The DEVICE_PROBE method should probe to see if the device is present. It should return 0 if the device exists, ENXIO if it cannot be found. If some other error happens during the probe (such as a memory allocation failure), an appropriate error code should be returned. For cases where more than one driver matches a device, a priority value can be returned. In this case, success codes are values less than or equal to zero with the highest value representing the best match. Failure codes are represented by positive values and the regular Unix error codes should be used for the purpose.
If a driver returns a success code which is less than zero, it must not assume that it will be the same driver which is attached to the device. In particular, it must not assume that any values stored in the softc structure will be available for its attach method and any resources allocated during probe must be released and re-allocated if the attach method is called. In addition it is an absolute requirement that the probe routine have no side effects whatsoever. The probe routine may be called more than once before the attach routine is called.
If a success code of zero is returned, the driver can assume that it will be the one attached, but must not hold any resources when the probe routine returns. A driver may assume that the softc is preserved when it returns a success code of zero.
A value equal to or less than zero indicates success, greater than zero indicates an error (errno). For values equal to or less than zero: zero indicates highest priority, no further probing is done; for a value less than zero, the lower the value the lower the priority, e.g. -100 indicates a lower priority than -50.
device(9), DEVICE_ATTACH(9), DEVICE_DETACH(9), DEVICE_IDENTIFY(9), DEVICE_SHUTDOWN(9)