Where no acceptable solution was ever found, but was at least recoverable sometimes using Ctrl, Alt, F1 to get to console, login, uninstall AMDGPU-PRO or AMDGPU drivers then reboot.
System Information (PC).
Motherboard = Asus Z87 Deluxe.
CPU = i7-4770K at stock clocks.
GPU 1 = Primary PCIe slot = Nvidia GTX 780Ti. Drives HP22xi, Dell ST2220T and HP22xi monitors. GPU 2 = Secondary PCie Slot = R9 Nano. Drives Avermedia LGX HDMI Passthrough Capture Card to HP22xi or Directly to HDMI input of the HP 22xi Monitor. RAM = 32 GB DDR3.
OS Disk = 512 GB Crucial SSD.
I decided to see what would happen if I tried to install AMD Driver 18.20 for Ubuntu 18.04 (Which has 'AMDGPU' drivers already installed in the distribution (the Cynic in me thinks this is simply the previous 'Radeon' drivers, rebadged).
In this case the default open source noveau drivers are installed with the Nvidia card.
Ubuntu 18.04 starts to boot up, I see the Ubuntu Logo then all screens go black.
The system becomes unresponsive and I can not even use Ctrl Alt F1 to get to a console, login, and uninstall your latest drivers. This seems very similar situation to the previous case of login loop on Ubuntu 16.04, but worse, since it requires a complete OS wipe and reinstall just to recover.
Moving the Nvidia card to be a secondary card with the AMD card as primary is not an acceptable solution.
The GTX780Ti will be replaced with a new high end Nvidia GPU and I want the Nvidia GPU as the primary card.
Does anyone even test running an Nvidia primary GPU with an AMD secondary GPU on Ubuntu in AMD?