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From: Grant Likely <grant.likely@secretlab.ca>
Date: Tue, 9 Feb 2010 10:28:15 -0700
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Subject: Re: [PATCH 3/3] of/gpio: Introduce of_put_gpio(), add ref counting
	for OF GPIO chips
To: Anton Vorontsov <avorontsov@ru.mvista.com>
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On Fri, Feb 5, 2010 at 1:50 PM, Anton Vorontsov
<avorontsov@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 that of_get_gpio()'s returned GPIO number will be no longer valid, or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: =A0 =A0 =A0 =A0 =A0 =A0 =A0 driver B: =A0 =A0 =A0 =A0 =A0 =A0 =
=A0driver C:
> --------- =A0 =A0 =A0 =A0 =A0 =A0 =A0 --------- =A0 =A0 =A0 =A0 =A0 =A0 =
=A0---------
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0gpiochip_add()
> gpio =3D of_get_gpio()
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0gpiochip_remove()
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 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@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.