linux-kernel - Re: [PATCH v5 1/4] gpiolib: Add "unknown" direction support

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Date:	Sun, 6 Mar 2011 23:52:53 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Peter Tyser <ptyser@...-inc.com>
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>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Joe Perches <joe@...ches.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Alexander Stein <alexander.stein@...tec-electronic.com>,
	Ryan Mallon <ryan@...ewatersys.com>
Subject: Re: [PATCH v5 1/4] gpiolib: Add "unknown" direction support

On Sun, Mar 06, 2011 at 08:43:05PM -0600, Peter Tyser wrote:
> > The thing about gpios is that how they are used is entirely dependant
> > on what they are wired up to.  There is no avoiding the fact that you
> > *absolutely* must understand the usage model before even considering
> > fiddling with a gpio (ignoring the "I wonder what this knob does"
> > use-case such as when reverse engineer a board).  So, while it is nice
> > to have an 'unknown' state for sysfs to report, it is certainly not
> > required.  The model still remains that the pin direction must be set
> > before (or at the same time as) reading/writing the pin.
> 
> As is, even if someone knows about the GPIO wiring on their board, they
> have to know Linux has a "rule" that "before I can use a GPIO, I have to
> explicitly set its direction, even if the current reported direction is
> what I want".  A number of our customers have tried to use a GPIO which
> states its an 'input' as an input after exporting it, which is
> completely logical.  But it doesn't work, because its not really an
> input...  Why not set the direction accurately as 'unknown' so users
> intuitively know they have to set the direction before using it?  You
> also mention that it would be a nice feature above, so why not include
> it?

I don't like what it does to the implementation, and I'd rather make
drivers provide accurate data at the outset.

g.

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