Re: [PATCH v5 2/4] gpiolib: Add ability to get GPIO direction

Grant Likely
2011-03-07T07:08:20Z

On Sun, Mar 06, 2011 at 09:07:21PM -0600, Peter Tyser wrote:
> On Sun, 2011-03-06 at 00:30 -0700, Grant Likely wrote:
> > On Tue, Mar 01, 2011 at 05:28:18PM -0600, Peter Tyser wrote:
> > > Add a new get_direction() function to the gpio_chip structure. This is
> > > useful so that the direction of a GPIO can be determined when its
> > > initially exported. Previously, the direction defaulted to "unknown"
> > > regardless of the actual configuration of the GPIO.
> > >
> > > If a GPIO driver implements get_direction(), it is called in
> > > gpio_request() to set the initial direction of the GPIO accurately.
> >
> > I don't like this approach. It effectively creates two methods for
> > determining the direction of a gpio; either ask the driver, or read
> > the flags value. Currently, only gpio_request() actually uses the
> > first option, but it still leaves the ambiguity.
> >
> > Instead, the driver should be able to preload the direction flag at or
> > shortly after gpiochip_add() time. No need for a new callback hook
> > from what I can see.
>
> The callback hook was used because the gpio_desc[] structure was local
> to gpiolib.c. The code would have to be restructured a bit to allow
> drivers to set the flags themselves. I can do that if you'd prefer, but
> don't think it'd be all that much cleaner.

Make it part of the gpiochip_add function, or add a function for
explicitly setting gpio directions. Should not require any
reorganization at all.

> Also, I thought it made sense to read the direction of the GPIO in
> gpio_request() because that's the point that the GPIO comes under the
> GPIO subsystem's control. Prior to that, there's a chance the direction
> could be changed by platform code, or another driver, etc, so reading
> the direction in gpiochip_add() may result in out-of-date directions.

That's the problem with gpiolib having sole responsibility of the
cache. It doesn't give the driver any recourse for changing things if
it needs to. The best solution might very well be to eliminate the
direction state flag entirely and force all gpiochip drivers to
populate a gpio_get_direction() callback, but that is a lot more work.

> The reading of the direction could also be put in chip drivers'
> request() function. That would get rid of the callback and still ensure
> the direction is up-to-date.

I don't object to a callback hook. My objection is how it is bodged
on to work around limitations to the direction being cached in the
flags variable. I want to see a solution that either depends entirely
on the callback, or completely fixes the problems with the cached
value by allowing the driver to update it.

> I think the current patch seems cleaner than the alternatives, but will
> change if you'd prefer.

Thanks,
g.