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drm/vc4: Make pageflip completion handling more robust.
Protect both the setup of the pageflip event and the latching of the new requested displaylist head pointer by the event lock, so we can't get into a situation where vc4_atomic_flush latches the new display list via HVS_WRITE, then immediately gets preempted before queueing the pageflip event, then the page-flip completes in hw and the vc4_crtc_handle_page_flip() runs and no-ops due to lack of a pending pageflip event, then vc4_atomic_flush continues and only then queues the pageflip event - after the page flip handling already no-oped. This would cause flip completion handling only at the next vblank - one frame too late. In vc4_crtc_handle_page_flip() check the actual DL head pointer in SCALER_DISPLACTX against the requested pointer for page flip to make sure that the flip actually really completed in the current vblank and doesn't get deferred to the next one because the DL head pointer was written a bit too late into SCALER_DISPLISTX, after start of vblank, and missed the boat. This avoids handling a pageflip completion too early - one frame too early. According to Eric, DL head pointer updates which were written into the HVS DISPLISTX reg get committed to hardware at the last pixel of active scanout. Our vblank interrupt handler, as triggered by PV_INT_VFP_START irq, gets to run earliest at the first pixel of HBLANK at the end of the last scanline of active scanout, ie. vblank irq handling runs at least 1 pixel duration after a potential pageflip completion happened in hardware. This ordering of events in the hardware, together with the lock protection and SCALER_DISPLACTX sampling of this patch, guarantees that pageflip completion handling only runs at exactly the vblank irq of actual pageflip completion in all cases. Background info from Eric about the relative timing of HVS, PV's and trigger points for interrupts, DL updates: https://lists.freedesktop.org/archives/dri-devel/2016-May/107510.html Tested on RPi 2B with hardware timing measurement equipment and shown to no longer complete flips too early or too late. Signed-off-by:Mario Kleiner <mario.kleiner.de@gmail.com> Reviewed-by: