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

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Message-ID: <20110221112240.GH2616@pengutronix.de>
Date:	Mon, 21 Feb 2011 12:22:40 +0100
From:	Wolfram Sang <w.sang@...gutronix.de>
To:	Alexander Stein <alexander.stein@...tec-electronic.com>
Cc:	Lars-Peter Clausen <lars@...afoo.de>,
	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 Mon, Feb 21, 2011 at 12:07:27PM +0100, Alexander Stein wrote:
> On Monday 21 February 2011, 10:47:56 Wolfram Sang wrote:
> > > We had exported our 5V_enable gpio to sysfs to allow a user-space
> > > application to enable/disable devices connected to 5V circuit.
> > > But on the other hand we had to read the current status of this gpio in
> > > the power-fail interrupt handler to distinguish between false-positive
> > > (5V disabled) and a correct detection.
> > 
> > What about gpio_export() (description in Documentation/gpio.txt)?
> 
> Ah, I didn't know about this. I just expected this is only used from sysfs 
> part. But you have to make sure your .ko is loaded before userspace is 
> accessing sysfs and tries to export the GPIO.

Eh? Userspace doesn't export the GPIO in that case.

> Or is it "allowed" by the API convention to gpio_request and gpio_export (and 
> set direction) a GPIO in the machine startup code which will later be used in 
> a different place?

different place = userspace? Well, that's the main intention of
gpio_export(). (I have the feeling we are missing each other here,
thoguh) I'd suggest looking a bit further in the docs/code. It should
make clear what is possible.

Regards,

   Wolfram

-- 
Pengutronix e.K.                           | Wolfram Sang                |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

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