compat-wireless and minstrel

[Adam Wozniak <awozniak@irobot.com>]

I have two systems under test, both Dell laptops (a Latitude D630 and an 
Inspiron 600m) both running Ubuntu 9.10 with the latest updates, and 
bleeding edge compat-wireless-2009-11-02.  I'm using identical AR9170 
based D-Link DWA-160 USB 802.11adapters.  I'm using nuttcp to measure 
throughput.  I'm running in ad-hoc mode.  Both machines have the same 
ar9170 files in /lib/firmware.  The machines are sitting about 5 feet 
apart in my office.

I'm having occasional problems where throughput drops through the floor 
(0.5Mbps - 1.5Mbps).  When I cat 
/sys/kernel/debug/ieee80211/*/stations/*/rc_stats, one of the machines 
lists the full set of rates, but the other only lists 1M and 54M.  After 
a period of time, that machine drops 54M and lists only one rate 
(1Mbps), and the throughput listed by nuttcp drops accordingly.  I 
assume that, for whatever reason, the rates drop off the list and 
minstrel uses the only one left available to it.

If I modify include/net/mac80211.h and force the inline function 
rate_supported to always return 1, this fixes the problem.  However, I 
think this is a band aid around some other issue.

Any clues or ideas what the real issue might be here?