linux-kernel - Re: [PATCH] arm64: dts: renesas: r8a779a0: falcon-cpu: Add SW46 switch support

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Date:   Thu, 23 Sep 2021 14:47:20 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Kieran Bingham <kieran.bingham@...asonboard.com>
Cc:     Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-input <linux-input@...r.kernel.org>
Subject: Re: [PATCH] arm64: dts: renesas: r8a779a0: falcon-cpu: Add SW46
 switch support

Hi Kieran,

On Thu, Sep 23, 2021 at 2:17 PM Kieran Bingham
<kieran.bingham@...asonboard.com> wrote:
> On 23/09/2021 08:32, Geert Uytterhoeven wrote:
> > On Wed, Sep 22, 2021 at 10:30 PM Kieran Bingham
> > <kieran.bingham@...asonboard.com> wrote:
> >> Add support for SW46-1 and SW46-2 as switches using the gpio-keys
> >> framework.
> >>
> >> Signed-off-by: Kieran Bingham <kieran.bingham@...asonboard.com>
> >>         keys_pins: keys {
> >> -               pins = "GP_6_18", "GP_6_19", "GP_6_20";
> >> +               pins = "GP_1_28", "GP_1_29",
> >> +                      "GP_6_18", "GP_6_19", "GP_6_20";
> >>                 bias-pull-up;
> >>         };
> >
> > This part is not needed, as the GPIOs connected to the slide switches
> > have external pull-up resistors (unlike the GPIOs connected to the
> > push switches, which are driven low by open-drain buffers, without
> > external pull-up resistors).
>
> Ah - for some reason I thought it was required to configure the PFC
> regardless, and show that these pins are acquired by the gpio function -
> but of course I'd expect 'getting' the gpio would do that..

That should work automatically, for a GPIO.

> Out of interest, is the OD buffer there to act as a hardware debounce or
> such? or is there another likely reason?

Perhaps to improve sharing of the GPIO through the expansion connector?
Other Renesas boards use the exact same input circuitry, with a
capacitor and resistor for debouncing, but without the OD buffer, and
they also provide access to the GPIO through an expansion connector.
It's even a plain buffer, without schmitt-trigger inputs.  Personally,
I would have taken one with schmitt-trigger functionality, if I would
have bothered with adding a buffer in the first place (but I'm not
a real hardware engineer ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds