Date: Mon, 29 Nov 2010 23:10:50 +0100 (CET)
From: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
To: Greg KH <greg@kroah.com>
cc: Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH/RFC] core: add a function to safely try to get device
 driver owner
In-Reply-To: <20101129214128.GA9691@kroah.com>
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 <20101129214128.GA9691@kroah.com>
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On Mon, 29 Nov 2010, Greg KH wrote:

> On Mon, Nov 29, 2010 at 09:54:10PM +0100, Guennadi Liakhovetski wrote:
> > Hi Jon
> > 
> > On Mon, 29 Nov 2010, Jonathan Corbet wrote:
> > 
> > > On Mon, 29 Nov 2010 20:43:28 +0100 (CET)
> > > Guennadi Liakhovetski <g.liakhovetski@gmx.de> wrote:
> > > 
> > > > When two drivers interoperate without an explicit dependency, it is often
> > > > required to prevent one of them from being unloaded safely by dereferencing
> > > > dev->driver->owner. This patch provides a generic function to do this in a
> > > > race-free way.
> > > 
> > > I must ask: why not, instead, make the dependency explicit?  In
> > > particular, this looks like an application for the proposed media
> > > controller code, which is meant to model the connections between otherwise
> > > independent devices.  The fact that your example comes from V4L2 (which is
> > > the current domain of the media controller) also argues that way.
> > 
> > Sorry, don't see a good way to do this. This function is for a general 
> > dependency, where you don't have that driver, we are checking for register 
> > with us, so, the only way to get to it is via dev->driver->owner.
> 
> Wait, what?  The device is already bound to a driver, right, so why
> would you care about "locking" the module into memory?  What could this
> possibly be used for?

To protect against rmmod -> driver_unregister -> dev->driver = NULL?

> > And I also don't want to move registering the device into the
> > dependant driver and then wait (with a timeout) for a driver to probe
> > with it... I just want to verify, whether a driver has attached to
> > that device and whether I can lock it down.
> 
> Who cares if a driver is attached to any device?  And again, why would
> you want to "lock it down"?

In my case I have two platform devices: CEU and CSI2. In some cases (with 
parallel sensors) CEU operates on its own. With serial (CSI-2) camera 
sensors we need the CSI2 driver. So, I want to 
try_module_get(csi2_dev->driver->owner) the CSI2 driver from my CEU 
driver. This call can Oops if not done safely. Am I missing something? Is 
there an easier way to achieve the same?

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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