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@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?
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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