video cards spoiling real-time

[frank mori hess <fmhess@students.uiuc.edu>]

Hello,

I saw some discussion about problems with video cards introducing latency
into real-time tasks, so I thought I would contribute my experiences in
the matter. I used rtlinux 3pre9 on 2.2.17 kernel, and my usual timing
error (from the examples/measurements test) is about 10usec.

Using XFree86 3.3.6, I tried three different cards,

PCI Trident 3dimage 9750: Scrolling in an xterm caused timing errors to
increase to about 50 usec. Dragging the xterm around the screen or
opening/closing windows caused errors of a few thousand usec.

AGP Cirrus Logic CL-GD5465:
Dragging around and opening/closing windows would cause occasional
timing errors of up to 1000 usec.

AGP SIS 6326:
No problems observed.


Setting the "no_accel" option in XF86Config made any timing problems
disappear.

Frank