I attach my Dell XPS 9350 (Iris Graphics 540) to a Dell U2711 display using a USB-C to HDMI cable. The laptop detects the display and claims it is sending a signal to it, but the display shows no picture.

The relevant output from xrandr --prop is:

DP1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 600mm x 340mm
    Content Protection: Undesired 
        supported: Undesired, Desired, Enabled
    Broadcast RGB: Automatic 
        supported: Automatic, Full, Limited 16:235
    audio: auto 
        supported: force-dvi, off, auto, on
    non-desktop: 0 
        range: (0, 1)
    link-status: Good 
        supported: Good, Bad
   1920x1080     60.00*+  60.00    50.00    59.94    24.00    23.98  
   1920x1080i    60.00    50.00    59.94  
   1600x1200     60.00  
   1680x1050     59.88  
   1280x1024     75.02    60.02  
   1280x800      59.91  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08 

This laptop and cable can drive a TV (Samsung UA55KU6000W) at 4k and 1080p.

The HDMI port of this display works fine when driven from the HDMI port of a different laptop (some yea old Macbook).

How do I get the laptop and display to work together?

The laptop is running Ubuntu 18.04 with the HWE kernel 4.18.0-17-generic, but (I think) non-HWE xserver-xorg (1:7.7+19ubuntu7.1) and xserver-xorg-video-intel (2:2.99.917+git20171229-1).

Update: I can drive the display at 2560x1440 if I execute the following commands:

xrandr --newmode 2560x1440 241.50 2560 2608 2640 2720   1440 1443 1448 1481 +hsync -vsync
xrandr --addmode DP1 2560x1440
xrandr --output DP1 --mode 2560x1440

I obtained the modeline for the --newmode command by connecting a different computer to this display's DisplayPort input and running xvidtune -show.

So this is progress. My guess is that this display outputs a different EDID on its HDMI connector as opposed to its DisplayPort connector, but when using a USB-C to HDMI cable we really need X to see the DisplayPort modelines. (This monitor famously only supports 2560x1440 via DP, not HDMI, although it seems to work using a custom modeline.)

Unfortunately, while I can make this work at 2560x1440, repeating the above sequence of commands with a 1920x1080 modeline does not work. I can add a new modeline (with a different name to 1920x1080), but if I try to use it, the original 1920x1080 modeline is used. And there is no way to delete the built-in modelines. E.g.

DP1 connected 2560x1440+1920+0 (normal left inverted right x axis y axis) 600mm x 340mm
   1920x1080     60.00 +  60.00    50.00    59.94    24.00    23.98  
   2560x1440     59.95* 
   1920x1080_U2711  60.00  
$ xrandr --output DP1 --mode 1920x1080_U2711
$ xrandr
DP1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 600mm x 340mm
   1920x1080     60.00*+  60.00    50.00    59.94    24.00    23.98  
   2560x1440     59.95  
   1920x1080_U2711  60.00  
$ sudo xrandr --delmode DP1 1920x1080
X Error of failed request:  BadAccess (attempt to access private resource denied)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  19 (RRDeleteOutputMode)
  Serial number of failed request:  41
  Current serial number in output stream:  42

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