Disconnecting and reconnecting HDMI/DVI connectors, or a loose connection, can
cause the driver to stall in a loop with the following messages being logged
approximately every 500ms in syslog.

Nov 25 16:13:30 dragon kernel: [363196.855813] radeon 0000:02:00.0: ring 0
stalled for more than 1710608msec
Nov 25 16:13:30 dragon kernel: [363196.855818] radeon 0000:02:00.0: GPU lockup
(current fence id 0x0000000000284cb5 last fence id 0x0000000000284cb6 on ring
0)

This causes the screen to blank. A reboot is required to clear this.

It would seem more sensible to reset the driver if it has been locked on the
same fence for a long time.

=====================================================================

(II) Module radeon: vendor="X.Org Foundation"        [    82.693]    compiled
for 1.19.3, module version = 7.10.0      [    82.693]    Module class: X.Org
Video Driver                  [    82.693]    ABI class: X.Org Video Driver,
version 23.0       [    82.693] (II) 

roger@dragon:/var/log$ sudo lshw -c video                                      
                            *-display
       description: VGA compatible controller
       product: RV770 [Radeon HD 4850]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:02:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:26 memory:d0000000-dfffffff memory:fbae0000-fbaeffff
ioport:b000(size=256) memory:c0000-dffff


Roger