Date: Wed, 6 Apr 2011 10:56:47 -0700
From: Greg KH <gregkh@suse.de>
To: Samuel Ortiz <sameo@linux.intel.com>
Cc: Grant Likely <grant.likely@secretlab.ca>,
        Andres Salomon <dilinger@queued.net>, linux-kernel@vger.kernel.org,
        Mark Brown <broonie@opensource.wolfsonmicro.com>, khali@linux-fr.org,
        ben-linux@fluff.org, Peter Korsgaard <jacmet@sunsite.dk>,
        Mauro Carvalho Chehab <mchehab@infradead.org>,
        David Brownell <dbrownell@users.sourceforge.net>,
        linux-i2c@vger.kernel.org, linux-media@vger.kernel.org,
        netdev@vger.kernel.org, spi-devel-general@lists.sourceforge.net,
        Mocean Laboratories <info@mocean-labs.com>
Subject: Re: [PATCH 07/19] timberdale: mfd_cell is now implicitly available
 to drivers
Message-ID: <20110406175647.GA8048@suse.de>
References: <20110401112030.GA3447@sortiz-mobl>
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 <20110401235239.GE29397@sortiz-mobl>
 <BANLkTi=bq=OGzXFp7qiBr7x_BnGOWf=DRQ@mail.gmail.com>
 <20110404100314.GC2751@sortiz-mobl>
 <20110405030428.GB29522@ponder.secretlab.ca>
 <20110406152322.GA2757@sortiz-mobl>
 <20110406155805.GA20095@suse.de>
 <20110406170537.GB2757@sortiz-mobl>
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On Wed, Apr 06, 2011 at 07:05:38PM +0200, Samuel Ortiz wrote:
> Hi Greg,
> 
> On Wed, Apr 06, 2011 at 08:58:05AM -0700, Greg KH wrote:
> > On Wed, Apr 06, 2011 at 05:23:23PM +0200, Samuel Ortiz wrote:
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -33,6 +33,7 @@ struct class;
> > >  struct subsys_private;
> > >  struct bus_type;
> > >  struct device_node;
> > > +struct mfd_cell;
> > >  
> > >  struct bus_attribute {
> > >  	struct attribute	attr;
> > > @@ -444,6 +445,8 @@ struct device {
> > >  	struct device_node	*of_node; /* associated device tree node */
> > >  	const struct of_device_id *of_match; /* matching of_device_id from driver */
> > >  
> > > +	struct mfd_cell	*mfd_cell; /* MFD cell pointer */
> > > +
> > 
> > What is a "MFD cell pointer" and why is it needed in struct device?
> An MFD cell is an MFD instantiated device.
> MFD (Multi Function Device) drivers instantiate platform devices. Those
> devices drivers sometimes need a platform data pointer, sometimes an MFD
> specific pointer, and sometimes both. Also, some of those drivers have been
> implemented as MFD sub drivers, while others know nothing about MFD and just
> expect a plain platform_data pointer.

That sounds like a bug in those drivers, why not fix them to properly
pass in the correct pointer?

> We've been faced with the problem of being able to pass both MFD related data
> and a platform_data pointer to some of those drivers. Squeezing the MFD bits
> in the sub driver platform_data pointer doesn't work for drivers that know
> nothing about MFDs. It also adds an additional dependency on the MFD API to
> all MFD sub drivers. That prevents any of those drivers to eventually be used
> as plain platform device drivers.

Then they shouldn't be "plain" platform drivers, that should only be
reserved for drivers that are the "lowest" type.  Just make them MFD
devices and go from there.

> So, adding an MFD cell pointer to the device structure allows us to cleanly
> pass both pieces of information, while keeping all the MFD sub drivers
> independant from the MFD core if they want/can.

They shouldn't be "independant", make them "dependant" and go from
there.

thanks,

greg k-h
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