linux-kernel - Re: Question about MFD devices.

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Message-ID: <20110527151746.GB24011@sirena.org.uk>
Date:	Fri, 27 May 2011 16:17:46 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Joachim Holst <jocke@...stenhof.se>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about MFD devices.

On Thu, May 26, 2011 at 05:26:24PM +0200, Joachim Holst wrote:

> In the process, I've been trying to figure out when these MFD devices are
> supposed to be used, but I don't see the logic behind them as of yet.
> Currently it seems to me that "if a chip has more than one function, it's
> an MFD" no matter what type of functions the chip supplies.

Yes.

> To me, MFD seems to somewhat duplicate the standard kernel driver
> structure, most implementations I've seen make it rather difficult to use
> exported interfaces without integrating your driver into the structure.

Right, and MFD driver is all about sharing the resources of a chip
between the various subsystems in Linux that implement the functionality
of the chip.  This means that the various subfunctions often know quite
a bit about how the core works.

> Personally, I really don't think that a simple driver that toggles a GPIO
> X times based on an ADC value should be integrated into an MFD device
> structure. A misc device seems a bit more appropriate for this.

This doesn't seem like an MFD to me - an MFD is more about sharing the
resources on a chip than pulling things together, that sounds like
taking multiple resources on the chip and gluing them together into a
single driver.
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