--- /dev/null
+From: Felix Fietkau <nbd@nbd.name>
+Date: Sat, 6 Feb 2021 16:33:14 +0100
+Subject: [PATCH] mac80211: minstrel_ht: rework rate downgrade code and
+ max_prob rate selection
+
+The current fallback code for fast rate switching on potentially failing rates
+is triggering too often if there is some strong noise on the channel. This can
+lead to wild fluctuations in the rate selection.
+Additionally, switching down to max_prob_rate can create a significant gap down
+in throughput, especially when using only 2 spatial streams, because max_prob_rate
+is limited to using fewer streams than the max_tp rates.
+In order to improve throughput without reducing reliability too much, use the
+rate downgrade code for the max_prob_rate only, and allow the non-downgraded
+max_prob_rate to use as many spatial streams as the max_tp rates
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+
+--- a/net/mac80211/rc80211_minstrel_ht.c
++++ b/net/mac80211/rc80211_minstrel_ht.c
+@@ -511,6 +511,14 @@ minstrel_ht_set_best_prob_rate(struct mi
+ int cur_tp_avg, cur_group, cur_idx;
+ int max_gpr_group, max_gpr_idx;
+ int max_gpr_tp_avg, max_gpr_prob;
++ int min_dur;
++
++ min_dur = max(minstrel_get_duration(mi->max_tp_rate[0]),
++ minstrel_get_duration(mi->max_tp_rate[1]));
++
++ /* make the rate at least 18% slower than max tp rates */
++ if (minstrel_get_duration(index) <= min_dur * 19 / 16)
++ return;
+
+ cur_group = MI_RATE_GROUP(index);
+ cur_idx = MI_RATE_IDX(index);
+@@ -532,11 +540,6 @@ minstrel_ht_set_best_prob_rate(struct mi
+ !minstrel_ht_is_legacy_group(max_tp_group))
+ return;
+
+- /* skip rates faster than max tp rate with lower prob */
+- if (minstrel_get_duration(mi->max_tp_rate[0]) > minstrel_get_duration(index) &&
+- mrs->prob_avg < max_tp_prob)
+- return;
+-
+ max_gpr_group = MI_RATE_GROUP(mg->max_group_prob_rate);
+ max_gpr_idx = MI_RATE_IDX(mg->max_group_prob_rate);
+ max_gpr_prob = mi->groups[max_gpr_group].rates[max_gpr_idx].prob_avg;
+@@ -594,40 +597,6 @@ minstrel_ht_assign_best_tp_rates(struct
+
+ }
+
+-/*
+- * Try to increase robustness of max_prob rate by decrease number of
+- * streams if possible.
+- */
+-static inline void
+-minstrel_ht_prob_rate_reduce_streams(struct minstrel_ht_sta *mi)
+-{
+- struct minstrel_mcs_group_data *mg;
+- int tmp_max_streams, group, tmp_idx, tmp_prob;
+- int tmp_tp = 0;
+-
+- if (!mi->sta->ht_cap.ht_supported)
+- return;
+-
+- group = MI_RATE_GROUP(mi->max_tp_rate[0]);
+- tmp_max_streams = minstrel_mcs_groups[group].streams;
+- for (group = 0; group < ARRAY_SIZE(minstrel_mcs_groups); group++) {
+- mg = &mi->groups[group];
+- if (!mi->supported[group] || group == MINSTREL_CCK_GROUP)
+- continue;
+-
+- tmp_idx = MI_RATE_IDX(mg->max_group_prob_rate);
+- tmp_prob = mi->groups[group].rates[tmp_idx].prob_avg;
+-
+- if (tmp_tp < minstrel_ht_get_tp_avg(mi, group, tmp_idx, tmp_prob) &&
+- (minstrel_mcs_groups[group].streams < tmp_max_streams)) {
+- mi->max_prob_rate = mg->max_group_prob_rate;
+- tmp_tp = minstrel_ht_get_tp_avg(mi, group,
+- tmp_idx,
+- tmp_prob);
+- }
+- }
+-}
+-
+ static u16
+ __minstrel_ht_get_sample_rate(struct minstrel_ht_sta *mi,
+ enum minstrel_sample_type type)
+@@ -1107,8 +1076,6 @@ minstrel_ht_update_stats(struct minstrel
+
+ mi->max_prob_rate = tmp_max_prob_rate;
+
+- /* Try to increase robustness of max_prob_rate*/
+- minstrel_ht_prob_rate_reduce_streams(mi);
+ minstrel_ht_refill_sample_rates(mi);
+
+ #ifdef CPTCFG_MAC80211_DEBUGFS
+@@ -1153,7 +1120,7 @@ minstrel_ht_txstat_valid(struct minstrel
+ }
+
+ static void
+-minstrel_downgrade_rate(struct minstrel_ht_sta *mi, u16 *idx, bool primary)
++minstrel_downgrade_prob_rate(struct minstrel_ht_sta *mi, u16 *idx)
+ {
+ int group, orig_group;
+
+@@ -1168,11 +1135,7 @@ minstrel_downgrade_rate(struct minstrel_
+ minstrel_mcs_groups[orig_group].streams)
+ continue;
+
+- if (primary)
+- *idx = mi->groups[group].max_group_tp_rate[0];
+- else
+- *idx = mi->groups[group].max_group_tp_rate[1];
+- break;
++ *idx = mi->groups[group].max_group_prob_rate;
+ }
+ }
+
+@@ -1183,7 +1146,7 @@ minstrel_ht_tx_status(void *priv, struct
+ struct ieee80211_tx_info *info = st->info;
+ struct minstrel_ht_sta *mi = priv_sta;
+ struct ieee80211_tx_rate *ar = info->status.rates;
+- struct minstrel_rate_stats *rate, *rate2;
++ struct minstrel_rate_stats *rate;
+ struct minstrel_priv *mp = priv;
+ u32 update_interval = mp->update_interval;
+ bool last, update = false;
+@@ -1233,18 +1196,13 @@ minstrel_ht_tx_status(void *priv, struct
+ /*
+ * check for sudden death of spatial multiplexing,
+ * downgrade to a lower number of streams if necessary.
++ * only do this for the max_prob_rate to prevent spurious
++ * rate fluctuations when the link changes suddenly
+ */
+- rate = minstrel_get_ratestats(mi, mi->max_tp_rate[0]);
++ rate = minstrel_get_ratestats(mi, mi->max_prob_rate);
+ if (rate->attempts > 30 &&
+ rate->success < rate->attempts / 4) {
+- minstrel_downgrade_rate(mi, &mi->max_tp_rate[0], true);
+- update = true;
+- }
+-
+- rate2 = minstrel_get_ratestats(mi, mi->max_tp_rate[1]);
+- if (rate2->attempts > 30 &&
+- rate2->success < rate2->attempts / 4) {
+- minstrel_downgrade_rate(mi, &mi->max_tp_rate[1], false);
++ minstrel_downgrade_prob_rate(mi, &mi->max_prob_rate);
+ update = true;
+ }
+ }
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