Re: [PATCH/RFC] core: add a function to safely try to get devicedriver owner

[Guennadi Liakhovetski]

On Tue, 30 Nov 2010, Greg KH wrote:

> On Tue, Nov 30, 2010 at 12:11:42AM +0100, Guennadi Liakhovetski wrote:
> > On Mon, 29 Nov 2010, Greg KH wrote:
> > 
> > > On Mon, Nov 29, 2010 at 11:10:50PM +0100, Guennadi Liakhovetski wrote:
> > > > 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@xxxxxx> 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?
> > > 
> > > But again, why would some other driver ever care about what some random
> > > dev->driver would be?
> > 
> > It's not a random one, call it a "companion device."
> 
> Ok, but again go back to Jon's original proposal to just call the
> functions in that driver from yours, causing the implicit module
> ordering issue to be automatically resolved.

Greg, in this specific case - yes, I could do this. But (1) there is no 
need for that - both drivers implement and use the v4l2-subdev API and 
thus stay generic. In the host driver this adds the convenience, that it 
doesn't have to call to the CSI2 driver explicitly at all - it just calls 
the v4l2-subdev function like "call .s_mbus_fmt for all subdev drivers" 
and the function is called for the sensor and the CSI2 driver. (2) what 
about the other location I pointed out earlier in the v4l2 core? There 
drivers are absolutely generic. I also suspect these are not the only 
cases, where this helper would come in handy. I added the media list to CC 
for any more opinions on this matter.

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