linux-kernel - Re: [PATCH v2] ARM: dts: imx6qdl-sabresd.dtsi: Add red led

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Date:	Thu, 6 Mar 2014 10:05:16 +0800
From:	Shawn Guo <shawn.guo@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Vincent Stehlé <vincent.stehle@...escale.com>,
	Sascha Hauer <kernel@...gutronix.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: dts: imx6qdl-sabresd.dtsi: Add red led

On Wed, Mar 05, 2014 at 07:39:26PM +0000, Russell King - ARM Linux wrote:
> On Wed, Mar 05, 2014 at 07:42:28PM +0800, Shawn Guo wrote:
> > It's not a hog pin, so shouldn't be added here.  (Right, most of the
> > existing pins shouldn't be here from the beginning)
> 
> On the subject of that kind of thing... I have an issue with the existing
> pinmux stuff.  This is a wider issue than your discussion above...
> 
> I regard the idea of requiring the pinmux to be configured as part of
> the driver probe a rather broken concept - yes, it makes sense in some
> scenarios, but for the vast majority of cases, it doesn't.

>From what I've seen, it's the opposite - the concept works for the
majority, and only some scenarios needs special handling.

> 
> Let's take an example.  A pin is wired to a SPDIF transceiver.  The
> out-of-reset SoC configuration results in the pin turning the LED on
> in the SPDIF transceiver.
> 
> With the way stuff is setup, with the driver having the pinmux settings
> for itself, the point at which that pin gets configured is when the
> driver loads, which may be a module - so that can happen quite late in
> the boot sequence.  A failure to boot can result in it not happening at
> all.
> 
> So, this brings up the obvious question: is this proper behaviour of the
> hardware, or is this something we should do better with - such as where
> we know what the settings should be for a platform, and these settings
> never change, we arrange for that to happen at kernel boot time as a
> once-off.
> 
> For example, in the above case, we know it's board XYZ, we know that this
> pin is connected to the SPDIF transceiver, so maybe we should configure
> it early on in the boot sequence whether or not the driver has been
> configured and/or loaded.

For such special case, I'm fine with using hog group to handle it.  But
IMO, bootloader is even a better place to correct such unreasonable
out-of-reset SoC configuration for particular board.

Shawn

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