iwn(4) driver showing poor performance, variable media selection on 10.0-BETA3

[R. Tyler Croy]

I recently installed FreeBSD 10.0-BETA3 onto my Thinkpad X200, everything is
largely running fine except the wireless performance leaves a lot to be
desired.

I've got an older Thinkpad T43 running -CURRENT next to the X200 in the same
exact room, same distance from the AP, but with an iwi(4) card in it.

The T43 will download large files between 100-200KB/s, and the "media" mode for
wlan0 is "OFDM/48Mbps" which seems non-crazy.

The X200 however will get between 20-90KB/s on the same files, and the "media"
mode will oscillate *dramatically* during the transfer. I've watched it go from
OFDM/54Mbps, down to 48Mbps, to 16Mbps, to DS/1Mbps which is where it seems to
idle.

I've started looking at the output of wlanstats(1) and nothing jumps out at me,
then again, I'm a noob so what do I know:

    [23:35:28] tyler:~ $ wlanstats
    8          rx frame too short
    30         rx from wrong bssid
    802156     rx discard 'cuz dup
    4          rx discard 'cuz mcast echo
    172272     rx beacon frames
    175177     rx element unknown
    996        rx frame chan mismatch
    61         rx seq# violation (TKIP)
    42         active scans started
    41         background scans started
    1072961    tkip crypto done in s/w
    405918     tkip tx MIC done in s/w
    667043     tkip rx MIC done in s/w
    173976     rx management frames
    667045     total data frames received
    663976     unicast data frames received
    3069       multicast data frames received
    406278     total data frames transmit
    405920     unicast data frames sent
    1M         current transmit rate
    22         current rssi
    -86        current noise floor (dBm)
    -64        current signal (dBm)

The only thing that looks removely suspicious is the "rx discard" segment, but
again, I plead noob.


Any pointers would be useful and appreciated!

- R. Tyler Croy
------------------
   Code: https://github.com/rtyler
Chatter: https://twitter.com/agentdero
         rtyler at jabber.org

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