odd problem with nouveau on 64-bit Ubuntu 12.04

[D. Hugh Redelmeier]

I've not had great luck with Ubuntu 12.04.  I've tracked down one of my 
problems and am reporting it here to help others.

On my notebook's hardware (64-bit AMD, 3G of RAM, nVidia GeForce Go 6100 
video chipset), Ubuntu uses the nouveau X video driver.  It doesn't work.  

Symptom (after all updates): LightDM allows login but Unity desktop only 
manages to paint the background.  Lots of crud in dmesg and 
/var/log/Xorg.0.log.  To find this, I needed to ssh in.

There were other symptoms before I applied the updates.

Forcing the VESA driver (with kernel parameter "nomodeset") allowed me to 
log in and thus to do updates and install the ssh server.

This seems to be <https://bugs.freedesktop.org/show_bug.cgi?id=46557>
That bug report suggests the work-around of adding the kernel parameter 
"mem=2g".  That worked for me.

Sure sounds like the nouveau driver has some sign-extension bug.

If you experience this, be sure to add your observations to the bugzilla 
entry.
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