linux-kernel - Re: mfd: Core driver for Winbond chips

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Message-ID: <20130410003632.GA25712@roeck-us.net>
Date:	Tue, 9 Apr 2013 17:36:32 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Wim Van Sebroeck <wim@...ana.be>
Cc:	Samuel Ortiz <sameo@...ux.intel.com>,
	linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
	Pádraig Brady <P@...igBrady.com>
Subject: Re: mfd: Core driver for Winbond chips

On Tue, Apr 09, 2013 at 07:31:15PM +0200, Wim Van Sebroeck wrote:
> Hi Guenter,
> 
> > > > I was waiting for feedback from Wim, who submitted a similar driver, about his
> > > > thoughts. Key question is how to reserve access to the shared resource - either
> > > > through an exported function in the mfd driver requesting a mutex, or through
> > > > request_muxed_region(). I am going back and forth myself on which one is better.
> > > > 
> > > > Maybe it does not really matter, but using a function has the slight advantage
> > > > that it auto-loads and locks the mfd module while one of its client modules
> > > > is loaded. If we use request_muxed_region, that is not the case and the client
> > > > module must use another means to request and lock the mfd module.
> > > > 
> > > > Maybe you have an opinion ?
> > > 
> > > This is indeed the main issue that has to be solved. Both options will work.
> > > I like the auto-load and lock, but I need to look at the request_muxed_region
> > > code again first before I can see what the possible drawbacks are :-).
> > > 
> > One drawback of using request_muxed_region is that it needs a return value
> > from superio_enter. Also, it needs some code in the client driver init function
> > to ensure that the mfd driver gets loaded, and possibly a call to __module_get()
> > in the client driver probe function to keep the mfd driver loaded.
> > 
> > winbond_superio_enter() would not need a return value and could use
> > devm_request_region. We could also consider allocating the hwmon memory space in
> > the mfd driver and pass it as resource to the client drivers, which would remove
> > a few more lines of code from those.
> > 
> > Overall I am slightly in favor of using an exported function.
> 
> I looked at commit 8b6d043b7ee2d1b819dc833d677ea2aead71a0c0 (which implements
> request_muxed_region). You indeed need some extra code for loading the lowl-level
> mfd driver. So I am also in favour of the exported function.
> 
So which way should we go ? Take your driver as a starting point or mine ?

One thing I'll want to add is support for both superio regions, as I have a use
case for it.

Thanks,
Guenter
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