Draft: RFC Introduce long-running completion fences

For long-running workloads, drivers either need to open-code completion waits, invent their own synchronization primitives or internally use dma-fences that do not obey the cross-driver dma-fence protocol, but without any lockdep annotation all these approaches are error prone.

So since for example the drm scheduler uses dma-fences it is desirable for a driver to be able to use it for throttling and error handling also with internal dma-fences tha do not obey the cros-driver dma-fence protocol.

Introduce long-running completion fences in form of dma-fences, and add lockdep annotation for them. In particular:

• Do not allow waiting under any memory management locks.
• Do not allow to attach them to a dma-resv object.
• Introduce a new interface for adding callbacks making the helper adding a callback sign off on that it is aware that the dma-fence may not complete anytime soon. Typically this will be the scheduler chaining a new long-running fence on another one.