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DEVCTL(4)		 BSD 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.	 Fu-
     ture 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 de-
     vice a character at a time, you will lose the rest	of the data.  Listen-
     ing 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 de-
     tach messages to flush data to the	device.	 The device is already gone.


BSD			       February	11, 2003			   BSD


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