linux-kernel - Re: [PATCH/RFC] hardware irq debouncing support

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Date:	Fri, 3 Oct 2008 11:22:14 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	David Brownell <david-b@...bell.net>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Victor <linux@...im.org.za>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH/RFC] hardware irq debouncing support

David Brownell <david-b@...bell.net> wrote:
> Hardware IRQ debouncing is common for IRQ controllers which are
> part of GPIO modules ... they often deal with mechanical switches,
> buttons, and so forth.  This patch:
> 
>  - Provides simple support for that in genirq
> 
>  - Includes sample implementations for some Linux systems
>    which already include non-generic support for this:
> 
>      * Atmel SOCs (AT91, AT32 -- the same GPIO module)
>      * OMAP2/OMAP3 (not quite as simple)
> 
> Control over how long to debounce is less common, and often applies
> to banks of GPIOs not individual signals ... not addressed here.
> 
> Drivers can request this (where available) with a new IRQF_DEBOUNCED
> flag/hint passed to request_irq():
> 
>     IF that flag is set when a handler is registered
> 	AND the relevant irq_chip supports debouncing
> 	AND the IRQ isn't already flagged as being debounced
>     THEN the irq_chip is asked to enable debouncing for this IRQ
> 	UNTIL the IRQ's last handler is unregistered.
>     ELSE
>         nothing is done ... the hint is ignored
>     FI
> 
> Comments?

Note that at least for AT91 and AVR32, the terminology used is "glitch
filtering", not "debouncing". The filtering period is very short (half
a clock cycle), so it probably won't help much when dealing with
mechanical switches and buttons.

What kind of guarantees should IRQF_DEBOUNCE provide? Filtering short
glitches may be useful, but if drivers start assuming it will do real
debouncing of badly filtered switches and buttons, I think we're in for
some trouble...

> Having this mechanism in genirq would let boards remove a bunch of
> nonportable code, and would let drivers like gpio_keys, gpio_mouse,
> and various touchscreens work more reliably.  It'd also let various
> SOC-specific MMC and CF card drivers switch over to more standard
> (and widely understandable) mechanisms.
> 
> I'd like to submit such updates for the 2.6.28 merge window, in
> part to let mainline avoid needing yet another driver-specific
> programming interface for IRQ debouncing.  (For TWL4030/TPS659x0,
> as used in most OMAP3 boards including the Gumstix Overo and the
> BeagleBoard.)

Given that the limitations of this interface are clearly documented, I'm
all for it.

What would be perhaps even more useful is generic software debouncing
support. Something like

  int request_debounced_irq(int irq, unsigned long debounce_us,
		void (*handler)(void *data), void *data);

which would set up a handler which disables the interrupt and sets up a
timer which will ack/unmask the interrupt and run the handler.

This would mean the "interrupt handler" really gets run in softirq
context, and shared interrupt would probably be impossible to support,
but I think it would be useful for certain kinds of interrupts.

What do you think?

Haavard
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