image corruption when in multiseat mode and "vga!=normal" kernel parameter

[Tomasz Chmielewski]

I have a multiseat system with one AGP card (nvidia GeForce FX 5200; 
"primary" card, BIOS displays here) and one PCI card (ATI), running Linux.

Normally, I use a "vga=791" bootloader parameter to get a reasonable 
console resolution.


If I start X in "multiseat mode" (X on both cards, two keyboards+mice), 
image is corrupted on nvidia AGP card:

- 30% of upper part of the screen turns black sometimes, or just some of 
this "upper 30% part"
- if it turns black, I can see the shapes of text written on the console 
(/etc/issue, login prompt, blinking cursor etc. - not very clear, as 
it's very small), repeated 4 times, in green colour
- if I enable GPM ("A mouse server for the Linux console") and move the 
mouse pointer, it leaves a trace in this upper part of the screen, 
repeated 4 times, as if I was painting something
- if I mark text with a mouse (i.e. in a web browser), the "upper 30% 
part" turns green as I move the mouse down
- the "black area" is repainted as I move the window or click a mouse in 
the corrupted area


The corruption happens with nv, nouveau and binary nvidia driver and a 
vga= mode different than normal (i.e. vga=791).
The corruption does _not_ happen when I don't use vga= parameter (or use 
vga=normal).
The corruption does not happen if I start X only on one of the cards.

ATI card does not show any corruption.


Is such behaviour expected?

It is 100% reproducible with Linux 2.6.27 and 2.6.28-rc8 and xorg server 
1.4 and 1.5.

If you want, I can send some screenshots/photos.


-- 
Tomasz Chmielewski
http://wpkg.org