Comment 16 for bug 507148

I have tried various things to help diagnose this. To enable/disable KMS, I pass radeon.modeset=1 or 0 (respectively). To enable compiz, I turn on desktop effects from within the System/Preferences/Appearance/Visual Effects in Ubuntu. Checking Xorg.0.log, with KMS, EXA is enabled by default. Without KMS, XAA is disabled by default. To force EXA or XAA, I create a simple xorg.conf and use AccelMethod. Performance based on general responsiveness and resizing firefox windows.

Here are my findings:

* KMS/EXA/compiz: compiz crash (out of memory) (this bug, fairly easy to reproduce with a couple of large applications (firefox and OO.o))
* KMS/EXA/no compiz: lockup, but much harder to reproduce (saw it once after several hours of use)
* no KMS/XAA/compiz: lockup and garbling (https://launchpad.net/bugs/513950, https://launchpad.net/bugs/513956)
* no KMS/XAA/no compiz (stable after several full days of use, but needs RenderAccel "off" due to https://launchpad.net/bugs/513968)
* no KMS/forced EXA/compiz: limited testing, seems stable, but performance is very poor. Had difficulty turning on compiz in all circumstances (see this comment: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/513950/comments/4)
* no KMS/forced EXA/no compiz: limited testing, seems stable, but performance is very poor

Performance with EXA and KMS is generally good. Performance with EXA and without KMS is very poor. Performance with XAA without KMS is better than EXA with KMS.

Note that I tried to force XAA when using KMS, by using AccelMethod "XAA", but EXA was used anyway. Xorg.0.log gave:
...
(**) RADEON(0): Option "AccelMethod" "XAA"
...
(II) RADEON(0): EXA: Driver will not allow EXA pixmaps in VRAM
...
(II) Loading sub module "exa"
(II) LoadModule: "exa"
(II) Loading /usr/lib/xorg/modules/libexa.so
(II) Module exa: vendor="X.Org Foundation"
        compiled for 1.7.3.902, module version = 2.5.0
        ABI class: X.Org Video Driver, version 6.0
(--) Depth 24 pixmap format is 32 bpp
(II) RADEON(0): [DRI2] Setup complete
(II) RADEON(0): Front buffer size: 5808K
(II) RADEON(0): VRAM usage limit set to 20577K
(==) RADEON(0): Backing store disabled
(II) RADEON(0): Direct rendering enabled
(II) RADEON(0): Render acceleration enabled for R100 type cards.
(II) RADEON(0): Setting EXA maxPitchBytes
(II) EXA(0): Driver allocated offscreen pixmaps
(II) EXA(0): Driver registered support for the following operations:
(II) Solid
(II) Copy
(II) Composite (RENDER acceleration)
(II) UploadToScreen
(II) DownloadFromScreen
(II) RADEON(0): Acceleration enabled
(==) RADEON(0): DPMS enabled
(==) RADEON(0): Silken mouse enabled
(II) RADEON(0): Set up textured video

This unsurprisingly resulted in the same out of memory bugs as KMS/EXA tests above.