Issues adding driver support for present extension
frank.binns at imgtec.com
Tue Apr 15 09:12:31 PDT 2014
I've been working on adding support for the present extension in the PVR
video driver and I've hit a couple of issues related to flipping:
1) When the display driver supports async flipping this mode of
operation is always used. This means that, when the swap interval is
none 0, the present extension relies on vblank events to get
synchronised flips. However, by the time we get the event, wait for any
outstanding rendering and do the flip we have probably missed the vblank
period meaning we get screen artefacts. Is there a good reason (other
than simplifying the code) why present isn't relying on the default
behaviour of drmModePageFlip() to get synchronised flips?
2) When the display driver doesn't support async flipping and the swap
interval is 0 we see swaps alternating between flipping and blitting.
This results in a back buffer continually being created and destroyed on
the client side, which is obviously highly undesirable.
My understanding is that, in the async case, a vblank event gets
requested for the next vblank. Any swaps that come in before this
event's received never get presented to the screen. In addition to this,
it's fairly unlikely that we catch things in the middle of a flip, i.e.
we've called drmModePageFlip() but not got back the flip event, meaning
that any queued vblank events, i.e. queued swaps, are going to be
completed via a flip. However, in the non-async case we're very likely
to catch things in the middle of a flip as a flip can only be completed
during a vblank period. The result is this ping-ponging between flipping
My current thoughts are that two things probably need to be done to fix
a) If present gets a swap for the same MSC as a pending flip then it
should never try to do the presentation and send back
PresentCompleteModeSkip in the reply.
b) If present gets a swap for an MSC in the future then it shouldn't be
prevented from completing the swap via a flip just because there is
currently a pending flip.
Does this sound reasonable, is there a better way of fixing this or am I
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