mac80211/minstrel_ht: improve accuracy of throughput metric at high data rates

At high data rates the average frame transmission durations are small
enough for rounding errors to matter, sometimes causing minstrel to use
slightly lower transmit rates than necessary.
To fix this, change the unit of the duration value to nanoseconds
instead of microseconds, and reorder the multiplications/divisions when
calculating the throughput metric so that they don't overflow or
truncate prematurely.
At 2-stream HT40 this makes TCP throughput a bit more stable.

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

net/mac80211/rc80211_minstrel_ht.c

@@ -26,11 +26,11 @@
 /* Number of symbols for a packet with (bps) bits per symbol */
 #define MCS_NSYMS(bps) ((MCS_NBITS + (bps) - 1) / (bps))
 
-/* Transmission time for a packet containing (syms) symbols */
+/* Transmission time (nanoseconds) for a packet containing (syms) symbols */
 #define MCS_SYMBOL_TIME(sgi, syms)					\
 	(sgi ?								\
-	  ((syms) * 18 + 4) / 5 :	/* syms * 3.6 us */		\
-	  (syms) << 2			/* syms * 4 us */		\
+	  ((syms) * 18000 + 4000) / 5 :	/* syms * 3.6 us */		\
+	  ((syms) * 1000) << 2		/* syms * 4 us */		\
 	)
 
 /* Transmit duration for the raw data part of an average sized packet */
@@ -64,9 +64,9 @@
 }
 
 #define CCK_DURATION(_bitrate, _short, _len)		\
-	(10 /* SIFS */ +				\
+	(1000 * (10 /* SIFS */ +			\
 	 (_short ? 72 + 24 : 144 + 48 ) +		\
-	 (8 * (_len + 4) * 10) / (_bitrate))
+	 (8 * (_len + 4) * 10) / (_bitrate)))
 
 #define CCK_ACK_DURATION(_bitrate, _short)			\
 	(CCK_DURATION((_bitrate > 10 ? 20 : 10), false, 60) +	\
@@ -211,7 +211,8 @@ static void
 minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
 {
 	struct minstrel_rate_stats *mr;
-	unsigned int usecs = 0;
+	unsigned int nsecs = 0;
+	unsigned int tp;
 
 	mr = &mi->groups[group].rates[rate];
 
@@ -221,10 +222,12 @@ minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
 	}
 
 	if (group != MINSTREL_CCK_GROUP)
-		usecs = mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);
+		nsecs = 1000 * mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);
 
-	usecs += minstrel_mcs_groups[group].duration[rate];
-	mr->cur_tp = MINSTREL_TRUNC((1000000 / usecs) * mr->probability);
+	nsecs += minstrel_mcs_groups[group].duration[rate];
+	tp = 1000000 * ((mr->probability * 1000) / nsecs);
+
+	mr->cur_tp = MINSTREL_TRUNC(tp);
 }
 
 /*
@@ -536,7 +539,7 @@ minstrel_calc_retransmit(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
 	mr->retry_updated = true;
 
 	group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
-	tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len;
+	tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len / 1000;
 
 	/* Contention time for first 2 tries */
 	ctime = (t_slot * cw) >> 1;