From: Joe Thornber <thornber@sistina.com>

There was a race associated with the 'wait for a significant event'
functionality.

Basically userland could read the status table, then wait for another
event, but the event it was waiting for could have occurred in the gap
between reading and waiting.

To solve this we assign identifiers to events, in order to successfully
wait for an event both userland and the kernel driver must be in agreement
about what the last event identifier was.  If they don't agree the wait
call will return immediately, allowing userland to re-read the status and
see what it missed.

The new ioctl interface will use this properly.