Out of curiosity, would it be reasonable to allow user-space (more precisely, the compositor) to set the deadline via an IOCTL without actually performing an atomic commit with the FB? Some compositors might want to wait themselves for FB fence completions to ensure a client doesn't block the whole desktop (by submitting a very costly rendering job). In this case it would make sense for the compositor to indicate that it intends to display the buffer on next vblank if it's ready by that point, without queueing a page-flip yet.
