Sometimes the automatically selected primary GPU isn't suitable with no way to make an well educated guess to do it better. To make it possible for the user to override the automatically calculated default, make it possible to override it using a udev rule.
E.g. to select /dev/dri/card1 as the primary GPU, add a file e.g. /usr/lib/udev/rules.d/61-mutter-primary-gpu.rules (path my vary depending on distribution) containing the fellowing line:
ENV{DEVNAME}=="/dev/dri/card1", TAG+="mutter-device-preferred-primary"
Reboot or manual triggering of udev rules to make it take effect may be required.
Related: !1057 (closed)