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    SubjectRe: [PATCH 1/6] gpiolib: Allow GPIO chips to request their own GPIOs
    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@linux.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?


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