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
[off-list ref] wrote:

On 23/09/2021 08:32, Geert Uytterhoeven wrote:

quoted

On Wed, Sep 22, 2021 at 10:30 PM Kieran Bingham
[off-list ref] wrote:

quoted

Add support for SW46-1 and SW46-2 as switches using the gpio-keys
framework.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.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@linux-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