linux-kernel - Re: [PATCH v3] gpio: interrupt consistency check for OF GPIO IRQs

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Date:	Tue, 24 Sep 2013 10:56:26 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Alexandre Courbot <acourbot@...dia.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Mark Brown <broonie@...nel.org>,
	Lars Poeschel <poeschel@...onage.de>,
	Lars Poeschel <larsi@....tu-dresden.de>,
	Grant Likely <grant.likely@...aro.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Kumar Gala <galak@...eaurora.org>,
	Pawel Moll <pawel.moll@....com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Enric Balletbo i Serra <eballetbo@...il.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Kevin Hilman <khilman@...aro.org>,
	Balaji T K <balajitk@...com>,
	Tony Lindgren <tony@...mide.com>,
	Jon Hunter <jgchunter@...il.com>, joelf@...com
Subject: Re: [PATCH v3] gpio: interrupt consistency check for OF GPIO IRQs

On 09/24/2013 02:26 AM, Linus Walleij wrote:
> On Mon, Sep 23, 2013 at 10:12 PM, Stephen Warren <swarren@...dotorg.org> wrote:
>> On 09/23/2013 01:53 PM, Linus Walleij wrote:
> 
>>> I think the kernel should prevent such things.
>>
>> It might be nice if it could do that.
>>
>> However, that is 100% unrelated to the problem at hand.
> 
> I don't think it is unrelated when the old OMAP boardfile-based
> code definately prevents such uses by its strict usage
> of gpio_request() for all IRQ-bound GPIOs.
> 
> I think not preventing it for the DT boot path is setting lower
> standards for DT code than for boardfile code which is not
> what we should be doing.

Semantics matter.

In the old board file code, the gpio_request()s were present to work
around the bug in the OMAP driver where request_irq() wouldn't configure
the IRQ signal correctly. That's the primary reason those calls were there.

Now, this had the side-effect of also preventing anything else from
calling gpio_request() on those GPIOs, but that wasn't the primary
motivation; just a convenient effect.

...
> Solving the issue that e.g. two different drivers competing about the
> same resource (as in one driver requesting an IRQ and another one
> requesting a GPIO) is not what I'm after here.
> 
> I'm more after the GPIO subsystem having knowledge of a certain
> GPIO line being requested for IRQ, and denying that line to be set
> as input.

s/input/output/ I assume.

...
> Maybe this can actually be achieved quite easily with
> an additional API? Like gpio_lock_as_irq(gpio) which flags this
> in .flags of struct gpio_desc and prevent such things?
> 
> Alexandre what do you think about this idea?
> 
>> Equally, I am actually not 100% sure we want the core to prevent this.
>> Why shouldn't two different drivers request the same IRQ? Why shouldn't
>> at least one driver, perhaps more, request the pin as a GPIO (assuming
>> it will only read the GPIO value, not flip the pin to output).
> 
> But I have already stated that this is OK?
> 
> Are we talking past each other now?

If all you want to do is prevent gpio_direction_input() on a GPIO that's
in use as a GPIO, then that's probably OK.

However, the interrupt consistency patch that was posted implemented
that restriction by calling gpio_request(), and the wording of most of
what you've written implies to me that implementing the restriction by
calling gpio_request() is what you're after. That approach imposes far
more restrictions than just preventing gpio_direction_input(). Imposing
those additional restrictions is what I'm objecting to.
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