RFC: Unpinned DMA-buf handling

[Christian König]

The basic idea stayed the same since the last version of those patches. The exporter can provide explicit pin/unpin functions and the importer a move_notify callback. This allows us to avoid pinning buffers while importers have a mapping for them.

In difference to the last version the locking changes were separated from this patchset and committed to drm-misc-next.

This allows drivers to implement the new locking semantics without the extra unpinned handling, but of course the changed locking semantics is still a prerequisite to the unpinned handling.

The last time this set was send out the discussion ended by questioning if the move_notify callback was really the right approach of notifying the importers that a buffer is about to change its placement. A possible alternative would be to add a special crafted fence object instead.

Let's discuss on the different approaches once more,
Christian.