mac80211: minstrel: store probability variance instead of standard deviation

This avoids the costly int_sqrt calls in the statistics update and moves
it to the debugfs code instead.
This also fixes an overflow in the previous standard deviation
calculation.

Signed-off-by: Thomas Huehn <thomas.huehn@evernet-eg.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 14 insertions, 11 deletions

diff --git a/net/mac80211/rc80211_minstrel.h b/net/mac80211/rc80211_minstrel.h
index 03716fddaf24..ea86e6784154 100644
--- a/net/mac80211/rc80211_minstrel.h
+++ b/net/mac80211/rc80211_minstrel.h
@@ -36,21 +36,16 @@ minstrel_ewma(int old, int new, int weight)
 }
 
 /*
- * Perform EWMSD (Exponentially Weighted Moving Standard Deviation) calculation
+ * Perform EWMV (Exponentially Weighted Moving Variance) calculation
  */
 static inline int
-minstrel_ewmsd(int old_ewmsd, int cur_prob, int prob_ewma, int weight)
+minstrel_ewmv(int old_ewmv, int cur_prob, int prob_ewma, int weight)
 {
-	int diff, incr, tmp_var;
+	int diff, incr;
 
-	/* calculate exponential weighted moving variance */
-	diff = MINSTREL_TRUNC((cur_prob - prob_ewma) * 1000000);
+	diff = cur_prob - prob_ewma;
 	incr = (EWMA_DIV - weight) * diff / EWMA_DIV;
-	tmp_var = old_ewmsd * old_ewmsd;
-	tmp_var = weight * (tmp_var + diff * incr / 1000000) / EWMA_DIV;
-
-	/* return standard deviation */
-	return (u16) int_sqrt(tmp_var);
+	return weight * (old_ewmv + MINSTREL_TRUNC(diff * incr)) / EWMA_DIV;
 }
 
 struct minstrel_rate_stats {
@@ -65,7 +60,7 @@ struct minstrel_rate_stats {
 	 *  prob_ewma - exponential weighted moving average of prob
 	 *  prob_ewmsd - exp. weighted moving standard deviation of prob */
 	unsigned int prob_ewma;
-	u16 prob_ewmsd;
+	u16 prob_ewmv;
 
 	/* maximum retry counts */
 	u8 retry_count;
@@ -151,6 +146,14 @@ struct minstrel_debugfs_info {
 	char buf[];
 };
 
+/* Get EWMSD (Exponentially Weighted Moving Standard Deviation) * 10 */
+static inline int
+minstrel_get_ewmsd10(struct minstrel_rate_stats *mrs)
+{
+	unsigned int ewmv = mrs->prob_ewmv;
+	return int_sqrt(MINSTREL_TRUNC(ewmv * 1000 * 1000));
+}
+
 extern const struct rate_control_ops mac80211_minstrel;
 void minstrel_add_sta_debugfs(void *priv, void *priv_sta, struct dentry *dir);
 void minstrel_remove_sta_debugfs(void *priv, void *priv_sta);