[ar9300] Disable unconditionally reducing transmit power in the case of FCC.
Ok, yeah, the commit title is a bit misleading.
This has to do with CDD (cyclic delay diversity) - how this and later
wifi hardware transmits lower rates over more antennas. Eg, if you're
transmitting legacy 11abg rates on 2 or 3 antennas, you COULD just
send them all at the same time or you could delay each by tens/hundreds
of nanoseconds to try and get some better diversity characteristics.
However, this has a fun side effect - the antenna pattern is no longer
a bunch of interacting dipoles, but are a bunch of interacting dipoles
plus a bunch of changing phases. And it's frequency dependent - 50-200nS
is not exactly the same fraction of a wavelength across all of 2GHz or 5GHz!
Thus the power spectral density and maximum directional gain that you're
effectively getting is not .. well, as flat as it once was.
For more information, look up FCC/OET 13TR1003 in the FCC technical report
database. It has pretty graphics and everything.
Anyway, the problem lies thusly - the CDD code just subtracts another 3dB
or 5dB for the lower rates based on transmit antenna configuration.
However, it's not done based on operating configuration and it doesn't
take into account how far from any regulatory limits the hardware is at.
It also doesn't let us do things like transmit legacy rates and frames
on a single antenna without losing up to 5dB when we absolutely don't
need to in that case (there's no CDD used when one antenna is used!)
This shows up as the hardware behaving even worse for longer distance links
at 20MHz because, well, those are the exact rates losing a bunch more
transmit power.
- For lower power NICs (ie the majority of what is out there!) it's highly unlikely we're going to hit anywhere near the PSD limits.
- It's doing it based on the existing limits from the CTL table (conformance testing limits) - this isn't the regulatory max! It's what the NIC is allowed to put out in each frequency and rate configuration! So things like band edges, power amplifier behaviour and maximum current draw apply here. Blindly subtracting 3 to 5dB from /this/ value is /very/ conservative..
- /and/ ath9k just plainly doesn't do any of this at all.
So, for now disable it and get the TX power back, thus matching what ath9k
in Linux is doing. If/once I get some more cycles I'll look at making it
a bit more adaptive and really only kick in if we're a few dB away from
hard regulatory limits.
Tested:
- AR9344 (2GHz + SoC, 2x2 configuration) - AP and STA modes
- QCA9580 (5GHz 2x2 and 3x3 configurations) - AP and STA modes