After inspecting the ath9k source, it seems the AR5416 and later MACs
don't take an explicit RTS/CTS duration. A per-scenario (ie, what multi-
rate retry became) rts/cts control flag and packet duration is provided;
the hardware then apparently fills in whatever details are required.
The per-rate sp/lpack duration calculation just isn't used anywhere
in the ath9k TX packet length calculations.
The burst duration register controls something different; it seems to
be involved with RTS/CTS protection of 11n aggregate frames and is set
via a call to ar5416Set11nBurstDuration().
I've done some light testing with rts/cts protected frames and nothing
seems to break; but this may break said RTS/CTS and CTS-to-self protection.
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