Using nVidia as an overpriced output adapter for AMD

xorg-nvidia-outputs.md

Original problem

• Monitor with only DVI free port that supports native resolution.
• Tired from nVidia tension with Linux and switching to AMD.
• New AMD cards and built-in have no DVI output and adapter will arrive late.

Solution: use reverse prime with nouveau

It is well described in https://wiki.archlinux.org/index.php/PRIME#Reverse_PRIME But in this case I want to use AMD as a primary and nVidia as a "Source Output" which nouveau supports.

Section "ServerLayout"
  Identifier     "layout"
  Screen      0  "primary-screen" # Screen for your primary GPU
  Inactive       "dGPU-nouveau"       # Device for your second GPU
EndSection

Section "Screen"
    Identifier "primary-screen"
    Device "dGPU-amd"
    Option "AllowEmptyInitialConfiguration" "true"
EndSection

(there were no need to specify PCIs for me)

Since AMD is not connected to any outputs need to wire them every time. E.g. https://wiki.archlinux.org/index.php/NVIDIA_Optimus#LightDM

But in my case I had terrible performance if xrandr doesn't have any mode enabled for AMD. Thus I had to modify start-up:

# Use outputs from nVidia (nouveau) for primary GPU
if xrandr --setprovideroutputsource nouveau 0 && xrandr --auto; then
  # force native resolution even that it cannot be shown through HDMI input of display
  # effectively turns off display, but still renders/clons through nouveau to DVI
  xrandr --addmode HDMI-A-1 2560x1440 && xrandr --output HDMI-A-1 --mode 2560x1440 || true
fi

In my case HDMI was the only port I was able to use for monitor, but it limited to 1920x1080. Thus this effectively turns of display over HDMI. It looks like this also fixed glitch when I saw only cursor for login window and had to switch back and forth on monitor beetween HDMI and DVI.