Sideport memory and KMS

[anonydroid at gmail.com (Nexus Droid)]

I used to have frequent X screen corruption when running under KMS
(kernels 2.6.32 to 2.6.37) and the Radeon X driver. I've identified
the error in dmesg as a "GPU lockup (waiting for * last fence id *)".
About a week ago I dug into the BIOS and found an option to turn off
the onboard Sideport memory of my motherboard. Now, over a week later
I've yet to experience the GPU lockup. Is this just a coincidence? Has
anyone else experienced any problems running KMS with integrated
Radeon graphics and Sideport memory?

Before last week, I set the onboard graphics to use the 128 MB
Sideport memory with various amounts of conventional RAM (which the
BIOS calls UMA), ranging from 64 to 512 MB. I would experience the X
freeze when running multiple simultaneous instances of software such
as Firefox and XPDF, which appear to do something funky to the screen.
I avoid or delay the onset of the freeze by running at most two or
three of these programs and using only simple Firefox themes. When the
freeze happens, I immediately press Ctrl-q (or its equivalent) to exit
the corrupted program. Continuing to use the program often leads to a
more permanent black screen of death that requires a hard reboot.

But since turning off Sideport memory and setting the conventional
shared memory to 256 MB, I've had an uncorrupted uptime of 7 days
while simultaneously running XPDF, three Firefox profiles, Chromium,
Mplayer (paused at -vo gl), and Gnome Terminal doing an IO-heavy file
transfer!