Just for reference - my xorg.conf
=================================

My OpenPOWER  (Raptor Blackbird) machine uses  an internal framebuffer.
to drive the monitor This thing is  known to have a bug which prevents.
it from autodetecting screen resolution. So it always run at 1280x1024.

To make it work one has to set up the "xorg.conf" file (on my Fedora it
is  the  "/etc/X11/xorg.conf.d/21-gpu-driver.conf"  and it  looks  like
this:

{{

# AST2500
Section "Device"
  Identifier     "GPU0"
  Driver         "modesetting"
  BusID          "PCI:2@5:0:0"
  VendorName     "ASpeed Corporation"
EndSection

Section "Monitor"
  Identifier     "Monitor0"
  Modeline "1920x1080" 172.80 1920 2040 2248 2576 1080 1081 1084 1118 -HSync +Vsync
  Modeline "1280x1024" 108.88 1280 1360 1496 1712 1024 1025 1028 1060 -HSync +Vsync
EndSection

Section "Screen"
  Identifier     "Screen0"
  Monitor        "Monitor0"
  Device         "GPU0"
  DefaultDepth    24
  SubSection "Display"
    Depth       24
    Modes      "1920x1080" "1280x1024" 
  EndSubSection
EndSection

}}


I put  it here mostly  for myself  - similar info  can be foudn  in the
Raptor Wiki.