linux-kernel - Re: [PATCH v3 2/4] gpiolib: Add ability to get GPIO pin direction

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Date:	Fri, 18 Feb 2011 11:36:26 -0600
From:	Peter Tyser <ptyser@...-inc.com>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	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>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Joe Perches <joe@...ches.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH v3 2/4] gpiolib: Add ability to get GPIO pin direction

On Fri, 2011-02-18 at 09:57 +0100, Uwe Kleine-König wrote:
> On Thu, Feb 17, 2011 at 05:03:17PM -0600, Peter Tyser wrote:
> > Add a new get_direction() function to the gpio_chip structure.  This is
> > useful so that the direction of a pin can be determined when its
> > initially exported.  Previously, the direction defaulted to "unknown"
> > regardless of the actual configuration of the GPIO pin.
> > 
> > If a GPIO driver implements get_direction(), it is called in
> > gpio_request() to set the initial direction of the pin accurately.
> IMHO the commit log is conceptually wrong, because it talks about a
> "pin".  Better use gpio here.

I don't follow.  I used "pin" to make it clear that the get_direction()
function operates on a pin-by-pin basis, and to help reduce any
ambiguity about if a gpio chip or gpio pin is being referred to.  Would
you prefer that the "pin" references are clarified to be "GPIO pin"?

> > Cc: Alek Du <alek.du@...el.com>
> > Cc: Samuel Ortiz <sameo@...ux.intel.com>
> > Cc: David Brownell <dbrownell@...rs.sourceforge.net>
> > Cc: Eric Miao <eric.y.miao@...il.com>
> > Cc: Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>
> > Cc: Mark Brown <broonie@...nsource.wolfsonmicro.com>
> > Cc: Joe Perches <joe@...ches.com>
> > Cc: Alan Cox <alan@...rguk.ukuu.org.uk>
> > Cc: Grant Likely <grant.likely@...retlab.ca>
> > Signed-off-by: Peter Tyser <ptyser@...-inc.com>
> > ---
> > Changes since v1:
> > - Add support for "unknown" direction
> > 
> > Changes since v2:
> >   Based on Wolfram's feedback:
> > - Use GPIOF_DIR_* flags as returns from get_direction()
> > - Call spin_lock_irqsave() to before setting flags
> > 
> >  drivers/gpio/gpiolib.c     |   23 +++++++++++++++++++++++
> >  include/asm-generic/gpio.h |    4 ++++
> >  2 files changed, 27 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > index eb74311..a656a2c 100644
> > --- a/drivers/gpio/gpiolib.c
> > +++ b/drivers/gpio/gpiolib.c
> > @@ -1174,6 +1174,7 @@ int gpio_request(unsigned gpio, const char *label)
> >  	struct gpio_desc	*desc;
> >  	struct gpio_chip	*chip;
> >  	int			status = -EINVAL;
> > +	int			dir;
> >  	unsigned long		flags;
> >  
> >  	spin_lock_irqsave(&gpio_lock, flags);
> > @@ -1214,6 +1215,28 @@ int gpio_request(unsigned gpio, const char *label)
> >  		}
> >  	}
> >  
> > +	if (chip->get_direction) {
> > +		/* chip->get_direction may sleep */
> > +		spin_unlock_irqrestore(&gpio_lock, flags);
> might_sleep_if(chip->can_sleep) ?

Makes sense.  I was following the lead of chip->request() in the same
function, which doesn't use might_sleep_if().  I assume might_sleep_if()
should be added to it as well in a separate patch?

> > +		dir = chip->get_direction(chip, gpio - chip->base);
> > +		spin_lock_irqsave(&gpio_lock, flags);
> > +		switch (dir) {
> > +		case GPIOF_DIR_OUT:
> > +			set_bit(FLAG_DIR_OUT, &desc->flags);
> > +			clear_bit(FLAG_DIR_IN, &desc->flags);
> > +			break;
> > +		case GPIOF_DIR_IN:
> > +			set_bit(FLAG_DIR_IN, &desc->flags);
> > +			clear_bit(FLAG_DIR_OUT, &desc->flags);
> > +			break;
> > +		default:
> > +			/* Direction isn't known */
> > +			clear_bit(FLAG_DIR_OUT, &desc->flags);
> > +			clear_bit(FLAG_DIR_IN, &desc->flags);
> > +			break;
> > +		}
> Alternatively to my suggestion for patch1:
> 	} else {
> 		clear_bit(FLAG_DIR_OUT, &desc->flags);
> 		clear_bit(FLAG_DIR_IN, &desc->flags);

I like this way better too.  I'll initialize dir = -1 and pull the
switch statement out of the conditional, like:

	if (chip->get_direction) {
		/* chip->get_direction may sleep */
		spin_unlock_irqrestore(&gpio_lock, flags);
		might_sleep_if(chip->can_sleep);
		dir = chip->get_direction(chip, gpio - chip->base);
		spin_lock_irqsave(&gpio_lock, flags);
	}

	switch (dir) {
	case GPIOF_DIR_OUT:
		set_bit(FLAG_DIR_OUT, &desc->flags);
		clear_bit(FLAG_DIR_IN, &desc->flags);
		break;
	case GPIOF_DIR_IN:
		set_bit(FLAG_DIR_IN, &desc->flags);
		clear_bit(FLAG_DIR_OUT, &desc->flags);
		break;
	default:
		/* Direction isn't known */
		clear_bit(FLAG_DIR_OUT, &desc->flags);
		clear_bit(FLAG_DIR_IN, &desc->flags);
		break;
	}

Thanks for the comments,
Peter

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