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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>
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56d1893d94
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2 changed files with 23 additions and 8 deletions
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@ -417,8 +417,8 @@ minstrel_rate_init(void *priv, struct ieee80211_supported_band *sband,
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tx_time_single = mr->ack_time + mr->perfect_tx_time;
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/* contention window */
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tx_time_single += t_slot + min(cw, mp->cw_max);
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cw = (cw << 1) | 1;
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tx_time_single += (t_slot * cw) >> 1;
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cw = min((cw << 1) | 1, mp->cw_max);
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tx_time += tx_time_single;
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tx_time_cts += tx_time_single + mi->sp_ack_dur;
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@ -464,6 +464,7 @@ minstrel_calc_retransmit(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
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const struct mcs_group *group;
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unsigned int tx_time, tx_time_rtscts, tx_time_data;
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unsigned int cw = mp->cw_min;
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unsigned int ctime = 0;
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unsigned int t_slot = 9; /* FIXME */
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unsigned int ampdu_len = MINSTREL_TRUNC(mi->avg_ampdu_len);
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@ -480,13 +481,27 @@ minstrel_calc_retransmit(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
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group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
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tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len;
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tx_time = 2 * (t_slot + mi->overhead + tx_time_data);
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tx_time_rtscts = 2 * (t_slot + mi->overhead_rtscts + tx_time_data);
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/* Contention time for first 2 tries */
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ctime = (t_slot * cw) >> 1;
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cw = min((cw << 1) | 1, mp->cw_max);
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ctime += (t_slot * cw) >> 1;
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cw = min((cw << 1) | 1, mp->cw_max);
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/* Total TX time for data and Contention after first 2 tries */
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tx_time = ctime + 2 * (mi->overhead + tx_time_data);
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tx_time_rtscts = ctime + 2 * (mi->overhead_rtscts + tx_time_data);
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/* See how many more tries we can fit inside segment size */
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do {
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cw = (cw << 1) | 1;
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cw = min(cw, mp->cw_max);
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tx_time += cw + t_slot + mi->overhead;
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tx_time_rtscts += cw + t_slot + mi->overhead_rtscts;
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/* Contention time for this try */
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ctime = (t_slot * cw) >> 1;
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cw = min((cw << 1) | 1, mp->cw_max);
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/* Total TX time after this try */
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tx_time += ctime + mi->overhead + tx_time_data;
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tx_time_rtscts += ctime + mi->overhead_rtscts + tx_time_data;
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if (tx_time_rtscts < mp->segment_size)
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mr->retry_count_rtscts++;
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} while ((tx_time < mp->segment_size) &&
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