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devctl - device event reporting and device control interface



      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 desir‐
      able in the long run, but will get a lot of hair out of this implementa‐
      tion.  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.  Lis‐
      tening programs are expected to cope.
      The sysctl and boot parameter hw.bus.devctl_disable is used 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.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.