[RFC PATCH] wifi: ath9k: clear destination mask for non-aggregate frames

[Tomas Tonhauser]

Frames to stations without an A-MPDU session (e.g. non-WMM/non-HT
clients) use the non-aggregate path. If the hardware returns such a
frame with ATH9K_TXERR_FILT, ath_tx_complete_buf() reports it to
mac80211 as IEEE80211_TX_STAT_TX_FILTERED. For a station that is not
in powersave, ieee80211_handle_filtered_frame() retries it once and
drops it if it is filtered again.

The aggregate path re-queues filtered subframes on tid->retry_q and
sets tid->clear_ps_filter, so they are retransmitted until they get
through.

The result is downlink loss to awake non-WMM stations, while WMM/HT
stations on the same radio are unaffected. On a QCA9563 AP with
ESP8266 clients this caused 20-45% ping loss, with two TXERR_FILT
completions per lost frame and no XRETRY failures.

Set CLRDMASK on every non-aggregate frame. This restores the
behaviour from before commit 5519541d5a5f ("ath9k: fix powersave
frame filtering/buffering in AP mode") for the non-aggregate path
only. The trade-off is that non-aggregate frames already queued in
hardware when a station enters powersave may be sent to the sleeping
station instead of being filtered.

Assisted-by: LLM
Signed-off-by: Tomas Tonhauser <t.tonhauser@xxxxxxxx>
---
RFC: is this direction acceptable, or is one of the alternatives
below preferred?

Setup: TP-Link EAP225 v3 (QCA9563, 2.4 GHz), OpenWrt 25.12.5,
backports 6.18.26. Clients: ESP8266 running Tasmota, modem sleep
disabled, associated without WMM (legacy rates).

Ping from the AP to one client, 30 packets, 14 lost:

  station "tx failed":             +14
  station "tx retries":            +2
  ath9k/xmit BE "TXERR Filtered":  +27
  ath9k/xmit BE "MPDUs XRetried":  +0

A second run: 12 lost, Filtered +28, XRetried +0.

Ruled out:
 - Client powersave: PS_STA never set in debugfs station flags, and a
   monitor capture showed no client frames with the PM bit set.
 - airtime_flags=0, aql_enable=0, nohwcrypt=1: no change.
 - RF: signal -50..-64 dBm; a WMM/HT client on the same radio has
   0 tx failed out of ~10k frames; the same ESP8266 hardware behind
   an ath10k AP has no loss.

With the patch: 500 pings to each of five ESP8266 clients, 0 lost.

Open question: what sets the destination mask for stations that are
awake and never hit XRETRY? 5519541d5a5f removed an unconditional
CLRDMASK commented "needed for crypto errors", so the hardware may
set it in other cases too.

Alternatives:
 1. Set CLRDMASK only when the station is not sleeping.
 2. Re-queue filtered non-aggregate frames in ath9k for awake
    stations, like the aggregate path does.
 3. Allow more than one software retry in
    ieee80211_handle_filtered_frame() for awake stations.

Related: 26a64259b702 ("ath9k: improve PS filter clearing and retry
counting for A-MPDU"), 315c457ff123 ("ath9k: improve powersave
filter handling").

The driver analysis and write-up were done with help from an LLM.
Tests / measurements were done by me.

 drivers/net/wireless/ath/ath9k/xmit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/ath/ath9k/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 3413bb4906d4..913aa280ae2a 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -1432,7 +1432,7 @@ static void ath_tx_fill_desc(struct ath_softc *sc, struct ath_buf *bf,
 			if (!sc->tx99_state)
 				info.flags = ATH9K_TXDESC_INTREQ;
 			if ((tx_info->flags & IEEE80211_TX_CTL_CLEAR_PS_FILT) ||
-			    txq == sc->tx.uapsdq)
+			    txq == sc->tx.uapsdq || !aggr)
 				info.flags |= ATH9K_TXDESC_CLRDMASK;
 
 			if (tx_info->flags & IEEE80211_TX_CTL_NO_ACK)
-- 
2.43.0