DEVICE_PROBE(9) FreeBSD Kernel Developer's Manual DEVICE_PROBE(9)


DEVICE_PROBEprobe for device existence


#include < sys/param.h>
#include < sys/bus.h>

DEVICE_PROBE( device_t dev);


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.

The following values are used by convention to indicate different strengths of matching in a probe routine. Except as noted, these are just suggested values, and there's nothing magical about them.

The device that cannot be reprobed, and that no possible other driver may exist (typically legacy drivers who don't follow all the rules, or special needs drivers).
The device is supported by a vendor driver. This is for source or binary drivers that are not yet integrated into the FreeBSD tree. Its use in the base OS is prohibited.
The device is a normal device matching some plug and play ID. This is the normal return value for drivers to use. It is intended that nearly all of the drivers in the tree should return this value.
The driver is a legacy driver, or an otherwise less desirable driver for a given plug and play ID. The driver has special requirements like when there are two drivers that support overlapping series of hardware devices. In this case the one that supports the older part of the line would return this value, while the one that supports the newer ones would return BUS_PROBE_DEFAULT.
The driver matches the type of device generally. This allows drivers to match all serial ports generally, with specialized drivers matching particular types of serial ports that need special treatment for some reason.
The driver matches all unclaimed devices on a bus. The ugen(4) device is one example.
The driver expects its parent to tell it which children to manage and no probing is really done. The device only matches if its parent bus specifically said to use this driver.


This manual page was written by Doug Rabson.
February 8, 2012 FreeBSD