in the queue -- do not attempt to requeue it. We only poll in two cases:
1) We have a non-interrupting controller. In this case, execution of the
previous command should have left the queue empty. (Perhaps there should be
a KASSERT() to this effect.)
2) We're in the shutdown path, either doing a cache sync or a dump. In these
cases, the retry behavior is useless, because we will no longer get
interrupts to notify us that the earlier commands completed. Instead we
just spin for a few seconds and fail anyway. (XXX We should probably clear
the queue explicitly so that the shutdown/dump will succeed.)
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