On Tuesday, June 12, 2007 7:53:13 Ian Romanick wrote:
> If an app is running with swap buffers synchronized to vblank, won't
> it go through the following:
>
> - Render scene.
> - Start to wait for vblank.
> - Enable vblank int.
> - Wait.
> - Disable vblank int.
> - Do swap.
> - Repeat.
>
> Isn't there some cost associated with the extra enable / disable of
> the vblank interrupt?
Yeah, but it's typically just one additional write as Michel pointed
out.
> In addition, the "fake" wall clock based vblank counter might have
> accuracy problems with periods of only one or two vblanks.
> Unfortunately, these are the cases where apps will care the most
> about accuracy.
Really? Refresh rates aren't very high relative to the event accuracy
we have in the kernel; we should be able to schedule an event with good
granularity, hopefully more than enough to hit a vblank window. But
like I said in my original post, drivers that don't have a vblank
counter register have a couple of options:
1) never disable interrupts, continue tracking events as usual
2) use a timer to keep the vblank counter accurate across interrupt
off periods
If it turns out that accuracy really is an issue, we can easily add the
delay Ville talked about to keep things easy for those drivers, but I
suspect option (1) is probably the best one for such hardware anyway.
Jesse
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