linux-kernel - Re: [PATCH] drivers: create a pin control subsystem v8

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Message-ID: <20111024073604.GA8708@ponder.secretlab.ca>
Date:	Mon, 24 Oct 2011 09:36:04 +0200
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Mike Frysinger <vapier.adi@...il.com>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Stephen Warren <swarren@...dia.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Barry Song <21cnbao@...il.com>, linux-kernel@...r.kernel.org,
	Joe Perches <joe@...ches.com>,
	Russell King <linux@....linux.org.uk>,
	Linaro Dev <linaro-dev@...ts.linaro.org>,
	Lee Jones <lee.jones@...aro.org>,
	David Brown <davidb@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org,
	Stijn Devriendt <highguy@...il.com>
Subject: Re: [PATCH] drivers: create a pin control subsystem v8

On Mon, Oct 24, 2011 at 09:26:38AM +0200, Linus Walleij wrote:
> On Sat, Oct 22, 2011 at 7:44 PM, Mike Frysinger <vapier.adi@...il.com> wrote:
> > On Tue, Oct 4, 2011 at 16:35, Grant Likely wrote:
> >> On Sat, Oct 01, 2011 at 12:39:21PM +0200, Linus Walleij wrote:
> >>> 2011/9/30 Grant Likely:
> >>> > I'm not convinced that the sysfs approach is
> >>> > actually the right interface here (I'm certainly not a fan of the gpio
> >>> > sysfs i/f), and I'd rather not be putting in unneeded stuff until the
> >>> > userspace i/f is hammered out.
> >>>
> >>> Actually, thinking about it I cannot see what would be wrong
> >>> with /dev/gpio0 & friends in the first place.
> >>>
> >>> Using sysfs as swiss army knife for custom I/O does not
> >>> seem like it would be long-term viable so thanks for this
> >>> observation, and I think we need /dev/gpio* put on some
> >>> mental roadmap somewhere.
> >>
> >> Agreed.  I don't want to be in the situation we are now with GPIO,
> >> where every time I look at the sysfs interface I shudder.
> >
> > the problem with that is it doesn't scale.  if i have a device with
> > over 150 GPIOs on the SoC itself (obviously GPIO expanders can make
> > that much bigger), i don't want to see 150+ device nodes in /dev/.
> > that's a pretty big waste.  sysfs only allocates/frees resources when
> > userspace actually wants to utilize a GPIO.
> 
> I was more thinking along the lines of one device per GPIO controller,
> then you ioctl() to ask /dev/gpio0 how many pins it has or so.

And there is also the question of whether it is even a good idea to
export pinctrl manipulation to userspace.

g.

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