linux-kernel - Re: [Patch v2 1/2] gpio: add GPIO hogging mechanism

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Date:	Mon, 12 Jan 2015 10:40:04 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Mark Brown <broonie@...nel.org>
Cc:	Alexandre Courbot <gnurou@...il.com>,
	Benoit Parrot <bparrot@...com>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Jiri Prchal <jiri.prchal@...ignal.cz>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [Patch v2 1/2] gpio: add GPIO hogging mechanism

On Tue, Dec 2, 2014 at 5:12 PM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Tue, Dec 02, 2014 at 03:29:46PM +0100, Linus Walleij wrote:
>> On Tue, Dec 2, 2014 at 3:13 PM, Alexandre Courbot <gnurou@...il.com> wrote:
>> > On Tue, Dec 2, 2014 at 1:36 AM, Maxime Ripard
>> > <maxime.ripard@...e-electrons.com> wrote:
>>
>> >> The only thing I'd like to have would be that the request here would
>> >> be non-exclusive, so that a later driver would still be allowed later
>> >> on to request that GPIO later on and manage it itself (ideally using
>> >> the usual gpiod_request function).
>> >
>> > Actually we have a plan (and I have some code too) to allow multiple
>> > consumers per GPIO. Although like Benoit I wonder why you would want
>> > to hog a GPIO and then request it properly later. Also, that probably
>> > means we should abandon the hog since it actively drives the line and
>> > would interfere with the late requested. How to do that correctly is
>> > not really clear to me.
>>
>> I don't get the usecase. A hogged GPIO is per definition hogged.
>> This sounds more like "initial settings" or something, which is another
>> usecase altogether.
>
> We do have one board where we have a pin (let's say GPIO14 of the bank
> A) that enables a regulator that will provide VCC the bank B.
>
> Now, both banks are handled by the same driver, but in order to have a
> working output on the bank B, we do need to set GPIO14 as soon as
> we're probed.
>
> Just relying on the usual deferred probing introduces a circular
> dependency between the gpio-regulator that needs to grab its GPIO from
> a driver not there yet, and the gpio driver that needs to enable its
> gpio-regulator.
>
> GPIO hogging needs to be the ideal solution for that, since we can
> just enforce the GPIO14 value as the driver is probed, which provides
> the guarantee that any driver using the bank B will actually drive the
> GPIO it might use.
>
> However, an exclusive request will prevent any representation of this
> as a regulator, which sounds a bit weird, since it really is just
> that.

Well that's elegant...

I think it's a hog in this case though, not a GPIO regulator,
definately not both. But let's check Mark's opinion on this.

Yours,
Linus Walleij
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