[PATCH v4 00/10] gpio: improve support for shared GPIOs

[Andy Shevchenko]

On Wed, Nov 19, 2025 at 1:24 AM Linus Walleij <linus.walleij at linaro.org> wrote:
> On Tue, Nov 18, 2025 at 12:15 PM Geert Uytterhoeven
> <geert at linux-m68k.org> wrote:

...

> > We have a long-standing use-case on various Renesas R-Car Gen3 boards
> > (e.g. Salvator-X(S) and ULCB[1]), where GPIOs are shared by LEDs and
> > key switches.  Basically, the GPIO is connected to:
> >   1. A key switch connecting to GND when closed, with pull-up.
> >   2. The gate of an N-channel MOSFET, turning on an LED when driven
> >      high.
> >
> > Hence:
> >   - In output mode, the LED can be controlled freely,
> >   - In input mode, the LED is on, unless the key is pressed,
> >   - Hence the switch state can only be read when the LED is turned on.
>
> Fantastic solution to a lack of GPIO lines.

I feel a poster "SARCASM" behind this line :-)
That's what happened when old-school (in a bad term) HW engineers who
try to enforce their experience on the modern SoC-based platforms that
run GP OSes in multi-tasking, multi-user manner.

> This reminds me of the Amiga 500 power LED which was connected
> to a GPIO which was cleverly also reused to control the audio filter,
> with the effect that when you turned off the audio filter the power LED
> went out and music toggling the filter off/on for effects would also
> give you an incidental stroboscope.
>
> > If you have any idea how to handle this, feel free to reply ;-)
>
> Isn't it pretty clear from the system-level DTS how the line
> is used?
>
> If it is connected to a gpio key it gets assigned for that usecase
> and handled by that driver and if it is connected to a gpio LED
> it is handled by that driver.
>
> For the input usecase the status of the LED is a byproduct and
> should not reflect in software I think. It surely should not be
> controllable and possible to set into output mode because
> that sounds like a recipe for HW damage if you drive it
> actively high and press the key at the same time.
>
> gpio_keys {
>     compatible = "gpio-keys";
>
>     button-ok {
>         gpios = <&gpio 0 GPIO_OPEN_DRAIN | GPIO_PULL_UP>;
>     };
> };

This is my understanding as well.

-- 
With Best Regards,
Andy Shevchenko