linux-kernel - Re: [PATCH v2 1/4] gpiolib: Add "unknown" direction support

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Message-Id: <201102171321.47704.alexander.stein@systec-electronic.com>
Date:	Thu, 17 Feb 2011 13:21:46 +0100
From:	Alexander Stein <alexander.stein@...tec-electronic.com>
To:	"Lars-Peter Clausen" <lars@...afoo.de>
Cc:	Eric Miao <eric.y.miao@...il.com>,
	Peter Tyser <ptyser@...-inc.com>, linux-kernel@...r.kernel.org,
	Alek Du <alek.du@...el.com>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	"Uwe Kleine-K?nig" <u.kleine-koenig@...gutronix.de>,
	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 v2 1/4] gpiolib: Add "unknown" direction support

On Thursday 17 February 2011, 13:03:54 Lars-Peter Clausen wrote:
> On 02/17/2011 08:33 AM, Eric Miao wrote:
> > On Thu, Feb 17, 2011 at 8:56 AM, Peter Tyser <ptyser@...-inc.com> wrote:
> >> Previously, gpiolib would unconditionally flag all GPIO pins as inputs,
> >> regardless of their true state.  This resulted in all GPIO output pins
> >> initially being incorrectly identified as "input" in the GPIO sysfs.
> >> 
> >> Since the direction of GPIOs is not known prior to having their
> >> direction set, instead set the default direction to "unknown" to prevent
> >> user confusion.  A pin with an "unknown" direction can not be written or
> >> read via sysfs; it must first be configured as an input or output before
> >> it can be used.
> > 
> > Hrm... that's why I don't like the original definition of gpio_request()
> > which is vague on the pin configurations.
> 
> Actually it doesn't say anything at all about the current configuration at
> all. Requesting a pin grants you exclusive access to that pin, if it
> succeeds. So it is solely about ownership and not about configuration.

Well, ownership is a bit misleading here. You must request a GPIO to change 
its direction. But to set or get a value this isn't required.
In general one could expect if you requested a GPIO you are the only one to 
call any function on it. On the other hand, this may be bad in some situations 
where you want to read a GPIO value from different places.

Alexander
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