On Tue, Mar 25, 2014 at 10:40:45AM +0100, David Herrmann wrote:
Hi
On Tue, Mar 25, 2014 at 9:01 AM, Daniel Vetter daniel@ffwll.ch wrote:
Besides the issue at hand though I think drivers need to make sure that the device they use for attaching does outlive the dma-buf. Which for real hotpluggin probably means that drivers need to drop all attachment on unplug (the dma mapping is useless anyway) and just keep all their imported gem objects alive with just a reference to the dma-buf object itself.
Drivers should never touch other drivers or even look at them. There is no reason i915 is responsible of keeping udl alive. That gets really messy and may introduce circular dependencies.
I'd like to see exported dma-bufs reference their drm-device owner. This way, the drm-device stays around until the dma-buf is removed. To avoid lazy device destruction, a driver can (during unplug) simply detach all dma-bufs if, and only if, they first made the dma-buf somehow stand-alone.
For example: udl can just move the allocated pages into the dma-buf, mark it as dead and detach it. The udl-device can get destructed and whenever i915 releases the dma-buf, the udl-dma-buf ops see it's dead and just deref it / release dma-buf resources. But this is all not needed if the exported dma-bufs just reference "drm_device", which is imho the easiest fix.
The issue at hand is the other way round, i.e. i915 exporting, udl importing. The i915/dma-buf falls over because the device struct referenced in the attachment disappeared. Dave fixed it by grabbing a reference, I think the better fix would be to simply not attach (since udl doesn't need it anyway). Either works. -Daniel