At the suggestion of a few drm developers, I'm looking at abstracting the buffer sharing mechanism away from the individual drm drivers and treating it as a low-level interface that kernel subsystems use to communicate, rather than as something drivers should be accessing directly. This would also mean that they wouldn't have to each implement their own set of dma_buf_ops, and the logic for things like detecting that you're importing your own dma_buf would be written once, in drm_prime.c and not in every driver's gem_prime_import function.
Of course, it's slightly difficult because each driver's implementation seems to be subtly different.
* i915 uses its own special locking function, i915_mutex_lock_interruptible.
* nouveau and radeon pin the pages when the dma_buf is created, while i915 pins them at map time.
* the vmap functions are different between i915 and radeon/nouveau, but it looks like all they use the dma_buf object for is to find the GEM object.
Does it make sense to try to abstract the dma_buf parts of this? For example, a hypothetical new drm_gem_map_dma_buf would call a hook that lets i915 do its i915_gem_wait_for_error thing, takes the lock, calls a new gem_get_pages driver hook, does the dma_map_sg call, and handles the unlocking? I'll come up with a more detailed proposal or patches if this sounds like a good idea.
-- Aaron