linux-kernel - Re: [PATCH] gpiolib: allow poll on gpio value

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Date:	Fri, 5 Jun 2009 23:01:39 -0700
From:	David Brownell <david-b@...bell.net>
To:	Ben Nizette <bn@...sdigital.com>
Cc:	Daniel Glöckner <dg@...ix.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gpiolib: allow poll on gpio value

On Friday 05 June 2009, Ben Nizette wrote:
> 
> Hey, good stuff Daniel.  There's a fair few common features missing but
> they can be added at a later date.
> 
> - Ability to honour rising and falling filters even if the hardware only
> supports both-edge (as lots of gpio interrupts do)

I suspect that's inadvisable.  Userspace code will need to know
what trigger model it's using, yes?  Lies are doubleplusungood.

> - Support for polling the gpio at some interval for gpios which don't
> support irqs at all

I had that thought.  Units ... seconds?  Milliseconds mapped to HZ?
Could come later.

> - Debounce support

Software?  Hardware capabilties vary *widely* ... three cases that
come quickly to mind:  (a) twl4030 fixed 30 msec delays, (b) at91
and avr32 "deglitch" filter, just syncs to a clock that's likely
from 30-100 MHz, (c) omap2/omap3, up to 255 cycles of 32 KiHz clock
but appplies entire GPIO banks, (d) DaVinci, no hardware support.

I can imagine a standard software filter option, but that would
need to be a separate sysfs mechanism since it wouldn't always
be desired.  (And separate patch, if needed.)

For hardware options ... do that by hardware-specific sysfs hooks
if they're really needed.

> - Reporting of number of changes since last read

Feels a more than bit overkilled by now.  ;)

> These are all things which exist in many out-of-tree or
> platform-specific implementations of this kind of thing and until
> they're there I reckon people will largely stick with what they've got.
> But that's really their problem of course, this is still valuable.
> 
> Regarding the code itself, not much but:
> 
> On Fri, 2009-06-05 at 16:36 +0200, Daniel Glöckner wrote:
> >  
> > +	"poll_edge" ... reads as either "none", "rising", "falling", or
> 
> IMO this is misleading, sounds like you're polling the gpio. 

So, just name the sysfs attribute "edge"?

> > +
> > +	struct sysfs_dirent	*value_sd;
> >  };
> 
> No CONFIG_ option to turn all this off?
> 
> What's the .text and .data impact of this?  Sure it's going to be small,
> but to some people (especially those likely to care about gpio) 1k
> of .data is something worth being able to turn off.

I think it's probably OK to have this covered by the current
GPIO_SYSFS flag.

> Using an IDR keyed to the gpio value and just allocating your useful
> data structures when poll_edge != "none" would help too.

Can do that without an IDR, I think...

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