linux-kernel - Re: [PATCH 1/3] soc_camera: Add the ability to bind regulators to soc

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Message-ID: <20101129154416.GA30926@sirena.org.uk>
Date:	Mon, 29 Nov 2010 15:44:18 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Alberto Panizzo <maramaopercheseimorto@...il.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Hans Verkuil <hverkuil@...all.nl>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Magnus Damm <damm@...nsource.se>,
	M?rton N?meth <nm127@...email.hu>, linux-media@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] soc_camera: Add the ability to bind regulators to
	soc_camedra devices

On Sun, Nov 28, 2010 at 08:05:06PM +0100, Guennadi Liakhovetski wrote:
> On Sun, 28 Nov 2010, Alberto Panizzo wrote:

> > In certain machines, camera devices are supplied directly
> > by a number of regulators. This patch add the ability to drive
> > these regulators directly by the soc_camera driver.

> IIRC, there has been a discussion a while ago, how to supply power to 
> cameras by regulators. Someone has tried to provide a .power() hook in the 
> platform code, but the problem was the order of driver loading / probing. 
> So, how about doing the following:

> 1. in your platform code you register a notifier like
> 	bus_register_notifier(&soc_camera_bus_type, &cam_notifier);

FWIW I'm looking at implementing a standard regulator API feature along
these lines in the background.  This should hopefully mean we don't need
driver support for most simple power control applications.  No ETA yet.

> The reasons why I do not want to add this to the core are: (1) I do not 
> want to have two methods for turning power on and off: a platform provided 
> .power() hook and and a set of regulators, (2) would anyone really want to 
> use several regulators for a camera sensor? If so, can it be the case, 
> that, for example, the regulators have to be switched off in the reverse 
> order to switching on? Or something else? (3) regulators can often do 
> more, than just set one of two power levels - for on and off. What if a 
> need arises to use other voltages?

The way MMC handled this was to provide a standard version of the hook
in the core which could be used by platforms with regulators supplying
the device - they just assign the appropriate function as their power()
operation AIUI.  That seems a fairly clean way of keeping stuff in the
core without giving up the flexibility.

> Is there any really good reason, why we _have_ to do this in soc-camera 
> core?

It does save everyone open coding stuff.
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