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

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Date:   Wed, 27 May 2020 07:31:20 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Kent Gibson <warthog618@...il.com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Hector Bujanda <hector.bujanda@...i.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: add GPIO_SET_DEBOUNCE_IOCTL

On Mon, May 25, 2020 at 5:17 PM Kent Gibson <warthog618@...il.com> wrote:

> > I suppose gpiolib would have to steal or intercept the interrupt
> > by using e.g. IRQF_SHARED and then just return IRQ_HANDLED
> > on the first IRQ so the underlying irq handler does not get called.
>
> And how would gpiolib ensure that it was first in the chain?

I don't know.

> Totally agree with the concept - just trying to work out how to
> implement it seemlessly given the existing API and usage, and given my
> limited knowledge of the kernel internals.

The irqchip maintainers certainly know the answer for the question
of shared interrupts at least.

> > Failure is an option! Sorry if I push too complex ideas.
>
> I'm not as concerned about complexity as I am about fragility.
>
> I don't see any problem adding debounce for gpiolib-cdev.
> Adding a more complete solution to gpiolib itself is certainly
> non-trivial, if it is possible at all.

I agree. It's just that I perceive it as more elegant if we can do that.

> The path I'll probably be taking is adding a debouncer to gpiolib-cdev,
> so at least we have a solution for userspace, then take a longer look at
> the more general solution.

That's fine! Thanks for looking into this.

Linus Walleij