Setting 3x3 NIC to 2x2

[Ben Greear]

Hello!

For this case:  2 PCs, one 3x3 peregrine ath10k NIC in each.  Once acting as AP,
the other as a station.  Testing UDP upload performance.

Someone reported a problem where they thought they had a 2x2 ath10k NIC
but in reality it was 3x3.  They had only the first two antenna connected.

Using stock Fedora 21 kernel and firmware-4.bin, performance was horrible..around 8-10Mbps
UDP throughput at best.  This was on 2.4 band, so VHT is disabled.

I think the first issue is that performance is so bad if you are missing a single
antenna.  (In my lab, a similar setup with 3 antenna connected was giving 120+Mbps
throughput).


The second problem I saw only on non-stock firmware & driver, so of course problem
could be related to that:

If I start up system in 3x3 mode, and then try to change to 2x2
by setting the tx-antenna mask with iw, then it *appears* that everything
is passed down to the firmware properly, but performance is horrible
(5-10Mbps).  Forcing rate-ctrl to use a higher fixed speed makes things worse,
so it's not just rate-ctrl messing up.

Restarting all network devices on that radio, which causes a soft NIC
reboot, does not fix the problem.  (I reset both AP and STA machines
radios multiple times)

But, if I do a full reboot, then the system comes up happy and will
negotiate up to 300Mbps nominal rate (ie, full 2x2 capabilities).

I suspect there may be bugs in setting up registers down in the
firmware, but I have not taken a close look at it at.

As always, I'm curious if others have seen similar problems (or, tried this
and NOT seen such problems.)

Thanks,
Ben

-- 
Ben Greear <greearb at candelatech.com>
Candela Technologies Inc  http://www.candelatech.com