linux-kernel - Re: [RFC v2 5/5] dt-bindings: gpio: Add bindings for pinctrl based generic gpio driver

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Date:   Tue, 17 Oct 2023 11:32:11 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Rob Herring <robh@...nel.org>, sudeep.holla@....com,
        cristian.marussi@....com, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.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 v2 5/5] dt-bindings: gpio: Add bindings for pinctrl based
 generic gpio driver

Hi Linus,

On Thu, Oct 12, 2023 at 09:25:20AM +0200, Linus Walleij wrote:
> On Tue, Oct 10, 2023 at 7:25???AM AKASHI Takahiro
> <takahiro.akashi@...aro.org> wrote:
> 
> > > We can probably mandate that this has to be inside a pin controller
> > > since it is a first.
> >
> > Yeah, my U-Boot implementation tentatively supports both (inside and
> > outside pin controller). But it is not a user's choice, but we should
> > decide which way to go.
> 
> OK I have decided we are going to put it inside the pin control node,
> as a subnode. (I don't expect anyone to object.)

While I'm still thinking of how I can modify my current implementation
to fit into 'inside' syntax, there are a couple of concerns:

1) invoke gpiochip_add_data() at probe function
Probably we no longer need "compatible" property, but instead we need to
call gpiochip_add_data() explicitly in SCMI pin controller's probe
as follows:

scmi_pinctrl_probe()
    ...
    devm_pinctrl_register_and_init(dev, ..., pctrldev);
    pinctrl_enable(pctrldev);

    device_for_each_child_node(dev, fwnode)
        if (fwnode contains "gpio-controller") {
            /* what pin_control_gpio_probe() does */
            gc->get_direction = ...;
            ...
            devm_gpiochip_data_add(dev, gc, ...);
        }

2) gpio-by-pinctrl.c
While this file is SCMI-independent now, due to a change at (1),
it would be better to move the whole content inside SCMI pin controller
driver (because there is no other user for now).

3) Then, pin-control-gpio.yaml may also be put into SCMI binding
(i.e. firmware/arm,scmi.yaml). Can we leave the gpio binding outside?

4) phandle in "gpio-ranges" property
(As you mentioned)
The first element in a tuple of "gpio-ranges" is a phandle to a pin
controller node. Now that the gpio node is a sub node of pin controller,
the phandle is trivial. But there is no easier way to represent it
than using an explicit label:
(My U-Boot implementation does this.)

scmi {
    ...
    scmi_pinctrl: protocol@19 {
        ...
        gpio {
            gpio-controller;
            ...
            gpio-ranges = <&scmi_pinctrl ... >;
        }
    }
}

I tried:
    gpio-ranges = <0 ...>; // dtc passed, but '0' might be illegal by spec.
    gpio-ranges = <(-1) ...>; // dtc passed, but ...
    gpio-ranges = <&{..} ...>; // dtc error because it's not a full path.

Do you have any other idea? Otherwise, I will modify my RFC
with the changes above.

-Takahiro Akashi


> It makes everything easier and clearer for users I think.
> 
> Yours,
> Linus Walleij