It looks like 2 or 3 separate bugs all introduced into radeon_drv.so
since Jaunty.  From latest karmic, if I do

  sudo dpkg -i --force-downgrade xserver-xorg-video-
radeon_6.12.1-0ubuntu2_i386.deb

everything is fine.  Compiz effects are a bit slow, but I don't get
segfaults or any screen corruption.  So I tracked through the git
versions...

1) A small amount of screen corruption around the window borders (with
DRI and compiz visual effects) seems to have been introduced between
xf86-video-ati-6.12.1 and xf86-video-ati-6.12.2.  (I think the
console/mode switching problems may have started during this period too,
but were later fixed.)

2) The segfault was introduced between xf86-video-ati-6.12.2 and 
xf86-video-ati-6.12.3 in commit 3c348091ae5d88ef3cb850889ca74674e0530b4e  (see 
attached) The conditions to produce this involve resizing a window or doing an 
operation larger than particular dimensions (with DRI and compiz visual 
effects), and it looks like the bad commit sets some dimension maxima too 
optimistically.  Changing a line to 
    xf86CrtcSetSizeRange (pScrn, 320, 200, 2560, 1200);
stops the segfault, but I guess problems may go deeper than just that line, 
because it shouldn't segfault at all.  This bug is also associated with screen 
corruption over large areas when a window with focus is dragged over one 
without focus.

3) The "tartan" patterns and font rendering problems (which only occur
*without* DRI and compiz visual effects) are not present in xf86-video-
ati-6.12.4, but are in the master head.  I don't know how to locate
these to an exact commit because of a merge and some compilation
problems.

Please let me know if I should do anything else, like report these
upstream.  I'd suggest this bug (421842) concentrates on the segfault
(number 2 above), and I could open another ubuntu bug for number 3).
Hope this isn't too confusing.

** Attachment added: "git outputs showing commit causing segfault, and a diff 
with working values"
   http://launchpadlibrarian.net/32483821/xf86-video-ati-bisect-segfault.txt

-- 
screen corruption and segfaults with radeon m6 ly
https://bugs.launchpad.net/bugs/421842
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