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>
diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index dbdebeda..3d09c58 100644
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -464,6 +464,7 @@
const struct mcs_group *group;
unsigned int tx_time, tx_time_rtscts, tx_time_data;
unsigned int cw = mp->cw_min;
+ unsigned int ctime = 0;
unsigned int t_slot = 9; /* FIXME */
unsigned int ampdu_len = MINSTREL_TRUNC(mi->avg_ampdu_len);
@@ -480,13 +481,27 @@
group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len;
- tx_time = 2 * (t_slot + mi->overhead + tx_time_data);
- tx_time_rtscts = 2 * (t_slot + mi->overhead_rtscts + tx_time_data);
+
+ /* Contention time for first 2 tries */
+ ctime = (t_slot * cw) >> 1;
+ cw = min((cw << 1) | 1, mp->cw_max);
+ ctime += (t_slot * cw) >> 1;
+ cw = min((cw << 1) | 1, mp->cw_max);
+
+ /* Total TX time for data and Contention after first 2 tries */
+ tx_time = ctime + 2 * (mi->overhead + tx_time_data);
+ tx_time_rtscts = ctime + 2 * (mi->overhead_rtscts + tx_time_data);
+
+ /* See how many more tries we can fit inside segment size */
do {
- cw = (cw << 1) | 1;
- cw = min(cw, mp->cw_max);
- tx_time += cw + t_slot + mi->overhead;
- tx_time_rtscts += cw + t_slot + mi->overhead_rtscts;
+ /* Contention time for this try */
+ ctime = (t_slot * cw) >> 1;
+ cw = min((cw << 1) | 1, mp->cw_max);
+
+ /* Total TX time after this try */
+ tx_time += ctime + mi->overhead + tx_time_data;
+ tx_time_rtscts += ctime + mi->overhead_rtscts + tx_time_data;
+
if (tx_time_rtscts < mp->segment_size)
mr->retry_count_rtscts++;
} while ((tx_time < mp->segment_size) &&