On 2022-02-19 07:32, Christoph Hellwig wrote:
So we are back to the callback madness instead of the nice and simple flag? Sigh.
TBH, I *think* this part could be a fair bit simpler. It looks like this whole callback mess is effectively just to decrement group->owner_cnt, but since we should only care about ownership at probe, hotplug, and other places well outside critical fast-paths, I'm not sure we really need to keep track of that anyway - it can always be recalculated by walking the group->devices list, and some of the relevant places have to do that anyway. It should be pretty straightforward for iommu_bus_notifier to clear group->owner automatically upon an unbind of the matching driver when it's no longer bound to any other devices in the group either. And if we still want to entertain the notion of VFIO being able to release ownership without unbinding (I'm not entirely convinced that's a realistically necessary use-case) then it should be up to VFIO to decide when it's finally finished with the whole group, rather than pretending we can keep track of nested ownership claims from inside the API.
Furthermore, If Greg was willing to compromise just far enough to let us put driver_managed_dma in the 3-byte hole in the generic struct device_driver, we wouldn't have to have quite so much boilerplate repeated across the various bus implementations (I'm not suggesting to move any actual calls back into the driver core, just the storage of flag itself). FWIW I have some ideas for re-converging .dma_configure in future which I think should probably be able to subsume this into a completely generic common path, given a common flag.
Robin.
