Command Section
DEVCTL(4)              FreeBSD Kernel Interfaces Manual              DEVCTL(4)

     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)

     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 format.

           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.


FreeBSD 11.1-RELEASE-p4         March 26, 2014         FreeBSD 11.1-RELEASE-p4
Command Section