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

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Date:   Fri, 26 Nov 2021 15:25:58 +0100
From:   Peter Seiderer <ps.report@....net>
To:     Felix Fietkau <nbd@....name>
Cc:     linux-wireless@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [RFC v2] mac80211: minstrel_ht: do not set RTS/CTS flag for
 fallback rates

Hello Felix,

On Fri, 26 Nov 2021 13:47:07 +0100, Felix Fietkau <nbd@....name> wrote:

> On 2021-11-16 22:28, Peter Seiderer wrote:
> > 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.
> The idea behind the this part of minstrel_ht code is to avoid the
> overhead of RTS/CTS for transmissions using the primary rate and to
> increase the reliability of retransmissions by adding it for fallback
> rates. This is completely unrelated to the RTS threshold.

How does it avoid RTS/CTS (if it is set independent by RTS threshold
evaluation mac80211 and/or hardware driver)?

>
> If you don't want this behavior, I'm fine with adding a way to
> explicitly disable it. However, I do think leaving it on by default
> makes sense.

I expected this (as otherwise the flag setting would not be there) ;-)

Any hint how to implement an additional RTS/CTS on/off feature despite
the RTS threshold one for use explicit by minstrel_ht? Configure option,
module option, ...?

Regards,
Peter

>
> - Felix