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.