPIO mode,
that is maybe not so optimal, because it's nice for the GPIO driver
(which will come up) to be able to figure out by e.g. string name
conventions that a pin is in GPIO mode, and which group and function
that will put it into GPIO mode.

If this generality is not desired, having standard names for GPIO
functions and groups is still going to be an upside, if it can be achieved.
But maybe this isn't attainable at this point?

Yours,
Linus Walleij