Sometimes after the first mode switch during booting our system, the LVDS output shows a distorted picture which after some seconds disappears completely. We were able to reproduce the problem in about 1 of 50 tries. The higher the resolution, the more likely the error was. We could reproduce it with xf86-video-ati 7.3.0 (Porteus Linux 3.0) and 7.5.0 (Porteus Linux 3.1rc2). It does NOT happen with the Catalyst driver we used before.

We found the following 100% working fix which we unconditionally apply on every boot (we cannot find out if the problem occured or not):

xrandr --output LVDS --off
sleep 1
xrandr --output LVDS --preferred

Omitting the "sleep 1" does NOT fix the problem. Because all of this, we suspect it has something to do with how the driver switches from one resolution to another, which also is a lot faster compared to when switching resolutions with the Catalyst driver.

The hardware we use are different congatec conga-EAF ETX boards (http://www.congatec.com/en/products/etx/conga-eaf.html) with AMD HD6320 and AMD HD 6250 graphics. The board uses an Analogix ANX3110 DisplayPort to LVDS converter, which we suspect cannot cope with the DisplayPort signal it gets. We can rule out our custom base board as the problem also appears when using the congatec evaluation board.