I'm fairly certain this is the well-known "screen corruption with XAA".
Indeed, EXA was made to replace XAA specifically because of bugs like
this.
So the real bug here is why is your system using XAA rather than EXA,
and that I believe is because in certain circumstances EXA has severe
problems, and so EXA is being quirked off in these cases. One situation
is where the video memory is too low. You can try forcing EXA via the
AccelMethod option. If it turns out that EXA works fine on your
hardware, perhaps the quirk can be adjusted to not be employed in that
case. If EXA does not work right, than probably we should focus efforts
on *that* issue, since it is extraordinarily unlikely anyone will be
fixing this problem with XAA at this point.
This quirk is a fairly recent thing. In Jaunty we (Ubuntu) forced all
ATI hardware to use EXA. I don't recall whether the quirk to use XAA
appeared in Karmic or only on Lucid, but this may explain why you see
this as a regression.
Alternatively, compiz could be quirked to not work for this graphics
card. I don't care for that option since it really just sweeps the bug
under the carpet, but it is something which could be considered if we
get down to release and the underlying issues haven't been resolved.
** Changed in: xserver-xorg-video-ati (Ubuntu Lucid)
Importance: Undecided => Low
** Changed in: xserver-xorg-video-ati (Ubuntu Lucid)
Status: New => Triaged
** Changed in: xserver-xorg-video-ati (Ubuntu Lucid)
Milestone: None => ubuntu-10.04-beta-2
--
[XAA] garbled screen with compiz but no KMS on ATI Radeon Mobility 7500
https://bugs.launchpad.net/bugs/513956
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