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     devctl — device event reporting and device control interface


     The devctl device is used to report device events from the kernel.  Future versions will
     allow for some device control as well.


     This design allows only one reader for /dev/devctl.  This is not desirable in the long run,
     but will get a lot of hair out of this implementation.  Maybe we should make this device a
     clonable device.

     Also note: we specifically do not attach a device to the device_t tree to avoid potential
     chicken and egg problems.  One could argue that all of this belongs to the root node.  One
     could also further argue that the sysctl(3) interface that we have now might more properly
     be an ioctl(2) interface.

     SIGIO support is included in the driver.  However, the author is not sure that the SIGIO
     support is done correctly.  It was copied from a driver that had SIGIO support that likely
     has not been tested since FreeBSD 3.4 or FreeBSD 2.2.8!

     The read channel for this device is used to report changes to userland in realtime.  We
     return one record at a time.  If you try to read this device a character at a time, you will
     lose the rest of the data.  Listening programs are expected to cope.

     The sysctl hw.bus.devctl_queue can be used to control queue length.  It is set to 0 to
     disable devctl when no devd(8) is running.


     The devctl device uses an ASCII protocol.  The driver returns one record at a time to its
     readers.  Each record is terminated with a newline.  The first character of the record is
     the event type.

     Type    Description
     !       A notify event, such as a link state change.
     +       Device node in tree attached.
     -       Device node in tree detached.
     ?       Unknown device detected.

   Message Formats
     Except for the first character in the record, attach and detach messages have the same

           Tdev at parent on location

     Part        Description
     T           + or -
     dev         The device name that was attached/detached.
     parent      The device name of the parent bus that attached the device.
     location    Bus specific location information.

     The nomatch messages can be used to load devices driver.  If you load a device driver, then
     one of two things can happen.  If the device driver attaches to something, you will get a
     device attached message.  If it does not, then nothing will happen.

     The attach and detach messages arrive after the event.  This means one cannot use the attach
     message to load an alternate driver.  The attach message driver has already claimed this
     device.  One cannot use the detach messages to flush data to the device.  The device is
     already gone.