So this is finally it. After all the work writing support for fences cross-dev synchronization is now possible. :-)
The last 2 patches of this series are not needed for cross-dev to work. But without it any waits on cross-device fences will be done synchronously. I've previously tested this with i915, but the patches for i915 fail to apply again with the execlist stuff, so I haven't tried with the latest drm-next changes.
I would like to have the first 2 patches applied on drm-next, and the radeon/nouveau specific patches when they go through their review.