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
.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 sysctl(3) interface that we have now might more properly be an ioctl(2) interface.
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
.Fx 3.4 or
.Fx 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 loose 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.
"Type Description" + 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.