mac80211: fix contention time computation in minstrel, minstrel_ht

When transmitting a frame, the transmitter waits a random number of
slots between 0 and cw. Thus, the contention time is (cw / 2) * t_slot
which we can represent instead as (cw * t_slot) >> 1. Also fix a few
other accounting bugs around contention time, and add comments.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/mac80211/rc80211_minstrel.c b/net/mac80211/rc80211_minstrel.c
index 778c604d7939..8adac67395f7 100644
--- a/net/mac80211/rc80211_minstrel.c
+++ b/net/mac80211/rc80211_minstrel.c
@@ -417,8 +417,8 @@ minstrel_rate_init(void *priv, struct ieee80211_supported_band *sband,
                         tx_time_single = mr->ack_time + mr->perfect_tx_time;
 
                         /* contention window */
-                        tx_time_single += t_slot + min(cw, mp->cw_max);
-                        cw = (cw << 1) | 1;
+                        tx_time_single += (t_slot * cw) >> 1;
+                        cw = min((cw << 1) | 1, mp->cw_max);
 
                         tx_time += tx_time_single;
                         tx_time_cts += tx_time_single + mi->sp_ack_dur;