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

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Message-ID: <CAD=FV=XS0dbE7zfgDGVdp84h4fHH6tfO8aN_HNh1LbOjYy51aQ@mail.gmail.com>
Date:	Tue, 19 Nov 2013 10:59:39 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Tomasz Figa <t.figa@...sung.com>,
	"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 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?
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