Commen=
t # 17
on bug 79223<=
/a>
from Michel D=C3=A4nzer
(In reply to comment #=
14)
> (In reply to comment #13)
> > (In reply to comment #12<=
/a>)
> > > Down to dri2_drawable_get_buffers() now. I assume I'll be hi=
tting a point
> > > where I'll have to switch over to looking in the X server so=
on...
> >=20
> > Yep, it's waiting for DRI2 buffer information from the X server, =
which is
> > delayed until the previous buffer swap actually finishes.
>=20
> Is that expected behaviour? I.e. am I chasing an already known limitat=
ion?
Yes, it's a limitation of DRI2 (at least without triple buffering, which is
hard to do with DRI2).
> The xbmc code seems to assume that those gl comm=
and will execute
> asynchronously as it is using querires to determine when the rendering=
and
> glReadPixels() is done.
glReadPixels is currently always synchronous with all Gallium based drivers=
, as
there's no hardware acceleration for PBOs yet.
> > FWIW, you might get somewhat less confusing=
timings if you call glFinish()
> > before glXSwapBuffers().
>=20
> I'll play around with it. But that doesn't sound like a good solution
> upstream as I guess it will remove parallelism for the drivers that ca=
n do
> all of this in the background?
Of course, this wasn't intended as a solution but just as a debugging aid to
try and make your timings correspond better to where the time is actually s=
pent
in the driver / hardware.
That said, in the scenarios you described, there would need to be at least a
glFlush() call before waiting for vblank, otherwise the driver / hardware m=
ay
not even start actually rendering the frame before the glXSwapBuffers call.
(In reply to comment #16)
> I guess the behaviour that xbmc is expecting is =
that the only time it will
> wait for a vblank, is that explicit vblank waiting in step 2.?
>=20
> Now is that an unreasonable expectation?
I'm afraid so. It would be better to use something like GLX_OML_sync_contro=
l's
glXSwapBuffersMscOML() for timing buffer swaps, instead of explicitly waiti=
ng
for vblank and then calling glXSwapBuffers().