Fixes for Xorg bugs on old ATI Radeon cards (gimp XOR green trails, EXA font woes)

Michael Shell list1 at michaelshell.org
Mon Jan 17 18:13:59 PST 2011



I recently spent a little time investigating two Xorg driver problems on
a quite old machine of mine (it runs LFS Xorg 7.2 with a now ancient ATI
Radeon 7200 / RV100 card). I wish I had discovered this info 3 years ago
when the Xorg bug first bit a lot of people:

https://bugs.freedesktop.org/show_bug.cgi?id=9877
https://bugs.launchpad.net/ubuntu/+source/gimp/+bug/156556
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=433230

But, maybe the info here might still help someone out. 

The first problem, which started with a new tool pointer outline XOR
drawing feature of the gimp 2.4 and later releases (this machine is
running gimp 2.6.6), manifests itself as green trails left behind
the gimp tool pointer.

One workaround is to place

(xor-color (color-rgb 1.0 1.0 1.0))

in the gimprc. However, this does not address the potential for problems
with other applications, although as far as I know only gimp seems to
be affected so far.

Also, newer Radeon cards do not seem to suffer the gimp problem. 
(e.g., RV300 is known to be OK)

Another workaround is to use Xorg EXA rather than the older XAA
acceleration architecture. However, on this old system EXA manifests
other, more serious bugs (e.g., fonts with missing pixels, screen areas
that do not update correctly, etc.). Also, Xorg EXA is known to suffer
from performance problems relative to XAA under older Radeon cards (e.g.,
slow folder opening under konqueror). I do not know if the EXA
performance problems on old Radeon cards have been (or will ever be)
fixed with newer Xorg releases.

Xorg developers will not fix XAA-only problems because XAA is now
considered obsolete, so even recent Xorg releases will manifest the
gimp green trail bug on RV100 Radeons under XAA (while XAA remains
available at all).

Anyway, after some experimenting, I discovered that adding the
xorg.conf driver option:

     Option      "XaaNoDashedTwoPointLine"

fixes the gimp problem without noticeably affecting performance. The
RV100 series probably has a hardware bug related to a certain
acceleration operation used only by the XAA DashedTwoPointLine, so
the Xorg ati driver probably should be patched to automatically disable
the XAA DashedTwoPointLine acceleration service with RV100 cards.

As for the EXA problems under the Radeon RV100, I found that 

     Option      "MigrationHeuristic"    "greedy"

fixes the EXA bugs as well. EXA performance is not as good as that of
XAA (but pretty close) on this older card and older Xorg. I do not
know if EXA performance issues still exist with the Radeon RV100 (or
even for much more recent Radeons for that matter) with the very
latest Xorg releases. 


Anyway, I just wanted to post this info in case it helps somebody.


  Cheers,

  Mike Shell




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