linux-kernel - Re: [PATCH] gpio: export 'debounce' attribute if supported by the gpio chip

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Date:	Fri, 7 Dec 2012 08:20:35 -0800
From:	Guenter Roeck <groeck-dsl@...global.net>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCH] gpio: export 'debounce' attribute if supported by the
 gpio chip

On Fri, Dec 07, 2012 at 06:59:55AM -0800, Guenter Roeck wrote:
> On Fri, Dec 07, 2012 at 09:07:28AM +0100, Linus Walleij wrote:
> > On Thu, Dec 6, 2012 at 7:32 AM, Guenter Roeck <linux@...ck-us.net> wrote:
> > 
> > > Create a 'debounce' attribute if debounce is supported by the gpio
> > > chip and a gpio pin is exported.
> > >
> > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > 
> > Can you describe the usecase for this?
> > 
> > I have this problem when working as a back-up GPIO maintainer that
> > I don't really understand the userspace apps doing this.
> > 
> > I would guess something like a userspace app reading a GPIO switch
> > and needing to set this to avoid key bounces, but it'd be nice to know
> > if this is really the case.
> > 
> Yes, that is one if the use cases. Button pressed on the chassis/board
> requesting user space action. Another is board presence detect pins which
> require rebounce support and are handled in user space. Yes, the later should be
> handled in the kernel, and most of them are, but there are some which don't need
> immediate kernel activity and are handled completely by applications.
> 
> There may be other use cases - there are hundreds of gpio pins in the system I
> am working on, and I have not looked into all of them.
> 
There are three use cases, all related to each other.

- board present (connector pin)
- board removal request (button)
- board voltage good (connector pin)

Guenter

> > If this is the usecase I am slightly concerned why these are not used:
> > drivers/input/keyboard/gpio_keys_polled.c
> > drivers/input/keyboard/gpio_keys.c
> > 
> > The latter even uses the in-kernel debounce interface.
> > 
> > I'd agree if this is not user input at all but something like a switch
> > in a factory production line.
> > 
> I could imagine declaring the activity request buttons to be "input", but for
> presence detects it is a bit far fetched and would add too much complexity.
> 
> Thanks,
> Guenter
> 
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