If you wish to override default selection, which is:
- Nvidia if installed and valid drivers
- /dev/dri/renderD128 device
The preference to set in Preferences.xml is: HardwareDevicePath="/dev/dri/device-name"
e.g.
HardwareDevicePath="/dev/dri/renderD129"
-or-
HardwareDevicePath="/dev/dri/renderD128" to force the Intel QSV vaapi
Also, if the group assignment for the device differs than that of the QSV device, user plex must be manually added to that group.