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SubjectRe: [PATCH 1/3] gpiolib: Add ability to get GPIO pin direction
On Tue, Feb 15, 2011 at 11:05:48AM -0600, Peter Tyser wrote:
> > > Also, in most cases I'd think that the BIOS/U-Boot/firmware should have
> > > configured the GPIO pins appropriately, which Linux should inherit in
> > > general. Linux currently inherits GPIO states that were set in firmware
> > > when a GPIO is requested, but it doesn't properly report those values
> > > via sysfs - that's the only bug I'm trying to fix.
> >
> > Yes - however you can't fix it unless you are prepared to admit that the
> > gpio has multiple states. At minimum you need to be able to report
> >
> > input/output/unknown/unavailable
> >
> > if you want to generalise it. Otherwise you don't solve the problem
> > because you are asking a question that driver cannot answer correctly.
>
> As far as the "unknown" state, I can update the patch to have the logic:
> + if (chip->get_direction) {
> + /* chip->get_direction may sleep */
> + spin_unlock_irqrestore(&gpio_lock, flags);
> + if (chip->get_direction(chip, gpio - chip->base) > 0)
> + set_bit(FLAG_IS_OUT, &desc->flags);
> + spin_lock_irqsave(&gpio_lock, flags);
> + } else {
> + set_bit(FLAG_IS_UNKNOWN, &desc->flags);
> + }
>
> This would have the side effect of having nearly all GPIO drivers
> default to an "unknown" direction until they implement the new
> get_direction() function, which I think is an improvement over the
> current system where they are all unconditionally shown as "input",
> often incorrectly. Are you OK with this Grant?

Not really, no. Defaulting to "input" may be incorrect, but it is
always safe, it it should only be a minor inconvenience to human users
of the sysfs interface. Actual usage of a gpio pin must always be to
explicitly set the direction before using a pin.


> Changing the logic to allow "unavailable" GPIO pins to be
> requested/exported would require larger changes to the code, and
> wouldn't provide much benefit without additional changes (eg an alt_func
> feature). So I'd vote to not add support for "unavailable" pins in this
> patch and rather wait until someone has a specific use for it, and add
> it then.

Agreed.

g.


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