NAME

     DEVICE_PROBE — probe for device existence

SYNOPSIS

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

     int
     DEVICE_PROBE(device_t dev);

DESCRIPTION

     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.

RETURN VALUES

     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.

     BUS_PROBE_SPECIFIC    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).

     BUS_PROBE_VENDOR      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.

     BUS_PROBE_DEFAULT     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.

     BUS_PROBE_LOW_PRIORITY
                           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.

     BUS_PROBE_GENERIC     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.

     BUS_PROBE_HOOVER      The driver matches all unclaimed devices on a bus.  The ugen(4) device is one
                           example.

     BUS_PROBE_NOWILDCARD  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.

SEE ALSO

     device(9), DEVICE_ATTACH(9), DEVICE_DETACH(9), DEVICE_IDENTIFY(9), DEVICE_SHUTDOWN(9)

AUTHORS

     This manual page was written by Doug Rabson.