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.\" Copyright (c) 2002 M. Warner Losh
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.Dd March 26, 2014
.Dt DEVCTL 4
.Os
.Sh NAME
.Nm devctl
.Nd "device event reporting and device control interface"
.Sh DESCRIPTION
The
.Nm
device is used to report device events from the kernel.
Future versions will allow for some device control as well.
.Sh IMPLEMENTATION NOTES
This design allows only one reader for
.Pa /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.
.Pp
Also note: we specifically do not attach a device to the
.Vt 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
.Xr sysctl 3
interface that we have now might more properly be an
.Xr ioctl 2
interface.
.Pp
.Dv SIGIO
support is included in the driver.
However, the author is not sure that the
.Dv SIGIO
support is done correctly.
It was copied from a driver that had
.Dv SIGIO
support that likely has not been
tested since
.Fx 3.4
or
.Fx 2.2.8 !
.Pp
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.
.Pp
The sysctl
.Va hw.bus.devctl_queue
can be used to control queue length. It is set to 0 to disable
.Nm
when no
.Xr devd 8
is running.
.Sh PROTOCOL
The
.Nm
device
uses an
.Tn 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.
.Pp
.Bl -column -compact "Type" "Description"
.Em "Type	Description"
!	A notify event, such as a link state change.
+	Device node in tree attached.
-	Device node in tree detached.
?	Unknown device detected.
.El
.Ss Message Formats
Except for the first character in the record, attach and detach
messages have the same format.
.Pp
.D1 Ar T Ns Ar dev Li at Ar parent Li on Ar location
.Pp
.Bl -column -compact "location" "Description"
.Em "Part	Description"
.It Ar T Ta "+ or -"
.It Ar dev Ta "The device name that was attached/detached."
.It Ar parent Ta "The device name of the parent bus that attached the device."
.It Ar location Ta "Bus specific location information."
.El
.Pp
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.
.Pp
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.
.Sh SEE ALSO
.Xr devd 8