RE: Changes in the network interface queueing handoff model

Attached is a patch that maintains the current if_start, but adds
if_startmbuf. If a device driver implements if_startmbuf and the global
sysctl net.startmbuf_enabled is set to 1, then the if_startmbuf path in the
driver will be used. Otherwise, if_start is used. I have modified the if_em
driver to implement if_startmbuf also. If there is no packet backlog in the
if_snd queue, it directly places the packet in the transmit descriptor ring.
If there is a backlog, it uses the if_snd queue protected by driver mutex,
rather than a separate ifq mutex.

I question whether you need a fallback software if_snd queue at all
for modern devices such as the Intel and Broadcom gigabit chips. The
hardware transmit descriptor rings typically have sizes of the order
of 256 descriptors. I think if the ring fills up, you could simply
drop the packet with ENOBUFS. That's what happens if the if_snd queue
fills up, and its maximum size is comparable to the sizes of modern
descriptor rings. It would simplify things quite a bit to eliminate
the if_snd queue entirely for such devices.

In any case, I'm glad you're looking at making this change. I think
it's the right thing to do.

John
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