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

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Message-Id: <201102211009.31381.alexander.stein@systec-electronic.com>
Date:	Mon, 21 Feb 2011 10:09:30 +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, 14:06:06 Lars-Peter Clausen wrote:
> > 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.
> 
> Well, it is not enforced in the actual code, but the GPIO API is based on
> convention and I would consider it a misusage of the API to call
> gpio_{set,get}_value without requesting the pin and having configured its
> direction prior to it.
> 
> > 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.
> 
> Sharing GPIOs in read-only mode, is indeed something that is not covered by
> the GPIO API. It might be worth adding a gpio_request_shared, which would
> only permit setting the direction to input. Futher gpio_request_shared
> calls would be allowed but gpio_request calls would fail.

gpio_request_shared sounds interesting, but in this case an implicit 
gpio_direction_input call has to be done in the first shared request. But the 
user must/should not call gpio_direction_input himself which is quite a bit 
different than using a normal gpio_requested gpio.

Alexander
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