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

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Message-ID: <74CDBE0F657A3D45AFBB94109FB122FF04AEA25368@HQMAIL01.nvidia.com>
Date:	Tue, 16 Aug 2011 10:12:17 -0700
From:	Stephen Warren <swarren@...dia.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	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 Tuesday, August 16, 2011 8:37 AM:
> On Tuesday 16 August 2011, Linus Walleij wrote:
> > One specific thing worries me: Grant asked me to make sure
> > to NOT create a global pin number space for the pinmuxes (and thus
> > pinctrl). This means that in order to proceed, mappings of pinmux
> > groups or pincontrol (such as bias) groups, each device using such
> > an entity need to reference the intended pincontroller/mux instance.
> >
> > Say mmc instance 0 need pingroup foo on pincontroller bar
> > means that there must be a specific reference from mmc.0:s
> > struct device * to pinctrl bar:s struct device *. Maybe this is
> > peanuts in DT, sorry not enough insight.
> 
> I think what you are looking for is the equivalent of the
> interrupt-parent property for pinmux. The idea is that each
> node in the device tree can point to a device managing the
> pinmux, so reference would point to a local number in that
> space. We have discussed this for the GPIO case already, and
> I suspect that the two should be identical (gpio-controller
> and pinmux-controller using the same device node and same
> property to refer to them). Since the pinmux-parent
> (gpio-parent, ...) property gets inherited by all child
> devices, you only need to set it once at the root of the
> device tree for the simple case where there is only one
> controller.

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...

-- 
nvpublic

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