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Re: iBook and playing DVDs

On Fri, 2002-05-17 at 04:42, Benjamin Herrenschmidt wrote:
> >The source buffer doesn't matter after we have filled the DMA buffers,
> >does it?
> Ah, you are right, I forgot about the fact we did an additional copy
> here. Too bad we can't just DMA from the source buffer, that sucks,
> remember the RAM throughput of most Macs isn't that good... This is
> probably one reason why DMA doesn't show that a significant perf
> improvement.

One might be able to DMA directly from the source buffer, but one would
have to walk the pages and set up descriptor tables with the bus
addresses. Do you think that could still be better? (Assuming one could
work out alignment etc.)

> >Anyway, I don't see any explicit synchronisation in the driver, so
> >probably the problem is the players calling XSync().
> Well, do we wait for DMA to finish or not ? If we do, then we are
> doing explicit sync.

Again, I don't see that in the driver, but maybe I'm just blind.

> If we don't, then we should, at least, wait
> on frame N+1 wait for frame N to finish. The point is to have at
> least one frame in advance to not busyloop when there is still
> work to do. The ideal case would be to block on command completion
> using an IRQ of course.
> XSync() can be faked. We can perfectly decide to buffer one frame
> in advance, can't we ? Then XSync on the next frame, thus we won't
> block on double buffer setup if the second buffer isn't filled. That
> make sure we only block (or spinloop) if the decoder is feeding
> us faster than the framerate, and only when we have the 2 buffers filled.
> That should smooth the whole data flow and avoid a lot of useless
> sleeps/busyloops.

Smells like a hack, this is stretching the XSync() semantics to say the
least, in particular I think XSync() is the only feedback the players
get for the timing, I wonder if such a change wouldn't have bad effects
of its own there.

Earthling Michel Dänzer (MrCooper)/ Debian GNU/Linux (powerpc) developer
XFree86 and DRI project member   /  CS student, Free Software enthusiast

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