linux-kernel - Re: [PATCH 12/19] twl4030: mfd_cell is now implicitly available to drivers

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Message-ID: <20110204104159.GA18714@pengutronix.de>
Date:	Fri, 4 Feb 2011 11:41:59 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Andres Salomon <dilinger@...ued.net>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	alsa-devel@...a-project.org, Samuel Ortiz <sameo@...ux.intel.com>,
	Takashi Iwai <tiwai@...e.de>,
	Brown <broonie@...nsource.wolfsonmicro.com>,
	Peter Ujfalusi <peter.ujfalusi@...ia.com>,
	linux-kernel@...r.kernel.org, Mark@...a-project.org,
	Jaroslav@...a-project.org, linux-input@...r.kernel.org,
	Timur Tabi <timur@...escale.com>,
	Liam Girdwood <lrg@...mlogic.co.uk>
Subject: Re: [PATCH 12/19] twl4030: mfd_cell is now implicitly available to
 drivers

Hello Andres,

On Wed, Feb 02, 2011 at 11:03:26PM -0800, Andres Salomon wrote:
> On Wed, 2 Feb 2011 22:53:39 -0800
> Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> 
> > On Wed, Feb 02, 2011 at 10:39:59PM -0800, Andres Salomon wrote:
> > > On Wed, 2 Feb 2011 22:05:21 -0800
> > > Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> > > 
> > > > On Wed, Feb 02, 2011 at 08:15:22PM -0800, Andres Salomon wrote:
> > > > >  static int __devinit twl4030_vibra_probe(struct platform_device
> > > > > *pdev) {
> > > > > -	struct twl4030_codec_vibra_data *pdata =
> > > > > pdev->dev.platform_data;
> > > > > +	struct twl4030_codec_vibra_data *pdata =
> > > > > platform_get_drvdata(pdev);
> > > > 
> > > > No, device's drvdata belongs to _this_ driver, and it is supposed
> > > > to manage it and use as it sees fit.
> > > 
> > > Right, so it's used to pass data to the probe function; once the
> > > probe function has obtained the pdata pointer, it's free to do with
> > > it what it will.
> > > 
> > > 
> > > > 
> > > > Note platform_set_drvdata(pdev, info) later in this function along
> > > > with platform_set_drvdata(pdev, NULL) in twl4030_vibra_remove(),
> > > > which means that with your change you will be able to bind the
> > > > device only once.
> > > > 
> > > 
> > > Hm, good point; if the driver is reloaded, the pdev that was
> > > created by mfd-core will have lost the pointer to pdata.
> > > 
> > > I wonder if I should be using mfd's driver_data instead. I used
> > > platform_data because a bunch of drivers had already made use of it
> > > to pass cell information..
> > 
> > Then they are doing it incorrectly. One possible way is to have parent
> > device carry relevant data in its drvdata and have children get it
> > from there.
> > 
> 
> I believe some drivers are even using the parent device already.  See
> drivers/leds/leds-mc13783.c, for example, whose parent device drvdata
> is used to pass around a struct mc13783 to its children.  Sounds
> like a possibility, will need to look into it further.
IMHO this isn't optimal done.  The led driver somehow needs access to a
struct mc13xxx because that one defines how to change the led-related
registers.

If you ask me, the most clean solution would be that the functions like
mc13xxx_lock and mc13xxx_reg_rmw wouldn't take a struct mc13xxx * as
first parameter but a struct device *.  Because in fact it's not the led
driver's business what the mfd driver stores in his driver data.

(Note, I said clean, neither easy nor effective nor best.)

Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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