linux-kernel - [RFC v2] mac80211: minstrel_ht: do not set RTS/CTS flag for fallback rates

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Date:   Tue, 16 Nov 2021 22:28:28 +0100
From:   Peter Seiderer <ps.report@....net>
To:     linux-wireless@...r.kernel.org
Cc:     Johannes Berg <johannes@...solutions.net>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Felix Fietkau <nbd@....name>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: [RFC v2] mac80211: minstrel_ht: do not set RTS/CTS flag for fallback rates

Despite the 'RTS thr:off' setting a wireshark trace of IBSS
traffic with HT40 mode enabled between two ath9k cards revealed
some RTS/CTS traffic.

Debug and code analysis showed that most places setting
IEEE80211_TX_RC_USE_RTS_CTS respect the RTS strategy by
evaluating rts_threshold, e.g. net/mac80211/tx.c:

 698         /* set up RTS protection if desired */
 699         if (len > tx->local->hw.wiphy->rts_threshold) {
 700                 txrc.rts = true;
 701         }
 702
 703         info->control.use_rts = txrc.rts;

or drivers/net/wireless/ath/ath9k/xmit.c

1238                 /*
1239                  * Handle RTS threshold for unaggregated HT frames.
1240                  */
1241                 if (bf_isampdu(bf) && !bf_isaggr(bf) &&
1242                     (rates[i].flags & IEEE80211_TX_RC_MCS) &&
1243                     unlikely(rts_thresh != (u32) -1)) {
1244                         if (!rts_thresh || (len > rts_thresh))
1245                                 rts = true;
1246                 }

The only place setting IEEE80211_TX_RC_USE_RTS_CTS unconditionally
was found in net/mac80211/rc80211_minstrel_ht.c.

As the use_rts value is only calculated after hitting the minstrel_ht code
preferre to not set IEEE80211_TX_RC_USE_RTS_CTS (and overruling the
RTS threshold setting) for the fallback rates case.

Signed-off-by: Peter Seiderer <ps.report@....net>
---
Changes v1 -> v2:
  - change from 'respect RTS threshold setting' to 'do not set RTS/CTS
    flag for fallback rates' (see commit message for reasoning)
---
 net/mac80211/rc80211_minstrel_ht.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index 72b44d4c42d0..f151acbe7bf5 100644
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -1355,11 +1355,9 @@ minstrel_ht_set_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
 
 	/* enable RTS/CTS if needed:
 	 *  - if station is in dynamic SMPS (and streams > 1)
-	 *  - for fallback rates, to increase chances of getting through
 	 */
-	if (offset > 0 ||
-	    (mi->sta->smps_mode == IEEE80211_SMPS_DYNAMIC &&
-	     group->streams > 1)) {
+	if (mi->sta->smps_mode == IEEE80211_SMPS_DYNAMIC &&
+	    group->streams > 1) {
 		ratetbl->rate[offset].count = ratetbl->rate[offset].count_rts;
 		flags |= IEEE80211_TX_RC_USE_RTS_CTS;
 	}
-- 
2.33.1