mac80211: minstrel_ht: fix rounding error in throughput calculation

On lower data rates, the throughput calculation has a significant rounding
error, causing rates like 48M and 54M OFDM to share the same throughput
value with >= 90% success probablity.

This is because the result of the division (prob_avg * 1000) / nsecs
is really small (8 in this example).

Improve accuracy by moving over some zeroes, making better use of the full
range of u32 before the division.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-10-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index 782b4668a3da07a270321159d36e045ad0093ab3..e35948f4e1bf94e619fbee6d9f71c7a0b50315e2 100644 (file)
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -445,10 +445,9 @@ minstrel_ht_get_tp_avg(struct minstrel_ht_sta *mi, int group, int rate,
         * (prob is scaled - see MINSTREL_FRAC above)
         */
        if (prob_avg > MINSTREL_FRAC(90, 100))
-               return MINSTREL_TRUNC(100000 * ((MINSTREL_FRAC(90, 100) * 1000)
-                                                                     / nsecs));
-       else
-               return MINSTREL_TRUNC(100000 * ((prob_avg * 1000) / nsecs));
+               prob_avg = MINSTREL_FRAC(90, 100);
+
+       return MINSTREL_TRUNC(100 * ((prob_avg * 1000000) / nsecs));
 }
 
 /*