How to configure radeonsi for clone mode?

[Boszormenyi Zoltan]

Hi,

for an NVIDIA based Xorg setup with the proprietary driver,
I have this file as /etc/X11/xorg.conf.d/nvidia.conf:

Section "Device"
        Identifier      "NVIDIA"
        Driver          "nvidia"
        Option          "AutoServerLayout" "on"
EndSection

Section "Screen"
        Identifier      "Screen0"
        Option          "TwinView" "on"
        Option          "TwinViewOrientation" "Clone"
        Option          "AutoServerLayout" "on"
EndSection

I would like to know how to setup the "clone mode" using the
radeon driver on a Zotac ZBOX NANO-AQ01 which uses the
radeonsi driver and has HDMI-0 and DisplayPort-0 connectors
according Xorg.0.log.

I came up with this using "man xorg.conf":

Section "Monitor"
    Identifier    "HDMI-0"
    Option        "AutoServerLayout" "on"
    Option        "Position" "0 0"
EndSection

Section "Monitor"
    Identifier    "DisplayPort-0"
    Option        "AutoServerLayout" "on"
    Option        "Position" "0 0"
EndSection

Section "Device"
    Identifier    "RADEON"
    Driver        "radeon"
    Option        "AutoServerLayout" "on"
    Option        "Monitor-HDMI-0" "HDMI-0"
    Option        "Monitor-DisplayPort-0" "DisplayPort-0"
EndSection

However, there is a small problem. I have connected a HD-ready
capable TV using a HDMI-DisplayPort converted to the DisplayPort
connector (native resolution 1366x768) and a FullHD capable
monitor using the HDMI connector. The result doesn't look good
and confirmed by Xorg.0.log:

[2853811.002] (II) RADEON(0): Output DisplayPort-0 using initial mode 1360x768 +0+0
[2853811.002] (II) RADEON(0): Output HDMI-0 using initial mode 1280x800 +0+0

If I connect the FullHD-capable monitor to the DisplayPort,
the result is more satisfying:

[2854212.540] (II) RADEON(0): Output DisplayPort-0 using initial mode 1920x1080 +0+0
[2854212.540] (II) RADEON(0): Output HDMI-0 using initial mode 1920x1080 +0+0

How can I setup clone mode in a way using the xorg.conf.d
extension file to make the system recognize the best resolution
and use that instead of limiting it to the monitor capabilities
connected to DisplayPort? It seems it's the primary connector.

I am using:

Xorg 1.16.3
Mesa 10.4.1
xf86-video-ati 7.5.0
libdrm 2.4.58

Best regards,
Zoltán Böszörményi