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My mini-PC has Intel i7 running Lubuntu-18.04.
It has 2 monitor outputs, one HDMI and one DisplayPort.
The HDMI port has a HDMI monitor (E2441) plugged into it.
The DisplayPort has a DisplayPort-HDMI converter in it, with a big HDMI TV screen plugged into it, and switched on.

But Menu > Preferences > Monitor Settings (lxrandr) only shows 1 monitor attached.

/var/log/syslog shows:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011ports:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-1: HDMI / DisplayPort 2 (priority 5800, latency offset 0 usec, available: yes)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.product.name = "E2441"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011active port: <hdmi-output-1>  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: 2 source(s) available.  

Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-0: HDMI / DisplayPort (priority 5900, latency offset 0 usec, available: no)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-1: HDMI / DisplayPort 2 (priority 5800, latency offset 0 usec, available: yes)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.product.name = "E2441"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-2: HDMI / DisplayPort 3 (priority 5700, latency offset 0 usec, available: no)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-3: HDMI / DisplayPort 4 (priority 5600, latency offset 0 usec, available: no)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011hdmi-output-4: HDMI / DisplayPort 5 (priority 5500, latency offset 0 usec, available: no)  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011properties:  
Oct 17 07:56:06 i7 pulseaudio[2107]: [pulseaudio] main.c: #011#011#011#011device.icon_name = "video-display"  

The "available: no" seems to be the problem.
Does it need a special driver?

$ xrandr  
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192  
DP-1 disconnected primary (normal left inverted right x axis y axis)  
HDMI-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 299mm  
   1920x1080     60.00*+  50.00    59.94    
   1920x1080i    60.00    50.00    59.94   
   1680x1050     59.88   
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   832x624       74.55  
   800x600       75.00    60.32    56.25  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
$ uname -a  
Linux i7 4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux  

This says it will load all HWE modules:
https://wiki.ubuntu.com/Kernel/LTSEnablementStack#Ubuntu_14.04_LTS_-_Trusty_Tahr says for Bionic:
$ sudo apt-get install --install-recommends linux-generic-hwe-18.04 xserver-xorg-hwe-18.04

I tried it, followed by a reboot - but still no change, apart from:

$ uname -a
Linux i7 5.0.0-31-generic #33~18.04.1-Ubuntu SMP Tue Oct 1 10:20:39 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

External monitor not detected on Ubuntu 18.04 is a similar problem with different hardware.
It seems to show that nobody agrees what fixes it.

What next?

I don't know what any of this means, but maybe somebody does.
$ sudo lshw -c video
name: i915
$ modinfo i915
filename: /lib/modules/5.0.0-32-generic/kernel/drivers/gpu/drm/i915/i915.ko
license: GPL and additional rights
description: Intel Graphics
author: Intel Corporation
...
parm: enable_dp_mst:Enable multi-stream transport (MST) for new DisplayPort sinks. (default: true) (bool)
parm: enable_dpcd_backlight:Enable support for DPCD backlight control (default:false) (bool)
parm: enable_gvt:Enable support for Intel GVT-g graphics virtualization host support(default:false) (bool)
(full listing at https://davekimble.net/problem.DP.txt)

/sys/class/drm/card0/card0-DP-1/status = "disconnected"
/sys/class/drm/card0/card0-HDMI-A-1/status = "connected"

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Does your miniPC have a DP++ icon? (see chapter "DisplayPort dual-mode" on DisplayPort wikipedia page) Laptops had it from the beginning, but first GPUs with DP needed active DP-HDMI adapters (which start at 20-30$). If you have access to DP cable and monitor, try it that way to check the port is working.

Maybe you could add info about your miniPC. I assume you are runnign from Intel IGP, but which gen?

5
  • No DP++ logo, but then they don't always use them. The screens are both 1920x1080@60Hz so I don't think it makes any difference. No DP cable or device, but the big screen works OK with HDMI and VGA. – Dave Kimble Oct 20 '19 at 1:12
  • PC is eGlobal with i7-8565U made in 2019. Documentation mentions Intel Iris Plus Graphics 640. – Dave Kimble Oct 20 '19 at 1:29
  • Trying to look at my thinkpad X260 PCB, I see 2 chips related to DP+HDMI. My thinking is that they may have cheapened out on components and they may not support 2 HDMI outputs at the same time. I remember when AMD introduced Eyefinity that you could not use more than 2 VGA/HDMI/DVI connections at once. For a 3rd monitor you required a DP output (whether a DP monitor or active DP-HDMI adapter). So, try to boot up with only the TV connected to the DP adapter. At least to see if DP is ++. – mathew7 Oct 20 '19 at 16:10
  • Trying a test on another eGlobal box with i5-8365U, with HDMI and VGA ports, adding the big screen on HDMI makes lxrandr show 2 monitors, as expected. – Dave Kimble Oct 21 '19 at 1:11
  • tried booting with only screen on DP, reported "no signal". /var/log/syslog didn't show any attempt to get the screen going. – Dave Kimble Oct 21 '19 at 1:57
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TL;DR Check if your DP cable firmly plugged in

As mine wasn't fully plugged in, so I had "no signal" message on my monitor. After I inserted the cable firmly I got the signal.

Before, I've tried to install mesa drivers and wake up DP manually with xrandr -d :0 --output DP-1 --auto

Eventually I removed all installed packages and it's working okay on blank Ubuntu 20.04.

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