linux-kernel - Re: [PATCH 2/2] gpiolib: Implement devm_gpio_request

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Message-ID: <20120407091636.GD3099@opensource.wolfsonmicro.com>
Date:	Sat, 7 Apr 2012 10:16:36 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] gpiolib: Implement devm_gpio_request_one()

On Sat, Apr 07, 2012 at 10:00:18AM +0100, Mark Brown wrote:
> On Fri, Apr 06, 2012 at 07:25:12PM -0700, Grant Likely wrote:

> > Actually, I looked at it again, and the _one version behaves subtly
> > different from the non-_one version.  That may be irrelevant though.

> It always does a direction set, yes.  This will make a difference if the
> direction call either fails or blocks future reconfigurations, what was
> puzzling me was if this was an issue or not and like I say I decided to
> assume it was and stick with the same pattern.

Just after sending this (isn't that always the way?) I realised there is
one important case where it makes a difference: if the code does this:

	gpio_request(foo, "bar");
	gpio_direction_output(foo, 1);

then if the GPIO was already driving out high we will maintain steady
state.  If, however, we were to do:

	gpio_request(foo, "bar");
	gpio_direction_input();
	gpio_direction_output(foo, 1);

where the first two calls are the natural result of implementing
gpio_request() in terms of gpio_request_one() then the GPIO would glitch
high Z during the process which might have unfortunate consequences for
whatever the GPIO is controlling.

I think instead of redoing the implementation we need start a campaign
to convert gpio_request() users to gpio_request_one() - coccinelle can
probably take a large chunk of the work but it'll need some manual help
I expect.

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