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@xxxxxxxxxxxxxxx>
> > ---
> > 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?
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