linux-kernel - Re: [PATCH] pinctrl: samsung: Allow pin value to be initialized using pinfunc.

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Date:	Wed, 20 Nov 2013 15:57:45 +0100
From:	Tomasz Figa <t.figa@...sung.com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	devicetree@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Heiko Stübner <heiko@...ech.de>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] pinctrl: samsung: Allow pin value to be initialized using
 pinfunc.

On Tuesday 19 of November 2013 10:59:39 Doug Anderson wrote:
> On Tue, Nov 19, 2013 at 10:46 AM, Stephen Warren <swarren@...dotorg.org> wrote:
> > On 11/19/2013 10:15 AM, Tomasz Figa wrote:
> >> This patch extends the range of settings configurable via pinfunc API
> >> to cover pin value as well. This allows configuration of default values
> >> of pins.
> >
> > Shouldn't there be a driver that acquires the GPIO that's output to the
> > pin, and configures the output value? IIRC there have been previous
> > discussions re: having a list of e.g. initial GPIO output values in DT,
> > and that was rejected, and this patch seems to be doing almost the exact
> > same thing, just at the pinctrl level rather than GPIO level.
> >
> > That all said, I admit this could be a useful feature...
> 
> I haven't followed all of the previous discussions, but I know I've
> run into scenarios where something like this would be useful.  The one
> that comes to mind is:
> 
> * We've got GPIOs that default at bootup to a pulled up input since
> the default state of the pin should be "high".
> 
> * These pins are really intended to be outputs, like an "enable",
> "reset", or "power down" line for a peripheral.  The pullup is strong
> enough to give us a good default state but we really want outputs.
> 
> * We'd like to provide this GPIO to a peripheral through device tree.
> ...and we'd like all the pinmux to be setup automatically so we use
> pinctrl-names = "default".
> 
> * If we set the pinmux up as "output" then there's a chance that the
> line will glitch at bootup since the pinmux happens (changing the pin
> to output) before the driver has a chance to run.
> 
> 
> Does that sound like the same scenario you're trying to solve Tomasz?

Yes. That's one of the use cases I had in my mind.

Best regards,
Tomasz

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