ath9k: Change rate control to use legacy rate as last MRR

In congested network, having all rate reties at MCS rates
is failing to transmit the frame offenly. By the time reaching
the success rate set, the application gets timed out. One such
scenario is that authentication time out during 4-Way handshake.
This patch uses a legacy rate as last retry sequnce for
unaggregated frames or if the first selected rate's PER is ~80%
of max limit. And also observed from the tx status that the frame
was trasmitted successfully by using legacy rates.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/rc.c

@@ -603,7 +603,8 @@ static u8 ath_rc_setvalid_htrates(struct ath_rate_priv *ath_rc_priv,
 static u8 ath_rc_get_highest_rix(struct ath_softc *sc,
 			         struct ath_rate_priv *ath_rc_priv,
 				 const struct ath_rate_table *rate_table,
-				 int *is_probing)
+				 int *is_probing,
+				 bool legacy)
 {
 	u32 best_thruput, this_thruput, now_msec;
 	u8 rate, next_rate, best_rate, maxindex, minindex;
@@ -624,6 +625,8 @@ static u8 ath_rc_get_highest_rix(struct ath_softc *sc,
 		u8 per_thres;
 
 		rate = ath_rc_priv->valid_rate_index[index];
+		if (legacy && !(rate_table->info[rate].rate_flags & RC_LEGACY))
+			continue;
 		if (rate > ath_rc_priv->rate_max_phy)
 			continue;
 
@@ -767,7 +770,7 @@ static void ath_get_rate(void *priv, struct ieee80211_sta *sta, void *priv_sta,
 	struct ieee80211_tx_rate *rates = tx_info->control.rates;
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
 	__le16 fc = hdr->frame_control;
-	u8 try_per_rate, i = 0, rix;
+	u8 try_per_rate, i = 0, rix, high_rix;
 	int is_probe = 0;
 
 	if (rate_control_send_low(sta, priv_sta, txrc))
@@ -786,7 +789,9 @@ static void ath_get_rate(void *priv, struct ieee80211_sta *sta, void *priv_sta,
 	try_per_rate = 4;
 
 	rate_table = ath_rc_priv->rate_table;
-	rix = ath_rc_get_highest_rix(sc, ath_rc_priv, rate_table, &is_probe);
+	rix = ath_rc_get_highest_rix(sc, ath_rc_priv, rate_table,
+				     &is_probe, false);
+	high_rix = rix;
 
 	/*
 	 * If we're in HT mode and both us and our peer supports LDPC.
@@ -822,17 +827,32 @@ static void ath_get_rate(void *priv, struct ieee80211_sta *sta, void *priv_sta,
 	}
 
 	/* Fill in the other rates for multirate retry */
-	for ( ; i < 4; i++) {
-		/* Use twice the number of tries for the last MRR segment. */
-		if (i + 1 == 4)
-			try_per_rate = 8;
+	for ( ; i < 3; i++) {
 
 		ath_rc_get_lower_rix(rate_table, ath_rc_priv, rix, &rix);
 		/* All other rates in the series have RTS enabled */
 		ath_rc_rate_set_series(rate_table, &rates[i], txrc,
 				       try_per_rate, rix, 1);
 	}
 
+	/* Use twice the number of tries for the last MRR segment. */
+	try_per_rate = 8;
+
+	/*
+	 * Use a legacy rate as last retry to ensure that the frame
+	 * is tried in both MCS and legacy rates.
+	 */
+	if ((rates[2].flags & IEEE80211_TX_RC_MCS) &&
+	    (!(tx_info->flags & IEEE80211_TX_CTL_AMPDU) ||
+	    (ath_rc_priv->per[high_rix] > 45)))
+		rix = ath_rc_get_highest_rix(sc, ath_rc_priv, rate_table,
+				&is_probe, true);
+	else
+		ath_rc_get_lower_rix(rate_table, ath_rc_priv, rix, &rix);
+
+	/* All other rates in the series have RTS enabled */
+	ath_rc_rate_set_series(rate_table, &rates[i], txrc,
+			       try_per_rate, rix, 1);
 	/*
 	 * NB:Change rate series to enable aggregation when operating
 	 * at lower MCS rates. When first rate in series is MCS2