[RFC] Implicit vs explicit user fence sync

[Christian König]

Hi guys,

with this patch set I want to look into how much more additional work it would be to support implicit sync compared to only explicit sync.

Turned out that this is much simpler than expected since the only addition is that before a command submission or flip the kernel and classic drivers would need to wait for the user fence to signal before taking any locks.

For this prototype this patch set doesn't implement any user fence synchronization at all, but just assumes that faulting user pages is sufficient to make sure that we can wait for user space to finish submitting the work. If necessary this can be made even more strict, the only use case I could find which blocks this is the radeon driver and that should be handle able.

This of course doesn't give you the same semantic as the classic implicit sync to guarantee that you have exclusive access to a buffers, but this is also not necessary.

So I think the conclusion should be that we don't need to concentrate on implicit vs. explicit sync, but rather how to get the synchronization and timeout signalling figured out in general.

Regards,
Christian.