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

From: Alexandre Courbot
Date: Thu Dec 04 2014 - 09:16:06 EST

On Wed, Dec 3, 2014 at 1:12 AM, Maxime Ripard
<maxime.ripard@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> wrote:
>> > On Tue, Dec 2, 2014 at 1:36 AM, Maxime Ripard
>> > <maxime.ripard@xxxxxxxxxxxxxxxxxx> 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?

> 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.

At this point I start wondering if such initial setup should not be
the job of the bootloader? GPIO hogging ought to be simple and
definitive, adding the possibility to have it just as an initial value
would considerably complexify it. E.g. when is the gpio chip driver
supposed to release the hogged descriptor in such a case?

Note that if the multiple GPIO consumer feature we are planning goes
through, you should be able to use both hogging *and* a regulator on
the same GPIO and achieve what you want. The expectation of multiple
consumers is that the board designers know what they are doing, and
this case would certainly fit (chip hogs the line and doesn't touch
the value after that, letting the regulator control it without any
conflict afterwards), although it would of course be better to solve
the issue through regular probing...
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