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

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Date:	Thu, 4 Dec 2014 15:27:41 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	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

Hi,

On Thu, Dec 04, 2014 at 11:15:38PM +0900, Alexandre Courbot wrote:
> On Wed, Dec 3, 2014 at 1:12 AM, 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.
> 
> I don't get it. According to what you said, the following order should
> go through IIUC:
> 
> 1) bank A is probed, gpio 14 is available
> 2) gpio-regulator is probed, acquires GPIO 14, regulator for Bank B is available
> 3) bank B is probed, grabs its regulator and turn it on, probes.
> 
> What am I missing?

It would be true if bank A and B were exposed through different
drivers (or at least different instances of the same driver), which is
not the case.

In our case, banks A and B are handled by the same instance.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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