Precise vblank timestamps and counting is needed for DRI2 sync and swap extensions and GLX_OML_sync_control, among other purposes. See this article for when the precise vblank timestamp support originally came to the Linux DRM drivers.
Published to the Nouveau mailing list is now a patch that brings the Nouveau support up to par. "This patch implements the drivers hooks needed for precise vblank timestamping. This is a complementary patch to Mario Kleiner's patches to to improve swap scheduling. With the complete patchset applied nouveau will be able to provide correct and precise pageflip timestamps (compliant to OML_sync_control spec)."