ena: exponential backoff exp limit

[osamaabb_amazon.com]

Summary

This patch limits the exponent in the exponential backoff
mechanism in order to avoid the value overflowing.

Approved by: cperciva
MFC after: 2 weeks
Sponsored by: Amazon, Inc.

[cperciva]

Dumb question, but where does the delay_us value which is passed to this come from? It looks like it come from ena_min_poll_delay_us which... as far as I can tell is never set? Is this just vestigal code or am I missing something?

[osamaabb_amazon.com]

In D46330#1064438, @cperciva wrote:

Dumb question, but where does the delay_us value which is passed to this come from? It looks like it come from ena_min_poll_delay_us which... as far as I can tell is never set? Is this just vestigal code or am I missing something?

You're correct, added such a patch in [1]
[1] https://reviews.freebsd.org/D46763