[PATCH 3/3] of/gpio: Introduce of_put_gpio(), add ref counting for OF GPIO chips

[Grant Likely]

On Fri, Feb 5, 2010 at 1:50 PM, Anton Vorontsov
<avorontsov at ru.mvista.com> wrote:
> OF GPIO infrastructure is using dynamic GPIO bases, so it is possible
> that of_get_gpio()'s returned GPIO number will be no longer valid, or
> worse, it may point to an unexpected GPIO controller.
>
> This scenario is possible:
>
> driver A:               driver B:              driver C:
> ---------               ---------              ---------
>                        gpiochip_add()
> gpio = of_get_gpio()
>                        gpiochip_remove()
>                                               gpiochip_add()
> gpio_request(gpio);
> gpio_set_value(gpio);
>
> That is, driver A assumes that it is working with GPIO from driver B,
> but in practice it may disappear and driver C will take its GPIO base
> number, so it will provide the same GPIO numbers.
>
> With this patch that situation is no longer possible. Though drivers
> will need to learn to put GPIOs back, so that GPIO controllers could
> be removed.
>
> Signed-off-by: Anton Vorontsov <avorontsov at ru.mvista.com>

Rather than having a lock at the device tree data pointer level which
mixes usage with potentially many other drivers; wouldn't it make more
sense to use a mutex at the of_gc subsystem context?

g.