carl9170: Unintentional transmission in monitor mode?

[Marco Fonseca <marco@xxxxxxxxxxxxxxx>]

Hello All,

I've noticed unexpected transmissions from the AR9170 while in monitor mode
and using tcpdump (and sometimes without tcpdump running).  These
transmissions degrade (pretty significantly) performance of other STA/AP on
the channel (as seen by iperf going from 60mbit/s to almost nill).  As best I
can tell these transmissions never coalesces into an actual frame in the air
(at least not one I'm able to pick up with a second different adapter in
monitor mode).

Are there any known reasons/bugs on this?  Is there anyway to completely
silence the transmitter of the AR9170 while still allowing reception?




Some additional information that may give some context:
- This seems to happen over a wide range of releases of compat wireless I've
  tried, I've tested between compat wireless 2.6.39-rc6-1 and 2012-7-03,
  various drivers and firmware (the latest being driver 1.9.4 and firmware
  1.9.5).  I've also seen the problem across multiple AR9170 adapters, as well
  as different linux hosts.

- I've put a few printk's in the carl driver & stubbed out (somewhat blindly)
  some of the tx functions in wlan.c in the firmware to see if I could silence
  or get a clue on whats going on, with not much success.  The problem still
  occurred (No prink's told me the kernel wasn't actively generating any
  pkts).

- Seems like the transmissions only occur when monitoring 802.11n speeds.
  (That is if I manual configure the AP to 54mbit/s, I don't see the
  issue).  Happens in both 2.4 or 5ghz band.

- I was able to also observe this behavior with a single ping.  Only seemed to
  occur with larger ICMP pings (would occur with -s 2000 pings, but not a
  standard (56 byte) ping).

- Along with iperf degradation, I've verified transmissions with a spectrum
  analyzer.


Thank you,

Marco Fonseca
--
To unsubscribe from this list: send the line "unsubscribe linux-wireless" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html