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/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
