Dual monitor setup, why does NVidia 340 Prime only detect one monitor?

Question

I'm using:

• Alienware M14xR2 laptop running Ubuntu 14.04
• 2 * DELL U2412M monitors attached via HDMI and Mini Display Port
• Optimus graphics (Intel + nVidia GeForce GT 650M) running with nvidea-340 and nvidia-prime, with Prime setup in nvidia-settings GUI to use the NVidia GPU
• built-in laptop monitor disabled

Unfortunately, the monitors only work successfully in the default mirrored setup - I have the following issues trying to get a dual external screen extended desktop to work:

• nvidia-settings GUI can only see one monitor on the X Server Display Config page. System Display GUI can see both displays but can only mirror the displays successfully. By fiddling with Display GUI and ARandR, I temporarily managed to get an extended desktop working, but NVidia GUI still only sees one of the monitors and if I try to reboot with 2 monitors attached, the GUI is totally glitched and unusable after login.
• During boot, the system just uses the left monitor (via HDMI i/f) for display, so I presume this is treated as the primary display. However, after boot, it appears the system switches to using the Display Port for the primary display and the system will not let me have the Ubuntu launcher only on the left (HDMI) display. The only options that work are for having the launcher on both or just on the right (Display Port) monitor. I tried to forcefully fix the issue with the primary monitor incorrectly being set to the right monitor by changing the primary monitor switch in ~/.config/monitors.xml, however this does not work and causes both monitors to appear at strange resolutions and glitch badly with uncoordinated mouse input.

Running /usr/lib/nvidia-340/bin/nvidia-xconfig --query-gpu-info gives

Number of GPUs: 1

GPU #0:
  Name      : GeForce GT 650M
  UUID      : GPU-...
  PCI BusID : PCI:1:0:0

  Number of Display Devices: 1

  Display Device 0 (DFP-0):
      EDID Name             : DELL U2412M
      Minimum HorizSync     : 30.000 kHz
      Maximum HorizSync     : 83.000 kHz
      Minimum VertRefresh   : 50 Hz
      Maximum VertRefresh   : 61 Hz
      Maximum PixelClock    : 170.000 MHz
      Maximum Width         : 1920 pixels
      Maximum Height        : 1200 pixels
      Preferred Width       : 1920 pixels
      Preferred Height      : 1200 pixels
      Preferred VertRefresh : 60 Hz
      Physical Width        : 520 mm
      Physical Height       : 320 mm

Running xrandr --query gives

Screen 0: minimum 8 x 8, current 3120 x 1920, maximum 16384 x 16384
DP-0 connected primary 1200x1920+1920+0 left (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200      60.0*+
   ...
DP-1 disconnected (normal left inverted right x axis y axis)
LVDS1 connected
   1600x900       60.0 +   40.0  
   ...
DP1 disconnected
DP2 disconnected
HDMI1 connected 1920x1200+0+0 518mm x 324mm
   1920x1200      60.0*+
   ...
HDMI2 disconnected
VGA1 disconnected
VIRTUAL1 disconnected

Running lspci | grep VGA gives

00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09) 01:00.0 VGA compatible controller: NVIDIA Corporation GK107M [GeForce GT 650M] (rev a1)

My xorg.conf is almost empty but I believe 14.04 automatically detects those settings.

Answer 1

Post-installation and removal Nvidia, I noticed a strange(and a good one at that) behavior. And that its that the GUI response in the second monitor has become much improved.

And I also noticed that the xorg.conf has a different setting, pasting it for your reference,

jpvel@jpvel-M11xR3:~/Downloads$ sudo cat /etc/X11/xorg.conf.04192015 

Section "ServerLayout"
    Identifier "layout"
    Screen 0 "nvidia"
    Inactive "intel"
EndSection

Section "Device"
    Identifier "intel"
    Driver "intel"
    BusID "PCI:0@0:2:0"
    Option "AccelMethod" "SNA"
EndSection

Section "Screen"
    Identifier "intel"
    Device "intel"
EndSection

Section "Device"
    Identifier "nvidia"
    Driver "nvidia"
    BusID "PCI:1@0:0:0"
    Option "ConstrainCursor" "off"
EndSection

Section "Screen"
    Identifier "nvidia"
    Device "nvidia"
    Option "AllowEmptyInitialConfiguration" "on"
    Option "IgnoreDisplayDevices" "CRT"
EndSection

I suspect that the existing nouveau driver seems to start honoring the configuration and appropriately instruct the graphics card to send the output to the external monitor. As

For those souls who tried using the DVI port or HDMI to VGA adapter, let me forwarn you, it does not work. As neutrino pointed out, the HDMI port is welded with Nvidia card in Alienware laptops

Edit from 2nd answer:

I went for the reinstallation and again tried using the latest Nvidia drivers from ppa:xorg-edgers/ppa for Nvidia-340, it ended up mucking the UI. So I decided to uninstall and live with the existing setup.

However, with the fresh installation, I am able to work with a dual monitor setup that is not perfect, but usable. I mirrored the displays and ensured that the intel's graphics driver could do the rendering(but it is lousy though I admit)

Answer 2

It seems that due to the unconventional way Dell wired up the 3 display ports, the Nvidia GFX and the on-board Intel GFX in the Alienware M14xR2, you cannot use HDMI and MDP for the external monitors. It is also not possible to disable the Intel GFX in bios as Dell wired up the Nvidia card to pass through the Intel card!

Using the HDMI and VGA ports for the monitors solves this issue and allows the use of an external dual monitor setup on Ubuntu 14.10 without mirroring.

However, at the time of writing, the Nvidia 340 and 346 series of drivers are not working for this dual monitor setup and will only work properly using just one monitor. Only Nvidia 331 series seems to support this dual setup (or Nouveau - but that driver shows a very noisy picture on the second monitor and its 3D acceleration does not work properly either). Bugs, bugs, bugs!