linux-kernel - RE: [PATCH 1/3] irq: If an IRQ is a GPIO, request and configure it

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Message-ID: <74CDBE0F657A3D45AFBB94109FB122FF049F171EF4@HQMAIL01.nvidia.com>
Date:	Fri, 5 Aug 2011 08:29:38 -0700
From:	Stephen Warren <swarren@...dia.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, Liam Girdwood <lrg@...com>,
	Chris Ball <cjb@...top.org>,
	"ccross@...roid.com" <ccross@...roid.com>,
	"olof@...om.net" <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: RE: [PATCH 1/3] irq: If an IRQ is a GPIO, request and configure it

Mark Brown wrote at Thursday, August 04, 2011 11:35 PM:
> On Thu, Aug 04, 2011 at 08:53:34PM -0700, Stephen Warren wrote:
> 
> > Well, things break. This is essentially the problem I was describing in
> > the PATCH 0 email, just with a slightly different motivation.
> 
> There's a bunch of existing code using that idiom.

Certainly.

> > I suppose that an alternative here would be to simply ignore any errors
> > from gpio_request. This might have the benefit of removing the need for
> > the other two patches I posted in the series. However, it seems a little
> > dirty; one benefit of the IRQ code calling gpio_request and honoring
> > errors would be to detect when some completely unrelated code had a bug
> > and had called gpio_request on the GPIO before. Such detection would be
> > non-existent if we don't error out on gpio_request. Perhaps some mechanism
> > is needed to indicate that the driver has explicitly already called
> > gpio_request for a legitimate shared purpose, and only then ignore
> > errors?
> 
> But it's not a bug to use a GPIO as an IRQ source, otherwise we wouldn't
> have gpio_to_irq() in the first place.

True, but I think there are two cases:

1) Some code legitimately uses a pin as both a GPIO and IRQ, and is fully
cognizant of the fact, just like in your example -> no bug.

2) Two pieces of unrelated code somehow accidentally get a GPIO and IRQ
number that map to the same resource, e.g. due to incorrect board files or
Device Tree content. This is probably a bug, but ends up looking exactly
the same as far as the IRQ code's gpio_request call failing in the patch I
posted.

> Feels like we need a backchannel
> between gpiolib and the IRQ code to do this.  Or perhaps the drivers
> that implement this should be taking care of setting up the GPIO mode?

-- 
nvpublic

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