linux-kernel - RE: [RFC PATCH v2 00/13] arm/tegra: Initialize GPIO & pinmux from DT

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Message-ID: <74CDBE0F657A3D45AFBB94109FB122FF04AF6F3062@HQMAIL01.nvidia.com>
Date:	Wed, 17 Aug 2011 23:22:16 -0700
From:	Stephen Warren <swarren@...dia.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Colin Cross <ccross@...roid.com>,
	Erik Gilling <konkers@...roid.com>,
	Olof Johansson <olof@...om.net>,
	Russell King <linux@....linux.org.uk>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Belisko Marek <marek.belisko@...il.com>,
	Jamie Iles <jamie@...ieiles.com>,
	Shawn Guo <shawn.guo@...escale.com>,
	Sergei Shtylyov <sshtylyov@...sta.com>
Subject: RE: [RFC PATCH v2 00/13] arm/tegra: Initialize GPIO & pinmux from DT

Arnd Bergmann wrote at Wednesday, August 17, 2011 5:24 AM:
> On Tuesday 16 August 2011, Stephen Warren wrote:
> > One issue here: There isn't always a single gpio/pinmux parent; as a
> > concrete example, the ALSA/ASoC driver for Tegra+WM8903 uses GPIOs both
> > from Tegra itself, and from the WM8903 audio codec.
> >
> > I could imagine the same being true in basically any case where one
> > device uses N GPIOs (e.g. SD controller with power, change-detect,
> > and read-only GPIOs; some could easily come from the SoC and some
> > from a GPIO expander).
> >
> > I'm not quite so sure that multiple parents would be useful for pinmux,
> > but I wouldn't say that it was impossible...
> 
> Hmm, how common do you think the example is?
> 
> It it's just a very rare case, we can still special-case it by
> introducing fake subdevices for the Tegra+WM8903 driver, each of
> which has only one gpio-parent. This is ugly but keeps the special
> case to the rare devices.
> 
> If we can expect more than a handful of these, we probably need
> a better design for the binding, which will cause a burden for
> all drivers.

It looks like DT already supports the generic case for GPIO; each GPIO
reference is of the form:

<&gpio 69 0>

and so each GPIO has a "parent" specified, and hence can come from a
different controller.

For drivers, this is all completely hidden by of_get_gpio().

I imagine we'd just use the same pattern for pinmux. This would both
support the general case, and end up being consistent with how GPIOs
already work, so this seems like a win both ways.

-- 
nvpublic

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