linux-kernel - Re: [PATCH v2 1/4] mfd: add viperboard driver

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Message-Id: <201210161143.19619.poeschel@lemonage.de>
Date:	Tue, 16 Oct 2012 11:43:19 +0200
From:	Lars Poeschel <poeschel@...onage.de>
To:	"Lars-Peter Clausen" <lars@...afoo.de>
Cc:	Lars Poeschel <larsi@....tu-dresden.de>, sameo@...ux.intel.com,
	linux-kernel@...r.kernel.org, jic23@....ac.uk, khali@...ux-fr.org,
	ben-linux@...ff.org, w.sang@...gutronix.de,
	grant.likely@...retlab.ca, linus.walleij@...aro.org
Subject: Re: [PATCH v2 1/4] mfd: add viperboard driver

On Tuesday 16 October 2012 at 10:40:26, Lars-Peter Clausen wrote:
> On 10/12/2012 04:34 PM, Lars Poeschel wrote:
> > [...]
> > +static void vprbrd_dev_release(struct device *dev)
> > +{
> > +	return;
> 
> A empty release callback is usually a good indicator that something is
> wrong. The release callback will be called once the last reference to the
> device has been called, so the memory associated with the device should not
> be freed before the release callback has been called, otherwise the memory
> might be accessed after it has been freed...
> 
> > +}
> > +
> > +static void vprbrd_free(struct vprbrd *dev)
> > +{
> > +	usb_put_dev(dev->usb_dev);
> > +	kfree(dev);
> 
> ..., so this kfree should be moved from here to the release callback.

Thank you for catching that one!
 
> Btw. I'm wondering why is the extra platform device required? Can't you not
> just use the usb device as the parent device for the mfd cells?

This is what I first did, but this does not work. You can read about my first 
thoughts why this is not working here: (To sum it up: The device is housed in 
an usb_device, not a platform_device and This usb_device has no mfd_cell 
member.)

https://lkml.org/lkml/2012/9/28/327

As I got a bit more deeper I also noticed, that mfd_add_devices (obviously) 
adds the devices "as childs" to the parent device. mfd_remove_devices then 
removes ALL "child" devices from the parent, not only those added by 
mfd_add_devices before. This does not work in the case of the usb parent 
device, because it has other childs that the usb layer added before (some 
endpoints and stuff). So I had to construct an "empty" (in sense of childs) 
mock platform_device between the usb and mfd.
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