The socketcanbackend.cpp opens the socket hardcoded:

if (Q_UNLIKELY((canSocket = socket(PF_CAN, SOCK_RAW | SOCK_NONBLOCK, CAN_RAW)) < 0)) {
  setError(qt_error_string(errno), QCanBusDevice::CanBusError::ConnectionError);
  return false;
}

Now what if you have a device that does not use CAN_RAW as the family. But instead uses a different implementation defined by the hardware vendor?

In my case, I have to pass "7" instead of whatever value CAN_RAW has, to get socketcan working on my device.

Probably the socketcan plugin can get made configurable so that you can pass an arbitrary value here?