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From:   Felix Fietkau <nbd@nbd.name>
To:     linux-wireless@vger.kernel.org
Cc:     johannes@sipsolutions.net
Subject: [PATCH v3 9/9] mac80211: minstrel_ht: fix rounding error in throughput calculation
Date:   Fri, 15 Jan 2021 13:02:42 +0100
Message-Id: <20210115120242.89616-10-nbd@nbd.name>
In-Reply-To: <20210115120242.89616-1-nbd@nbd.name>
References: <20210115120242.89616-1-nbd@nbd.name>
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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>
---
 net/mac80211/rc80211_minstrel_ht.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index 6496aa39f60b..9e010c8dd063 100644
--- 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));
 }
 
 /*
-- 
2.28.0