linux-kernel - Re: [PATCH 1/3] gpiolib: Add ability to get GPIO pin direction

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Date:	Sun, 6 Mar 2011 00:53:34 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Peter Tyser <ptyser@...-inc.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	Alek Du <alek.du@...el.com>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Eric Miao <eric.y.miao@...il.com>,
	Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [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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