Can't start the X server with multiple GPUs

If you have two or more GPUs and the X server has trouble starting, you may need to specify which should be used by X for rendering. You can do this by writing the following to a file, e.g. ~/xorg.conf:

Section "Device"
    Identifier "Device0"
    Driver "<driver>"
    BusID "PCI:<device>"
EndSection

• Replace <driver> with amdgpu, nouveau or nvidia based on the driver you installed.

• Replace <device> with the corresponding identifier for the PCI device. You can list all devices with lspci, but note that lspci displays the numbers in hexadecimal and the last number separated with a dot, like 00:1f.5; the numbers must be translated to decimal and the dot replaced with a colon, like PCI:0:31:5.

Then, you must tell the display manager to use the configuration. For example, LightDM has a xserver-config option in /etc/lightdm/config that can be set to point to the file.