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SubjectRe: [PATCH 1/3] gpiolib: Add ability to get GPIO pin direction
On Mon, Feb 14, 2011 at 07:35:02PM +0000, Alan Cox wrote:
> > What is the use-case for alt_func? From the point of view of a GPIO
> > driver, I don't think it cares if the pin has been dedicated to
> Currently it doesn't.
>
> However the moment it starts setting input and output itself on requests
> then it does because it may kick the pin out of alt_func mode when you
> merely want to request it so you know which pin to stick into alt_func
> mode, or to find a mapping. The current situation is an "ignorance is
> bliss" one, but making it smarter backfires. We have the same problem
> with unknown state - if I have a set of pins some of whose state is known
> at boot time then I can't now provide a get_direction interface because
> of the lack of states. At the very least we need input/output/godknows
> where the latter means the gpio_request code keeps its nose out.

Not quite; the gpio api is only about discrete gpios. If a particular
pin is dedicated to another non-gpio purpose, then from the POV of the
gpio api, the pin is disconnected from the outside world and any
twiddling of it just won't do anything. If an alt_func has any driver
behaviour impact, then it needs to be handled internal to the driver.

>
> reconfigure_resource();
> see_if_we_own_it()
>
> is simply the wrong order for a resource.

Yes, this is broken. gpio_request() should not change the state of
the resource. I don't see anything in gpiolib that currently does
this.

> The second problem is that in many cases you need to call gpio_request to
> know you have the pin yourself before you can set the direction. That
> means forcing the direction in gpio_request is daft - you force an
> undefined value that cannot yet safely be set in all cases.
>
> At the moment the lack of alt_func also has some strange effects and you
> end up with code like
>
> foo_firmware_update()
> {
> /* Reserve the line for alt_func use for the moment */
> gpio_request(GPIO_FOO, "blah");
> random_gpio_driver_specific_altfunc_foo();
> do stuff();
> random_gpio_driver_specific_altfunc_foo();
> gpio_free(GPIO_FOO);
>
> /* Now available again for its normal GPIO use */
> }
>
>
> and that means you can't generalise dynamic access to a shared GPIO pin
> without extra hardcoded knowledge.

I don't follow the argument. Of course you have to do weird hardcoded
things when a gpio pin has to be shared between multiple purposes, but
that is also a weird corner case that won't fit any kind of common
pattern. I don't see the above code snippet as a problem.

g.


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