Richard Cooper wrote:
Any ideas how I might sync to bit 3 of port 0x3DA, the video card's
vertical sync signal?
Well, I don't know if anyone cares, but since I asked the question here,
I figure it's appropriate to let everyone know what I came up with.
As best I can tell, it's just not possible under Linux. Everything
that does vertical refresh syncing in Linux does it with a
CPU-time-eating busy loop. It seems it isn't possible to give up the
CPU for a period of time shorter than 10-20ms. Since vertical retraces
occur every 17ms, and 17 < 20, if you want to have the CPU when the
next retrace occurs, you can't do any kind of sleeping. This is the
case even under "realtime scheduling," which seems to be a misnomer for
what would be better called "priority scheduling."
What kernel series are you using? Did you compile it yourself? What is
HZ set at? 100? 250? 1000? If it's a 2.6 kernel, did you try enabling
preemption? With HZ at 100, it is impossible for the kernel to give you
better than 10 ms resolution. With HZ at 1000, it can achieve 1 ms
resolution (in theory). What else is running on your machine? These
questions could help answer whether it's possible.
Stephen
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