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) 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
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
hw.bus.devctl_queue can be used to
control queue length. It is set to 0 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. |
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 detach messages to flush data
to the device. The device is already gone.
devd(8)