linux-kernel - Re: [PATCH 1/3] dt-bindings: pinctrl: Add bindings for mscc,ocelot-sgpio

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Date:   Mon, 18 May 2020 22:49:56 +0200
From:   Lars Povlsen <lars.povlsen@...rochip.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Lars Povlsen <lars.povlsen@...rochip.com>,
        SoC Team <soc@...nel.org>, "Rob Herring" <robh+dt@...nel.org>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH 1/3] dt-bindings: pinctrl: Add bindings for mscc,ocelot-sgpio


Linus Walleij writes:

> On Wed, May 13, 2020 at 4:11 PM Lars Povlsen <lars.povlsen@...rochip.com> wrote:
>
>> This adds DT bindings for the Microsemi SGPIO controller, bindings
>> mscc,ocelot-sgpio and mscc,luton-sgpio.
>>
>> Reviewed-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
>> Signed-off-by: Lars Povlsen <lars.povlsen@...rochip.com>
>
>> +  microchip,sgpio-ports:
>> +    description: This is a 32-bit bitmask, configuring whether a
>> +      particular port in the controller is enabled or not. This allows
>> +      unused ports to be removed from the bitstream and reduce latency.
>> +    $ref: "/schemas/types.yaml#/definitions/uint32"
>
> I don't know about this.
>
> You are saying this pin controller can have up to 32 GPIO "ports"
> (also known as banks).
>
> Why can't you just represent each such port as a separate GPIO
> node:
>
> pinctrl@nnn {
>     gpio@0 {
>         ....
>     };
>     gpio@1 {
>         ....
>     };
>     ....
>     gpio@31 {
>         ....
>     };
> };
>
> Then if some of them are unused just set it to status = "disabled";
>
> This also makes your Linux driver simpler because each GPIO port
> just becomes a set of 32bit registers and you can use
> select GPIO_GENERIC and bgpio_init() and save a whole
> slew of standard stock code.
>

Linus, thank you for your input.

The controller handles an array of 32*n signals, where n >= 1 && n <=
4.

The problem with the above approach is that the ports are disabled
*port*-wise - so they remove all (upto) 4 bits. That would be across the
banks.

You could of course have the "implied" semantics that a disabled port at
any bit position disabled all (bit positions for the same port).

But I don't know if this would be easier to understand, DT-wise.

What do you think...?

> Yours,
> Linus Walleij

-- 
Lars Povlsen,
Microchip