I have a desktop running vanilla ubuntu 16.04 with 4 monitors at work. Its always been a bit buggy with starting all 4 monitors, but after holidays its gotten worse. Normally I just reboot or turn monitor in question on-off a few times and its gets it in 2-3 trys.
2 monitors (the left 2 in pic) where unplugged for holidays. This is the only thing that changed. PC was off for 3 weeks. Problem started before I did any updates. Updates now caught up. problem persists.
Now the far left one only works in grub. Once I get to log in screen it drops out. I've rebooted and switched monitor off and on a bunch of times. Its not getting detected.
Don't have a spare DP-HDMI cable to try, but did try changing ports on the monitor (which is actually a TV with 2 hdmi inputs) and the RX580. Same results. So I don't think its the cable which is quite new and plugged in firmly (this has been a problem in past with HDMI which is why I tripple and double checked its in firmly).
I did however swap the HDMI with the monitor running on HDMI port. This does swap which monitor works. I have tried DP-1&2 Results are the same.
When I turn PC on it shows Bios and Grub on the TV connected to Display Port 1 and on the monitor connected to DVI-1. Then as soon as grub closes the DP-1 monitor drops out and the others all come on.
It has never displayed BIOS or grub on all the monitors, usually 1-2 monitors and often changes which ones each boot. I don't care about this, as long as one comes on so I can chose a grub option in the rare event I need to....
One last thing to mention is the tv/monitor had over-scanning problem. So I gave it custom resolution. For a while I was doing this every boot, via xrandr, then I eventually edited a startup file to include the new resolution, sorry I cant remember which file it was, can probably google up instructions I followed if needed. I don't think this is related though. Except its still showing the custom resolution for DP-1...
I've rebooted about 20 times now its not even looking like working after ubuntu starts.
Here is my xrander output:
:~$ xrandr Screen 0: minimum 320 x 200, current 4480 x 1080, maximum 16384 x 16384 DisplayPort-0 connected primary 1280x1024+3200+0 (normal left inverted right x axis y axis) 376mm x 301mm 1280x1024 60.02*+ 75.02 1280x800 60.02 1152x864 75.00 1280x720 60.02 1024x768 75.03 60.00 800x600 75.00 60.32 640x480 75.00 59.94 720x400 70.08 DisplayPort-1 disconnected (normal left inverted right x axis y axis) 1800x1012_60.00 60.00 DisplayPort-2 disconnected (normal left inverted right x axis y axis) HDMI-A-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm 1920x1080 60.00*+ 60.00 50.00 59.94 1680x1050 59.88 1280x1024 75.02 60.02 1440x900 74.98 59.90 1280x960 60.00 1280x800 59.91 1152x864 75.00 1280x720 60.00 50.00 59.94 1152x720 59.97 1024x768 75.03 60.00 832x624 74.55 800x600 75.00 60.32 720x576 50.00 720x480 60.00 59.94 640x480 75.00 60.00 59.94 720x400 70.08 1800x1012_60.00 60.00 DVI-D-0 connected 1280x1024+1920+0 (normal left inverted right x axis y axis) 376mm x 301mm 1280x1024 60.02*+ 75.02 1280x800 60.02 1152x864 75.00 1280x720 60.02 1024x768 75.03 60.00 800x600 75.00 60.32 640x480 75.00 59.94 720x400 70.08