[Make-wifi-fast] [PATCH RFC/RFT 1/4] mac80211: Rearrange ieee80211_tx_info to make room for tx_time_est

[Johannes Berg]

Hi,

Sorry for the long time to review here ...

On Thu, 2019-09-19 at 14:22 +0200, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke at redhat.com>
> 
> To implement airtime queue limiting, we need to keep a running account of
> the estimated airtime of all skbs queued into the device. Do to this
> correctly, we need to store the airtime estimate into the skb so we can
> decrease the outstanding balance when the skb is freed. This means that the
> time estimate must be stored somewhere that will survive for the lifetime
> of the skb.
> 
> Fortunately, we had a couple of bytes left in the 'status' field in the
> ieee80211_tx_info; and since we only plan to calculate the airtime estimate
> after the skb is dequeued from the FQ structure, on the control side we can
> share the space with the codel enqueue time. And by rearranging the order
> of elements it is possible to have the position of the new tx_time_est line
> up between the control and status structs, so the value will survive from
> when mac80211 hands the packet to the driver, and until the driver either
> frees it, or hands it back through TX status.

Seems reasonable to me, if we end up needing it and don't have an out-
of-band path (that you seem to have been discussing in this long thread
too)

johannes