WRT54g / b43 / mac802.11 BREAKTHROUGH

Hauke Mehrtens hauke at hauke-m.de
Wed Aug 22 12:21:26 EDT 2012

On 07/18/2012 01:56 PM, Bastian Bittorf wrote:
> hi devs!
> yesterday we had a breaktrough debugging b43 in our hackspace maschinenraum/m18[1,2]
> at weimar.freifunk.net[3,4] - since a long time our darling wrt54g suffers from a
> hanging wifi and bad performance[5], but the workaround is easy: now it's up to
> you to fix the rootcause.
> our testsetup, where we could _reproduce_ reliably a stopping TX is like this:
> laptop ---lan--- "A"-wrt54g/adhoc ~~~   air  ~~~ "B"-anyrouter/adhoc
> now make a testdownload with the laptop from B.
> wireless will stop working after ~10 seconds.
> wifi up will reanimate and our freifunk-cron.minutely-check
> will do this automagically. (read further, this is not the solution)
> we tried to limit the rateset to e.g. lower rates, but this does NOT change
> the behaviour. what works is: define a rateset on BOTH router which makes
> it impossible to change the band, e.g.:
> iw dev $WIFIDEV set bitrates legacy-2.4 1 2 5.5 11
> OR
> iw dev $WIFIDEV set bitrates legacy-2.4 6 9 12 18 24 36 48 54
> now we had a great performance, 10 Gigabytes of wireless transfer,
> no stopping TX anymore and an empty box of beer. three things to do now:
> 1) why does a band change (can be seen through minstrel) is a problem?
> 2) we have to make in config-option to force a band, or a rateset:
> 	e.g. uci set wireless.radio0.hwmode=11g
> 	e.g. uci set wireless.radio0.bitrates='6 9 12 18 24 36 48 54'
> 3) spread to word:
>    there is a great frustration in the community about b43,
>    but the old drivers _have_ to die, it's about time - really.
> thanks for your work,
> bye storchi, andi, bastian + m18 crew
> [1] http://blog.maschinenraum.tk/
> [2] http://blog.maschinenraum.tk/2012/07/15/bitcoin-vending-machine-exchange-euro-coins-for-bitcoin-wallets/
> [3] http://wireless.subsignal.org
> [4] http://wireless.subsignal.org/index.php?title=Main_Page_en
> [5] https://dev.openwrt.org/ticket/7552

I tried this on some of my devices (BCM4318 G-PHY and BCM4322 N-PHY) and
it is easily reproducible on all tested devices when restricting the
rates to 11 and 12 MBit/s.

I have the Broadcom device working as an Access point (on a MIPS SoC)
and a Laptop with an Intel Wifi device is connected to it. I generated
the traffic with iperf. If the Access Points sends the traffic the
problem occurs, if it is just receiving there is not problem.

After the problem occurred b43_generate_txhdr is called rarely ( every
~10 seconds) and I am not able to stay connected or connect to the
network any more. I am not familiar with the internal flow in b43 or
mac80211 could someone give me a hint where to look.

I can not see any special changes between CCK and OFDM rates before it
goes down there are many changes without a problem before it goes down.

Currently I do not have a Broadcom wifi card running in a x86 device
just mips devices could someone try to reproduce the problem on a x86

I added the b43 mailing list and Rafał.


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