Date:	Tue, 30 Nov 2010 21:43:09 +0100
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Greg KH <greg@...ah.com>
Cc:	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
	Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH/RFC] core: add a function to safely try to get device driver owner

Hi Greg,

On Tuesday 30 November 2010 19:32:25 Greg KH wrote:
> On Tue, Nov 30, 2010 at 06:55:54PM +0100, Laurent Pinchart wrote:
> > On Tuesday 30 November 2010 18:15:09 Greg KH wrote:
> > > On Tue, Nov 30, 2010 at 06:09:46PM +0100, Guennadi Liakhovetski wrote:
> > > > 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:
> > [snip]
> > 
> > > > > > > > > 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.
> > > 
> > > I agree, it probably would not solve all of the different issues that
> > > people might have for this type of thing, and this isn't the first time
> > > I've heard it be requested either.
> > > 
> > > But, this patch is just trying to increment a module owner of a device
> > > that is bound to a driver, which is the wrong level to be thinking of
> > > it.
> > > 
> > > If you request a module to be loaded, what would possibly cause it to
> > > be unbound that you need to have this "safely" in place?  Why would
> > > the module be unloaded?  And if it was unloaded, doesn't that imply
> > > that someone else wanted it unloaded so keeping that from happening
> > > would be a bit rude, right?
> > 
> > It depends on your definition of rude. I would consider the kernel even
> > more rude if it accepted my unload request and then crashed.
> 
> I totally agree, and that is a bug that should be fixed, but shouldn't
> have anything to do with this proposed interface (i.e. locking the
> module in place is not the proper fix.)
> 
> > I've recently run into a problem similar to Guennadi's with the OMAP3 ISP
> > driver. The driver instantiates several V4L2 I2C sub-devices for the
> > camera sensors and the lens and flash controllers. The sub-device
> > drivers get platform data when they're probed, and receive callbacks to
> > the board code to turn power on/off and configure clocks (it's a bit
> > more complex than just that, but you get the idea). The board code
> > callbacks then call to the OMAP3 ISP driver to configure clocks, because
> > the sensor clock is provided by the OMAP3 ISP.
> > 
> > Now, when the user opens the sensor's subdev device node
> > (/dev/v4l-subdev*), the subdev open function will turn the sensor clock
> > on. To do that it will call the OMAP3 ISP driver through board code. If
> > the OMAP3 ISP driver is unloaded at that point things will go pretty
> > bad.
> 
> Then the interface to call that driver should be properly reference
> counted, right?  That has nothing to do with the driver core locking
> modules into place.
> 
> > The way we deal with this is to try_module_get() on the OMAP3 ISP driver
> > in the subdev open() handlers. I'm of course opened to alternatives.
> 
> Do it like the rest of the kernel does it, lock the module in place with
> the module pointer it passed to you before calling open in that module.
> Nothing new here at all.

That doesn't work in this case, because we have two modules. Module A is the 
master and instantiates an I2C device handled by module B. Module B creates a 
character device and sets itself as the owner. When the corresponding device 
node is opened, module B's refcount is incremented, but module A refcount 
isn't, even though module B can call to module A through board code using 
function pointers provided in the platform data.

-- 
Regards,

Laurent Pinchart
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