linux-kernel - Re: [PATCH v4 4/6] gpio: introduce lock mechanism for gpiochip

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Date:	Wed, 30 May 2012 12:10:58 +0800
From:	Dong Aisheng <dongas86@...il.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Dong Aisheng <b29396@...escale.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linus.walleij@...ricsson.com,
	swarren@...dotorg.org, devicetree-discuss@...ts.ozlabs.org,
	rob.herring@...xeda.com
Subject: Re: [PATCH v4 4/6] gpio: introduce lock mechanism for gpiochip_find

Hi Grant,

On Fri, May 25, 2012 at 06:25:00PM -0600, Grant Likely wrote:
> On Fri, 25 May 2012 21:36:18 +0800, Dong Aisheng <b29396@...escale.com> wrote:
> > From: Dong Aisheng <dong.aisheng@...aro.org>
> > 
> > The module lock will be automatically claimed for gpiochip_find function
> > in case the gpio module is removed during the using of gpiochip instance.
> > Users are responsible to call gpiochip_put to release the lock after
> > the using.
> > 
> > Signed-off-by: Dong Aisheng <dong.aisheng@...aro.org>
> 
...
> Also, it doesn't do anything to protect against the gpio_chip being
> removed after the gpio number is resolved, which means the gpio number
> may no longer be valid, or may no longer point to the same gpio chip.
> It looks like the locking protection needs to be wider to be useful.
> 
I understand the issue now.
It's correct that we did not lock gpio_chip before calling gpio_request
after the gpio number is resolved.

I thought about adding a new API called of_gpio_request to hide the lock
to users like:
int of_gpio_request(..)
{
	spin_lock_irqsave(&gpio_lock, flags);
	ret = of_get_named_gpio(..);
	if (ret < 0)
		do_err..
	ret = gpio_request(..)

	spin_unlock_irqrestore(&gpio_lock, flags);
	return ret;
}
But it seems it does not work since the gpio_request may sleep and we may
need a new sleepable lock rather using the exist gpio_lock.

In the same time, i'm also thinking about a question that do we really
need to do this to protect gpio_chip being removed afer gpio number is
resolved?
My doubts is that gpio lib really does not block the gpiochip to be removed
before calling gpio_request, so why we need to do that for dt?
Maybe just let gpio_request to detect if gpio number is valid is already ok
for dt.

what's your suggestion on it?

Regards
Dong Aisheng
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