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I have 18.04 installed on a Mac Mini from (I think) 2015. I just installed two identical 4K monitors, one through the built-in HDMI port, the other through the built-in Thunderbolt port via a MDP->DP cable.

Both monitors are detected and seem to work. The one with the DP connection displays in 4K. The one with the HDMI connection has a maximal resolution of 1920x1080.

I'm a relative newbie and am not really sure how to start troubleshooting this. I'm not even certain if the problem is linux configuration or an intrinsic limitation in the hardware. Help? Thanks...

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You can check, if your graphics card is able to handle 8K (2x4K) by opening a terminal and issuing the following command:

xrandr

It will display some data like: (excerpt from my PC)

Screen 0: minimum 8 x 8, current 4480 x 1440, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1920x1080+0+360 (normal left inverted right x axis y axis) 598mm x 336mm..

.

The first line shows your max resolution, in my case it is maximum 16384 x 16384

Since both displays work, you should have two Monitors connected (In the example you can see that "DVI-I-1" is connected. And it will show the possible resolutions for each monitor:

DVI-D-0 connected 2560x1440+1920+0 (normal left inverted right x axis y axis) 597mm x 336mm
   2560x1440     59.95*+  69.93  
   2048x1280     60.00  
   1920x1200     59.88  
   1920x1080     60.00    59.94    50.00    60.05    60.00    50.04  
   1680x1050     59.95  
   1600x1200     60.00  
   1600x900      60.00  
   1440x900      59.89  
   1280x1024     75.02  
   1280x720      60.00    59.94    50.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    59.94    59.93  

This shows, that my second monitor is connected as DVI-D-0, its highest resolution is 2560x1440, (framerate is 69.93 hz) but the entries below could be selected as well.

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  • Thank you. I get Screen 0: minimum 320 x 200, current 4480 x 1440, maximum 16384 x 16384 which I think suggests that I should be able to run two 4K screens. However, the max resolution for the HDMI screen is 1920x1200, while the DP screen is 2560x1440. Does this suggest that something is wrong with the way the system is autodetecting the monitor's capability? – grichard Jul 2 '20 at 20:21
  • Yes, it seems that the EDID data of your second monitor can't be read. More Infos might help you to tackle this problem – kanehekili Jul 3 '20 at 22:26

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