linux-kernel - Re: [PATCH] gpiolib: add GPIO_SET_DEBOUNCE

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Message-ID: <CACRpkdaeXFW5K=Npy2ubWsffc7aepEQ5kSJ2HrkrESjaTy_psQ@mail.gmail.com>
Date:   Wed, 29 Apr 2020 14:38:13 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Hector Bujanda <hector.bujanda@...i.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kent Gibson <warthog618@...il.com>
Subject: Re: [PATCH] gpiolib: add GPIO_SET_DEBOUNCE_IOCTL

On Wed, Apr 29, 2020 at 2:06 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:

> I understand the need to set debounce time to make line events
> reliable. As I see it: there'll be a couple steps to add this.

I think there is a serious user-facing problem here though, because
not all GPIO controllers supports debounce, so the call may return
"nope" (error code).

I think that is unavoidable with things like pull-up/down or drive
strength, but for debounce I think we could do better.
drivers/input/keyboard/gpio_keys.c contains generic
debounce code using kernel timers if the GPIO driver
cannot provide debouncing, and I have thought for a long
time that it would be nice if we could do this generic, so that
we always provide debouncing if requested, even for in-kernel
consumers but most certainly for userspace consumers,
else userspace will just start to reinvent this too.

Yours,
Linus Walleij