linux-kernel - Re: [PATCH 1/6] gpiolib: Allow GPIO chips to request their own GPIOs

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Date:	Wed, 26 Feb 2014 14:47:58 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Lv Zheng <lv.zheng@...el.com>, Alan Cox <alan.cox@...el.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] gpiolib: Allow GPIO chips to request their own GPIOs

On Wednesday, February 26, 2014 11:05:42 AM Mika Westerberg wrote:
> On Tue, Feb 25, 2014 at 03:10:24PM +0100, Rafael J. Wysocki wrote:
> > On Monday, February 24, 2014 06:00:06 PM Mika Westerberg wrote:
> > > Sometimes it is useful to allow GPIO chips themselves to request GPIOs they
> > > own through gpiolib API. One usecase is ACPI ASL code that should be able
> > > to toggle GPIOs through GPIO operation regions.
> > > 
> > > We can't really use gpio_request() and its counterparts because it will pin
> > > the module to the kernel forever (as it calls module_get()). Instead we
> > > provide a gpiolib internal functions gpiochip_request/free_own_desc() that
> > > work the same as gpio_request() but don't manipulate module refrence count.
> > > 
> > > Since it's the GPIO chip driver who requests the GPIOs in the first place
> > > we can be sure that it cannot be unloaded without the driver knowing about
> > > that. Furthermore we only limit this functionality to be available only
> > > inside gpiolib.
> > > 
> > > Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > > ---
> > >  drivers/gpio/gpiolib.c | 57 +++++++++++++++++++++++++++++++++++++++++++-------
> > >  drivers/gpio/gpiolib.h |  3 +++
> > >  2 files changed, 53 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > > index f60d74bc2fce..489a63524eb6 100644
> > > --- a/drivers/gpio/gpiolib.c
> > > +++ b/drivers/gpio/gpiolib.c
> > > @@ -1458,7 +1458,8 @@ EXPORT_SYMBOL_GPL(gpiochip_remove_pin_ranges);
> > >   * on each other, and help provide better diagnostics in debugfs.
> > >   * They're called even less than the "set direction" calls.
> > >   */
> > > -static int gpiod_request(struct gpio_desc *desc, const char *label)
> > > +static int __gpiod_request(struct gpio_desc *desc, const char *label,
> > > +			   bool module_refcount)
> > >  {
> > >  	struct gpio_chip	*chip;
> > >  	int			status = -EPROBE_DEFER;
> > > @@ -1475,8 +1476,10 @@ static int gpiod_request(struct gpio_desc *desc, const char *label)
> > >  	if (chip == NULL)
> > >  		goto done;
> > >  
> > > -	if (!try_module_get(chip->owner))
> > > -		goto done;
> > > +	if (module_refcount) {
> > > +		if (!try_module_get(chip->owner))
> > > +			goto done;
> > > +	}
> > 
> > I'm wondering why you're not moving the module refcount manipulation entirely
> > to gpiod_request()?
> > 
> > I guess that's because of the locking, but I suppose that desc->chip will never
> > be NULL in gpiochip_request_own_desc(), so you don't even need to check it there?
> > 
> > So it looks like gpiochip_request_own_desc() can do something like
> > 
> > 	lock
> > 	__gpiod_request(stuff)
> > 	unlock
> > 
> > where __gpiod_request() is just the internal part starting at the "NOTE" comment.
> 
> Sounds good. Only thing I'm not sure about is the fact that
> __gpiod_request() releases the lock when it calls chip driver callbacks
> (and takes it back of course). Is that acceptable practice to take the lock
> outside of a function and release it inside for a while?

Yes, you can do that.

There even are sparse annotations for that: __releases() and __acquires()
(__rpm_callback() in drivers/base/power/runtime.c uses them among other things).

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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