[Make-wifi-fast] [RFC/RFT 5/5] ath9k: Count RX airtime in airtime deficit

[Toke Høiland-Jørgensen]

>> I initially thought that using the timestamp put into the frame by the
>> hardware could be a way to get timing. But there's only a timestamp of
>> the first symbol in rs_tstamp, and getting a time to compare it with is
>> difficult; by the time the frame is handled in the rx tasklet, way too
>> much time has been spent on interrupt handling etc for the current time
>> to be worth comparing with.

As an aside, I'm no longer sure this explanation for why I got wrong
timings for this way of measuring RX time is correct. It may simply be
that I was counting the same time interval more than once because I
didn't realise that the handler would be called for each frame. See
below.

> I think rs_tstamp in rx-status is different for first MPDU and last
> MPDU in an A-MPDU meaning you should be able to compute the entire
> duration (if you track it, and this should be fairly straightforward
> as you can't really rx interleaved MPDUs from different
> A-MPDUs/stations). I'm not sure if the last MPDU defines the tstamp of
> first symbol or last one.

I actually went down this path again last night, but haven't had a
chance to test it yet.

So what I have now is this:

       /* Only count airtime for last frame in an aggregate. FIXME: Should
        * this be only the first frame? */
       if (!rs->rs_isaggr || !rs->rs_moreaggr)
               airtime = (tsf & 0xffffffff) - rs->rs_tstamp;

Which was under the assumption that rs_tstamp will be the time of the
first symbol *of the whole ampdu* for all the frames in that aggregate.
However, if rs_tstamp differs between frames, tracking it at the first
frame might be the right things to do?

Is the entire A-MPDU received before the RX handler is called for the
first frame?

-Toke