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

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Date:	Mon, 9 Dec 2013 13:57:00 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Kevin Bracey <kevin@...cey.fi>,
	Kyungmin Park <kmpark@...radead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Heiko Stübner <heiko@...ech.de>,
	Tomasz Figa <t.figa@...sung.com>,
	Doug Anderson <dianders@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] pinctrl: samsung: Allow pin value to be initialized using pinfunc.

On Fri, Dec 6, 2013 at 12:54 AM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 12/03/2013 02:29 AM, Linus Walleij wrote:

(skipped the conversation on weak hogs, we are on the same page
here, just waiting for someone to start working on it ...)

> Related, I prefer to put /all/ static pinctrl configuration into the
> pinctrl device's "default" state (i.e. use a hog) rather than
> configuring the static pinctrl setup per device, for another reason too:
>
> If a particular IO controller's signals can be routed to n different
> (sets of) pins, then we need to do *both* of the following when setting
> up the pinmux:
>
> a) Configure the pins we want to host those signals to route to/from
> that particular IO controller.
>
> b) Configure any other pins that could route to/from that particular IO
> controller as some other function; either disabled, or routed to/from
> some different IO controller.
>
> That is so that the IO controller's RX/input signals are not connected
> from two different sets of pins at once, which would cause two things to
> driver them. Depending on HW, this could cause on of:
>
> 1) Multiple drivers -> high power usage, or even Silicon damage.
>
> 2) Inconsistent configuration, with the "wrong" set of pins driving the
> IO controller's inputs, and hence the signals on the "correct" pins
> being ignored -> hard to find bug.

I'm following, I think what we need here is to think about additional
behaviours and electronic constraints we can encode into the drivers
and/or the pin tables to safeguard pin states from electronically
unsound states.

That is to say, I prefer the subsystem to be conscious about the
electronic constraints and navigate around them or put up a road
block, rather than trying ti avoid driving into the roadblocks by means
of carefully crafted tables if you get the picture ...

Yours,
Linus Walleij
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