OK, so this was utterly painful and I needed to share what finally worked for me. I have 2 x Nvidia GTX480 cards. I have 3 x monitors. I wanted to be able to run all three off the same box, using it as a single X Screen (i.e. so I can move windows between screens and it acts correctly). No matter what I tried, it wouldn't let me do so with the proprietary driver. I asked on Stack Exchange and got somewhere closer. Here's what worked:

  • Use driver 319.60 or 337.12. 331.20 did not work for me.
    • Update (28th August 2015): I've now had success with driver 346, so this tutorial is still relevant.
  • Install the SLI bridge (hardware). Even though you are not using SLI, you need to put the bridge in and enable it.
  • Use SLIMosaic and BaseMosaic options. Ignore the fact the manual implies it won't work for this card. It does.

My XConfig looks like this:

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0"
EndSection

Section "Monitor"

    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "Ancor Communications Inc ASUS VS247"
    HorizSync       24.0 - 83.0
    VertRefresh     50.0 - 75.0
    Option         "DPMS"
EndSection

Section "Monitor"

    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "LG Electronics W2243"
    HorizSync       30.0 - 83.0
    VertRefresh     56.0 - 75.0
    Option         "DPMS"
EndSection

Section "Monitor"

    # HorizSync source: unknown, VertRefresh source: unknown
    Identifier     "Monitor2"
    VendorName     "Unknown"
    ModelName      "AOC LM720/LM720A"
    HorizSync       30.0 - 83.0
    VertRefresh     55.0 - 75.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 480"
    BusID          "PCI:1:0:0"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 480"
    BusID          "PCI:1:0:0"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"		
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "MultiGPU" "Off"
    Option         "SLI" "on"
    Option         "SLIMosaic" "True"
    Option         "BaseMosaic" "True"
    Option         "MetaModes" "GPU-1.CRT-0: 1280x1024+0+0, GPU-0.CRT-0: 1920x1080+1280+0, GPU-0.DFP-2: 1920x1080+3200+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Totally weirdly, you don't need to have more than one device listed (my second card is at "PCI:1:0:0").

Note also that, despite this then yielding two providers, when it's working xrandr 1.4 (which supports multiple GPUs) only lists 1 provider:

xrandr --listproviders
Providers: number : 1
Provider 0: id: 0x279 cap: 0x1, Source Output crtcs: 4 outputs: 10 associated providers: 0 name:NVIDIA-0"

Anyway, it now works!

References

This reference list has been generated, in many cases, through an automated process and may therefore contain errors or incomplete metadata. It is provided purely as assistance for finding the items mentioned within this post.

  1. Stack Exchange: xrandr dual Nvidia triple-head question <http://unix.stackexchange.com/questions/123862/how-can-i-get-xrandr-to-detect-both-nvidia-cards-2-x-gtx480-triple-head>