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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 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.
! A notify event, such as a link state change.
+ Device node in tree attached.
- Device node in tree detached.
? Unknown device detected.
Except for the first character in the record, attach and detach messages
have the same format.
Tdev at parent on location
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.