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

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Message-ID: <CACRpkdYesD9sRQZXQNEaBY2Ouu3bjKKGWpRtU-Lpa4AcjyPwXw@mail.gmail.com>
Date:   Mon, 25 May 2020 10:50:22 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Lars Povlsen <lars.povlsen@...rochip.com>
Cc:     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

On Mon, May 18, 2020 at 10:50 PM Lars Povlsen
<lars.povlsen@...rochip.com> wrote:
> 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).

I don't understand this, you would have to elaborate...

In any case microchip,sgpio-ports is probably not the right thing,
use ngpios which is documented and just divide by 32 to get the
number of ports I think? But that is just in case they get
enabled strictly in sequence, otherwise you'd need a custom
property.

Yours,
Linus Walleij