There's still a lot of random issues to sort out with the radio side of
things and AMPDU RX handling (and completely missing AMPDU TX handling!)
but if people wish to give this a go and assist in debugging the
issues, they can define ATH_DO_11N to enable it.
I'm just re-iterating - this is here to allow people to assist in
further 11n development; it is not any indication that the 11n support
is complete and functional.
Important notes:
* This doesn't support 1-stream cards yet - (eg AR9285) - the various bits
that negotiate TX/RX MCS don't know not to try >1 stream TX or negotiate
1-stream RX; so don't enable 11n unless you've first taught the rate
control module and the net80211 stack to negotiate 1-stream stuff;
* The only rate control module minimally 11n aware is ath_rate_sample;
* ath_rate_sample doesn't know about HT/40; so airtime will be incorrectly
calculated;
* The AR9160 and AR9280 radio code is unreliable at the higher MCS rates for
some reason; this will definitely impact 11n performance;
* AMPDU-TX isn't yet implemented;
* AMPDU-RX may be a bit buggy still and will definitely suffer from the
radio unreliability mentioned above (ie, don't expect 150/300mbit
RX just yet.)
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