On Mon, Jun 13, 2016 at 10:54:37AM +0900, Michel Dänzer wrote: > On 10.06.2016 23:43, Daniel Vetter wrote: > > On Fri, Jun 10, 2016 at 05:57:12PM +0900, Michel Dänzer wrote: > >> From: Michel Dänzer <michel.daenzer at amd.com> > >> > >> If userspace wants a page flip to take effect during vblank sequence n, > >> it has to wait for vblank seqno n-1 before calling the > >> DRM_IOCTL_MODE_PAGE_FLIP ioctl. > >> > >> This change makes sure that we do not program the flip to the hardware > >> before the end of vblank seqno n-1 in this case, to prevent the flip > >> from taking effect too early. > >> > >> On the other hand, if the DRM_IOCTL_MODE_PAGE_FLIP ioctl is called > >> during vblank, but userspace didn't wait for the current vblank seqno > >> before, this change would still allow the flip to be programmed during > >> the current vblank seqno. > > > > This just sounds like you're sending vblank events out a bit too early. > > And watching vblank waits that userspace does works, but it's fragile, > > add-hoc and I don't really jump in joy about adding that to the vblank > > core. Is there no way you can adjust sending out the vblank events > > similarly, to make sure userspace can never sneak in a pageflip too early? > > What you call "too early" is actually "during the vertical blank period > waited for". IMHO only notifying userspace of a vertical blank period > when it's already over would defeat the purpose.
Afaiui the rules are: - The timestamp for vblank event needs to agree with whatever oml_sync requries. - The event delivery itself needs to be consistent with what page_flip takes, i.e. if userspace sees an event and immediately issues a page_flip then it should not be able to hit the same vblank with that pageflip. - The event needs to be after the old buffers are not longer used and can be reused for rendering. - There's no requirement at all that the event gets delivered at a specific point in the vblank, hardware is too different for that to work - that kind of precision is why we have a separate timestamp. I assume you're goal is to not delay page_flips unecessarily, without breaking requirement 2 here. Imo a simpler fix would be to delay the vblank handling to end of vblank. Fixes everything without hacks, no single-use driver oddity in the core, and you still can page_flip as late as you want without forcing a delay to the next vblank if your already past the start of vblank. And since you have high-precision timestamp support the vblank core will fudge the timestamp as needed. Ofc this assumes that amd hw has a vblank_end irq, but most desktop hw seems to have that (didn't check tbh whether amd has it). So all that's needed I think is to enable the vblank_end irq (in lockstep with the start one) and move drm_handle_vblank() from its current place to that new irq. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
