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

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Message-ID: <CAAVeFuJTjWPzQYYkV3jW3+h6uigzzeKS9d7pY2cQ-dK119msXg@mail.gmail.com>
Date:	Thu, 4 Dec 2014 23:49:19 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.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

On Thu, Dec 4, 2014 at 11:27 PM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> 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.

Ok, so both banks A and B are part of the same device/DT node. Now I
think I understand the issue. You need to hog the pin so that bank B
will work right after the device is probed.

But you will still have the problem that the regulator device will
*not* be available when your device is probed, so you cannot call
regulator_get() for bank B anyway. I guess your only choice is to hog
that pin and leave it active ad vitam eternam.
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