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I've installed Ubuntu 16.04, and I had all three of my monitors working gracefully (it didn't quite work out of the box, but a little monkeying with the drivers soon straightened everything out). I've installed i3-wm, but it doesn't recognize my third monitor. I had i3-wm working without any problems on my previous Ubuntu 14.04 installation, and whenever I log out of i3-wm, my computer recognizes my third monitor again (i.e. I don't think there's any fault with the hardware).
I've tried unplugging / replugging my third monitor, restarting i3-wm, and rebooting several times without any success. Any thoughts on what might be problem? Thanks!

P.S. My third monitor is 4k (rotated) and connected through Display Port. My other two (working) monitors, both 1080p, connect through HDMI and DVI.

Edit: I ran xrandr, and here's the output:

Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 16384 x 16384
DVI-I-0 disconnected primary (normal left inverted right x axis y axis)
DVI-I-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 509mm x 286mm
   1920x1080     60.00*+
   1680x1050     59.95  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1280x800      59.81  
   1280x720      60.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   640x480       75.00    59.94  
DP-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 509mm x 286mm
   1920x1080     60.00*+  59.94    50.00    60.00    50.04  
   1680x1050     59.95  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1280x800      59.81  
   1280x720      60.00    59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    59.94    59.93  
   480x576       50.00  
   480x480       59.94  
DP-1 connected (normal left inverted right x axis y axis)
   3840x2160     60.00 +  29.98  
   2560x1440     59.95  
   1920x1200     59.88  
   1920x1080     60.00    59.94    50.00    23.97    60.00    50.04  
   1680x1050     59.95  
   1600x1200     60.00  
   1280x1024     75.02    60.02  
   1280x800      59.81  
   1280x720      60.00    59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00    50.08  
   720x480       59.94    60.05  
   640x480       75.00    59.94    59.93

Notably, "DP-0 disconnected (normal left inverted right x axis y axis)" (DP would be Display Port - meaning that my 4k display port monitor is not connected). Interestingly enough, there is a DP-1 option which is connected.
I also ran arandr, but like my i3-wm, it only recognized the 1080p monitors. I find it very strange that xrandr detects a monitor (connected through DP-1) that arandr doesn't.

QuantumFool
  • 151

1 Answers1

2

After some poking, I got my configuration to work.
First, I had to reboot my computer. I don't know why that was necessary, but apparently it was.
The next time I ran arandr, I noticed that there was now a DP-1 entry that I could click on: (I suspected that this would be the case, since xrandr's output indicated that there was a connected DP-1 monitor)

DP-1 available

When I expanded the DP-1 option, I was able to click "Active", and my third monitor turned on successfully.

DP-1 active

As a final step, I selected "Save As", saved my settings in an automatically generated file which I called layout.sh, and then followed the directions in the highest voted (not the accepted) answer to this post.

P.S. I didn't exactly follow the instructions in the post's answer. My actual /etc/lightdm/lightdm.conf file looks like:

[Seat:*]
autologin-guest=false
autologin-user=my_user_name
autologin-user-timeout=0
display-setup-script=/path/to/layout.sh
session-setup-script=/path/to/layout.sh

as opposed to the suggestion in the post:

[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
# for your login screen, e.g. LightDM (Ubuntu 11.10) or GDM (11.04 or earlier)
display-setup-script=/path/to/layout.sh
# for your desktop session
session-setup-script=/path/to/layout.sh

I think the new format of lightdm.conf comes with Ubuntu 15.10 and onwards (I'm using Ubuntu 16.04 - the version number claim is a guess based on the difference noted on this site).
P.P.S. I also changed some settings between the "Active" and "Save Configuration" steps - such as rotating my third monitor and moving it to the place it physically is relative to my other two monitors on my desk.

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QuantumFool
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