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

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Date:	Mon, 14 Feb 2011 12:46:03 -0600
From:	Peter Tyser <ptyser@...-inc.com>
To:	Grant Likely <grant.likely@...retlab.ca>
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 Mon, 2011-02-14 at 11:04 -0700, Grant Likely wrote:
> On Mon, Feb 14, 2011 at 10:45 AM, Peter Tyser <ptyser@...-inc.com> wrote:
> >> > We need four states for a gpio pin direction though. A pin can be
> >> >
> >> > - input
> >> > - output
> >>
> >> There are actually multiple output modes that a specific gpio
> >> controller could implement, but the gpio api only has a boolean
> >> understanding of output.  I don't know if it is really worthwhile to
> >> try and encode all the possible configurations in this API.
> >>
> >> > - unknown (hardware lacks get functionality and it has not been set by
> >> >  software yet)
> >
> > I'm not sure how we could handle unknown directions in a better way.  We
> > really should know the direction by this point for most (all?) GPIO
> > chips.  I'd think the proper fix would be to make sure we can detect a
> > direction for all chips - either by reading hardware bits or by having
> > the platform code let us know (eg pdata->n_latch in pcf857x.c).  If you
> > have a suggestion about how unknown pins should be used, I can look into
> > it and submit a follow up patch.
> 
> Does it really matter?  Sure it's *nice* to have the current status
> information if the driver can easily get it, but it probably isn't
> critical.  Any user of the pin should be putting the pin in the mode
> it needs regardless of the initial state.

I don't think it matters too much since I haven't encountered a driver
where you can't determine a pin direction yet.  But if it were
impossible to determine a direction, it would throw people off.  They'd
have to know that the pin direction couldn't be trusted initially, which
is a big leap (this is the bug I'm trying to fix with this patch).  Eg
if someone sees a pin direction as "input" via sysfs, what are the odds
that they'll run "echo input > direction" just to make sure...

Anyway, I don't think its a critical issue either since its a corner
case with an easy workaround.  If we did want to add support for
allowing a direction of "unknown", it could be in a follow up patch
since its technically a bigger issue than this patch.

Peter

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