[RFC PATCH wireless] wifi: mt76: mt7925: get the survey airtime from the firmware

[Devin Wittmayer]

Chris wrote:
> One thing on offset 20: in monitor mode it doesn't look like all of
> rx.

You're right, it isn't all of rx. Your numbers made me curious, so I
tried it here with an A9000 as a 20 MHz monitor next to an Alfa. It
seems to come down to how strong the signal is. Share of the time:

                                         offset 20   decoded frames
  one strong neighbour (-22 dBm)            2.3%          2.0%
  only weak neighbours (-73 dBm or less)    0.0%          3.6%
  my upload, monitor on a side channel     54.6%          7.5%

Strong signals land in 20 even when the monitor can't decode them,
including spill from the next channel, and weak ones are left out.
That would fit what you saw on 5 GHz and on channels 2 to 4.

As a station the A9000's own downlink does show up in 20, so I'd keep
it as the receive time.

Thanks for the long run against the Wi-Spy, and for the Tested-by.

Devin