Problem with multiple screens and vesa

[Paulo Zanoni]

Hi!

In my university we have a lot of multihead X servers, and some computers have
the following problem:

When users start drawing things on the screen, sometimes the screen gets
incorrectly drawed. This is easier to reproduce if multiple users are using
multiple screens at the same time. Dragging windows through the screen sometimes
makes ANOTHER screen draw incorrectly. This issue can happen with _any_ screen
(or more than one at the same time).

This problem happens only with computers that have the VIA 0x3344 video card
(btw, this is the only card that we have which uses the VESA driver).
I could reproduce it with the VESA driver and with the OPENCHROME driver (via
and unichrome don't even work for this device). Setups like a four-head with
via + sis + sis + sis reproduce the problem, but I couldn't reproduce it
on dual-head machines with a very new nvidia card and the VIA 0x3344
(using open source drivers, not binary). As I said, although the problem happens
only with machines that use this VIA card, any screen can get incorrectly drawed
(and possibly multiple screens), not only the VIA (vesa) screen.

I've tried disabling DRI, INT10 and GLX and still could reproduce the problem.

One thing that I noted is that when I'm using Xephyr and the problem happens, if
I mouse-over the incorrectly-drawn areas of the screen, they get drawed
correctly. When I'm not using Xephyr, doing this doesn't fix the problem.

It looks like one video card is writing stuff on another video card, but looking
at the Xorg output, it seems that RAC is enabled and, for instance, in the via +
sis + sis + sis setup, RAC says that all the screens decode both IO
and MEM.

Any clues about what could cause this problem? Maybe the drivers are not using
RAC correctly? Maybe it's just a bug on vesa? Maybe it's a hardware problem?

Thanks,
Paulo.

-- 
Paulo R. Zanoni