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.\" Copyright (c) 2002 M. Warner Losh
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.Dd February 11, 2003
.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 /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 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 interface that we have
not might more properly be an ioctl interface.
.Pp
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 hasn't been
tested since 3.4 or 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 loose
the rest of the data.
Listening programs are expected to cope.
.Pp
The sysctl and boot parameter hw.bus.devctl_disable is used to disable
.Nm
when no devd is running.
.Sh PROTOCOL
.Nm
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.
.Pp
.Bl -column -compact "Type" "Description"
.Em "Type	Description"
+	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.
.D1 Tdev at parent on location
.Bl -column -compact "Part" "Description"
.Em "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
.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'll get a device
attached message.
If it doesn't, 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