[Dri-devel] Combined vblank-waiting and buffer swapping

[Felix <fx...@gm...>]

Hi list,

I've been thinking about ways to implement a combined "wait for vblank
and buffer swap" operation. But somehow I can't get the pieces together
partly because I have a pretty vague understanding what the hardware
does or can do on a low level.

First a short summary of the problem. Currently the user space driver
waits for a vblank and then issues a buffer swap ioctl. That ioctl puts
the actual swap instructions on the command queue of the 3D hardware.
After that it may take an arbitrary time before the swap is actually
executed depending on how many rendering instructions were pending in
the queue when the swap was issued. Therefore you can observe tearing
even with vsyncing enabled.

A simple and stupid workaround is to do something equivalent to glFinish
before waiting for the next vblank. This way the application has to wait
a lot. With an ideal solution the application wouldn't have to wait at
all, though it could wait for a certain buffer swap to complete. The
OML_sync_control extension spec describes this in much detail.

Here is one possible way to do this, though I'm not sure it is actually
possible to do with real hardware ;-):

The 3D engine finishes rendering a frame. Then it generates an
interrupt. At this point the engine has to suspend _somehow_ until the
buffer swap has completed. The kernel waits for the software interrupt.
If the deadline for the frame has passed it executes a buffer swap
immediately. Otherwise it also waits for a specified vblank interrupt
and then executes the buffer swap. When the swap is completed it
_somehow_ makes the 3D engine resume executing rendering commands for
the next frame.

"_somehow_" marks the places I'm not sure about. Could someone comment
on this please? Even if this is all crap, maybe you have a better idea.

Regards,
  Felix

------------    __\|/__    ___     ___       -------------------------
 Felix       ___\_e -_/___/ __\___/ __\_____   You can do anything,
   Kühling  (_____\Ä/____/ /_____/ /________)  just not everything
 fx...@gm...       \___/   \___/   U        at the same time.