nvidia binary drivers fail to see display in discrete mode

[Tessa]

I'm having a really weird issue since reinstalling Ubuntu 19.04 the other day. This is on a system76 laptop with a bios toggle between nvidia prime mode (intel graphics connected to the display, nvidia card must be trunked through it) and discrete mode (intel graphics deactivated, nvidia card connected directly to the display).

I'd reinstalled when the laptop was in prime mode, and that seemed to work ok. However, I discovered that the nvidia binary driver's vulkan support doesn't work properly in this configuration, just get black windows in every vulkan app. So I went and switched my bios to discrete mode.... and gdm3 appears, but after login I get dumped to a blank tty with a stuck cursor.

Did a bunch of digging and discovered gdm3 is starting on nouveau or some sort of fb driver in wayland, and when X starts it reports that it can't figure out anything about my laptop display:

162:[ 291.239] (II) NVIDIA(0): Validated MetaModes:
163:[ 291.239] (II) NVIDIA(0): "NULL"

If I uncomment the line in /etc/gdm3/custom.conf which says "WaylandEnable=false", then gdm3 just hangs at a blank tty the same way as my user's X session.

If I uninstall the nvidia binary drivers, gdm3 starts but is extremely slow, my X session seems to work ok with greatly reduced performance.

The weirdest part with the binary drivers installed: if I switch to a tty, login as root, stop gdm3, start lightdm, lightdm runs x fine... but then hangs when I try to login. but if I then kill lightdm and start gdm3.... X starts correctly and sees my display, and the Xorg log for my user shows that it's no longer showing metamodes as null! but as soon as I reboot, it's all broken again.

So. this feels like something I can't seem to find in the Ubuntu stack about how things are setup for prime mode that aren't being correctly reset when I set my laptop to discrete mode, and somehow lightdm resets them even if it doesn't work properly itself. No idea what I need to do to make this work properly in discrete mode, but it's a pretty broken state of affairs right now where I don't have any performant way of using my video card.

short of fully reinstalling Ubuntu while the laptop is in discrete mode, do you have any suggestions on how to debug and fix this?

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: nvidia-driver-418 418.56-0ubuntu1
ProcVersionSignature: Ubuntu 5.0.0-20.21-generic 5.0.8
Uname: Linux 5.0.0-20-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.10-0ubuntu27.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Mon Jul 22 01:31:07 2019
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_CA.UTF-8
 SHELL=/bin/bash
SourcePackage: nvidia-graphics-drivers-418
UpgradeStatus: No upgrade log present (probably fresh install)