Running “xrandr --verbose” tells me (among other things):
eDP-1-1 connected 1920x1080+0+0 (0x1ff) normal (normal left inverted right x axis y axis) 344mm x 194mm
That’s for the internal flat panel of a laptop, driven from an Intel iGPU. It gives me the DRM identifier, so I can match this connector up with the corresponding one returned by the libdrm APIs.
This laptop also has an Nvidia dGPU driving a HDMI output, which is listed by xrandr, but doesn’t have CONNECTOR_ID set. Would it be possible to add this property to a future driver release?