Re: [PATCH v2 1/4] mac80211: Rearrange ieee80211_tx_info to make room for tx_time_est

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Johannes Berg <johannes@sipsolutions.net> writes:

> On Fri, 2019-10-18 at 12:15 +0200, Toke Høiland-Jørgensen wrote:
>
>> However, there's a nice juicy 'u16 ack_frame_id' at the start of
>> ieee80211_tx_info. Could we potentially use that? We could use the top
>> bit as a disambiguation flag; I think we're fine with 15 bits for the TX
>> time itself (a single packet won't exceed 8ms or TX time), so if we can
>> live with 15 bits of ACK frame ID space, that could be a way forward?
>
> I was going to say that should work as we only ever have a handful of
> ACK frame IDs, but ... you still need the airtime even for a frame that
> userspace wants to know the ACK status of, no?

Oh, right.

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. 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?

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 could even split 6/10 and only need to truncate the TX time at
rates below 13 Mbps... Or we could sacrifice a bit to the flag and only
truncate if the ACK status flag is set.

Think it mostly depends on what is the smallest ID space for ACK IDs we
can live with? :)

-Toke