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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Johannes Berg <johannes@sipsolutions.net>,
        Kan Yan <kyan@google.com>
Cc:     linux-wireless@vger.kernel.org,
        make-wifi-fast@lists.bufferbloat.net, ath10k@lists.infradead.org,
        John Crispin <john@phrozen.org>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Felix Fietkau <nbd@nbd.name>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kevin Hayes <kevinhayes@google.com>
Subject: Re: [PATCH v2 1/4] mac80211: Rearrange ieee80211_tx_info to make room for tx_time_est
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Date:   Fri, 18 Oct 2019 16:01:09 +0200
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Johannes Berg <johannes@sipsolutions.net> writes:

> On Fri, 2019-10-18 at 15:31 +0200, Toke H=C3=B8iland-J=C3=B8rgensen wrote=
:
>
>> Well, let's try to do the actual math... A full-size (1538 bytes) packet
>> takes ~2050 microseconds to transmit at 6 Mbps. Adding in overhead, it's
>> certainly still less that 4096 us, so 12 bits is plenty.
>
> What about A-MSDUs? But I guess maximum continous transmissions are at
> most 4ms anyway, so a single packet should never be longer.

Ah yeah, those could be a bit bigger, but yeah, 4ms should at least be
enough.

>> That leaves
>> four bits for the ACK status ID if we just split the u16; if we only
>> ever have "a handful", that should be enough, no?
>
> It's how many are in flight at a time, 16 doesn't seem likely to happen,
> but I don't really know what applications are doing with it now.
> Probably only wpa_s for the EAPOL TX status.

Right. Well in that case, let's try it. As long as we fail in a
reasonable way, we can just see if we run into anything that breaks? I
guess in this case that means rejecting requests from userspace if we
run out of IDs rather than silently wrapping and returning wrong data :)

>> We could also split 5/11. That would support up to 32 ACK IDs, and we
>> can just truncate the airtime at 2048 us, which is not a big deal I'd
>> say.
>
> We can also play with the units of the airtime, e.g. making that a
> multiple of 2 or 4 us? Seems unlikely to matter much?

Sure, that's a good point! Increments of 4us means we can fit 4ms is 10
bits, leaving plenty of space for ACK IDs (hopefully).

I'll rework the series to use that instead :)

-Toke