Forwarding this bug from Ubuntu reporter Andreas Modinos:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/565835

[Problem]
Corrupted graphics and lockup when doing user switching with compiz enabled.  With compiz disabled it freezes with a black screen.

[Original Description]
Steps to repeat:

1. Create two user accounts
2. Log in as one or the other
3. Attempt to switch users by selecting the other user's name or by selecting 'Switch user...' from the power menu.

What happens:

I get a screen with corrupted graphics that does not respond to input other than showing mouse movement (cursor is a huge block instead of a pointer). See attached screenshot for more details. If I use Ctrl+Alt+Fx key kombo I can succesfully switch to any of the text mode terminals, but will have 2-3 corrupt graphical terminals in range F7-F9. One terminal will host an unresponsive gnome screensaver dialog, one a blank screen with an arrow, and one the corrupt graphics or just a blinking text cursor. The blank screen with the arrow is the working x-session, because it will take commands. For example I was able to reboot by doing a Alt+F2, then typing gnome-terminal, then typing sudo reboot and then typing my password and hitting enter.

What should happen:
I should either get a login screen or the screensaver password dialog for the other user!

Tried with the radeon.agpmode parameter. I discovered that the default setting on my system is 4.

None of the tried settings (-1, 1, 2, 4) did anything to the but I noted that the graphics corruption was different with the -1 setting.

I noted a couple of things in my dmesg that I am not sure are important:
First, it says that it detects 64M of VRAM and later says that 32M of VRAM is actually ready.
Secondly, the aperture size stays at 128M for all agpmode settings except -1, when it changes to 512M.

A couple of times, what has happened since I disabled Compiz is that the screen will remain black with only the cursor visible. It seems to happen quite randomly, or at least I have not been able to detect a pattern.


DistroRelease: Ubuntu 10.04
Package: xserver-xorg 1:7.5+5ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-21.32-generic 2.6.32.11+drm33.2
Uname: Linux 2.6.32-21-generic i686
Architecture: i386
Date: Sun Apr 18 12:36:36 2010
DkmsStatus: Error: [Errno 2] Tiedostoa tai hakemistoa ei ole
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Alpha i386 (20100224.1)
Lsusb:
 Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 002 Device 002: ID 413c:8000 Dell Computer Corp. BC02 Bluetooth Adapter
 Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: Dell Computer Corporation Latitude D600
PccardctlIdent:
 Socket 0:
   no product info available
 Socket 1:
   product info: "O2Micro", "SmartCardBus Reader", "V1.0", ""
   manfid: 0xffff, 0x0001
PccardctlStatus:
 Socket 0:
   no card
 Socket 1:
   5.0V 16-bit PC Card
   Subdevice 0 (function 0) [unbound]
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-21-generic root=UUID=99ec916b-a551-4e75-85d2-60d165d6e870 ro quiet splash
ProcEnviron:
 LANG=fi_FI.utf8SourcePackage: xorg
dmi.bios.date: 06/29/2005
dmi.bios.vendor: Dell Computer Corporation
dmi.bios.version: A16
dmi.board.name: 0D2125
dmi.board.vendor: Dell Computer Corporation
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Computer Corporation
dmi.modalias: dmi:bvnDellComputerCorporation:bvrA16:bd06/29/2005:svnDellComputerCorporation:pnLatitudeD600:pvr:rvnDellComputerCorporation:rn0D2125:rvr:cvnDellComputerCorporation:ct8:cvr:
dmi.product.name: Latitude D600
dmi.sys.vendor: Dell Computer Corporation
system: codename:           lucid
 architecture:       i686
 kernel:             2.6.32-21-generic