mac80211: minstrel_ht: improve sample rate skip logic

There were a few issues that were slowing down the process of finding
the optimal rate, especially on devices with multi-rate retry
limitations:

When max_tp_rate[0] was slower than max_tp_rate[1], the code did not
sample max_tp_rate[1], which would often allow it to switch places with
max_tp_rate[0] (e.g. if only the first sampling attempts were bad, but the
rate is otherwise good).

Also, sample attempts of rates between max_tp_rate[0] and [1] were being
ignored in this case, because the code only checked if the rate was
slower than [1].

Fix this by checking against the fastest / second fastest max_tp_rate
instead of assuming a specific order between the two.

In my tests this patch significantly reduces the time until minstrel_ht
finds the optimal rate right after assoc

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index 370d677b547b..46ce08ed70b5 100644
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -924,6 +924,7 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
         struct minstrel_rate_stats *mrs;
         struct minstrel_mcs_group_data *mg;
         unsigned int sample_dur, sample_group, cur_max_tp_streams;
+        int tp_rate1, tp_rate2;
         int sample_idx = 0;
 
         if (mi->sample_wait > 0) {
@@ -945,14 +946,22 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
         mrs = &mg->rates[sample_idx];
         sample_idx += sample_group * MCS_GROUP_RATES;
 
+        /* Set tp_rate1, tp_rate2 to the highest / second highest max_tp_rate */
+        if (minstrel_get_duration(mi->max_tp_rate[0]) >
+            minstrel_get_duration(mi->max_tp_rate[1])) {
+                tp_rate1 = mi->max_tp_rate[1];
+                tp_rate2 = mi->max_tp_rate[0];
+        } else {
+                tp_rate1 = mi->max_tp_rate[0];
+                tp_rate2 = mi->max_tp_rate[1];
+        }
+
         /*
          * Sampling might add some overhead (RTS, no aggregation)
-         * to the frame. Hence, don't use sampling for the currently
-         * used rates.
+         * to the frame. Hence, don't use sampling for the highest currently
+         * used highest throughput or probability rate.
          */
-        if (sample_idx == mi->max_tp_rate[0] ||
-            sample_idx == mi->max_tp_rate[1] ||
-            sample_idx == mi->max_prob_rate)
+        if (sample_idx == mi->max_tp_rate[0] || sample_idx == mi->max_prob_rate)
                 return -1;
 
         /*
@@ -967,10 +976,10 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
          * if the link is working perfectly.
          */
 
-        cur_max_tp_streams = minstrel_mcs_groups[mi->max_tp_rate[0] /
+        cur_max_tp_streams = minstrel_mcs_groups[tp_rate1 /
                 MCS_GROUP_RATES].streams;
         sample_dur = minstrel_get_duration(sample_idx);
-        if (sample_dur >= minstrel_get_duration(mi->max_tp_rate[1]) &&
+        if (sample_dur >= minstrel_get_duration(tp_rate2) &&
             (cur_max_tp_streams - 1 <
              minstrel_mcs_groups[sample_group].streams ||
              sample_dur >= minstrel_get_duration(mi->max_prob_rate))) {