mac80211/minstrel_ht: improve rate selection stability

Under load, otherwise stable rates can easily fluctuate because of
collisions. In my tests on a clean channel, the success probability of
the max throughput rate often stays somewhere between 90% and 100% under
load. This can cause some unnecessary switching to lower rates.
This patch improves stability by treating success probability values
between 90% and 100% the same.
In my tests on a 3x3 HT20 link with lots of TCP traffic, it improves the
average throughput by a few mbit/s.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -202,14 +202,23 @@ minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
 	struct minstrel_rate_stats *mr;
 	unsigned int nsecs = 0;
 	unsigned int tp;
+	unsigned int prob;
 
 	mr = &mi->groups[group].rates[rate];
+	prob = mr->probability;
 
-	if (mr->probability < MINSTREL_FRAC(1, 10)) {
+	if (prob < MINSTREL_FRAC(1, 10)) {
 		mr->cur_tp = 0;
 		return;
 	}
 
+	/*
+	 * For the throughput calculation, limit the probability value to 90% to
+	 * account for collision related packet error rate fluctuation
+	 */
+	if (prob > MINSTREL_FRAC(9, 10))
+		prob = MINSTREL_FRAC(9, 10);
+
 	if (group != MINSTREL_CCK_GROUP)
 		nsecs = 1000 * mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);