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I recently updated my laptop to Ubuntu 18.04 from 16.04. When I tried to use my external monitor with it for the first time, everything worked well and I set the external monitor as the primary display. However, for the second time I tried to use my laptop with it, the external monitor only shows a black screen, even though it shows the connected source (it displays a text saying "HDMI", since I connect the monitor via HDMI). Since the laptop recognizes it and the setting was not modified since the last time I used it, the built-in screen is turned off.

When the external monitor is connected, xrandr produces the following output (note that I could only run it "blindly", since both the screens are black whenever the second monitor is connected):

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
LVDS-1 connected (normal left inverted right x axis y axis)
   1366x768      60.01 +
   1360x768      59.80    59.96  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   800x512       60.17  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   720x450       59.89  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   680x384       59.80    59.96  
   640x400       59.88    59.98  
   576x432       60.06  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
VGA-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 299mm
   1920x1080     60.00*+  50.00    59.94  
   1680x1050     59.88  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1440x900      59.90  
   1280x800      59.91  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    72.81    66.67    60.00    59.94  
   720x400       70.08  
DP-1 disconnected (normal left inverted right x axis y axis)

I tried the following things, to no avail:

  • add GRUB_CMDLINE_LINUX_DEFAULT="quiet splash amdgpu.dc=0" to /etc/default/grub (see this answer)
  • change the driver, currently I have: enter image description here
  • install a new nvidia driver, this caused login loop so I had to completely revert this
  • "restart" the external screen by running the script shown here
  • run xrandr --output HDMI-1 --mode 1920x1080 --rate 60 as described here

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