[PATCH ath-next v4 2/9] wifi: ath11k: enable airtime queue limits

[Julius Bairaktaris]

ath11k does not advertise NL80211_EXT_FEATURE_AQL, and every user of AQL in
mac80211 is gated on that bit: the airtime charge in
ieee80211_tx_dequeue(), ieee80211_txq_airtime_check(),
ieee80211_sta_update_pending_airtime(), and the per-station aql file in
debugfs, which is not even created.

That matters here more than it would elsewhere, because AQL is the only
brake. ath11k uses ieee80211_handle_wake_tx_queue(), whose
wake_tx_push_queue() drains a selected txq whole into the driver, and the
check at the top of ieee80211_tx_dequeue() is what stops that drain.
Without the feature bit nothing does, so mac80211 holds no backlog and
every queued byte of a saturated download sits in the hardware TX rings,
where there is no AQM and no flow separation.

Measured on an IPQ8074 AP with a TCP download forwarded from a wired
host to one 1x1 VHT80 station at 101 Mbit/s, sampled once a second from
that station's own aql file: BE pending airtime sits at the 12000 us
default high limit, 12024 us at the median, and at 1008 to 1024 us when
the limit is lowered to 500/1000 us. Pending airtime stands a frame
above the limit it is tested against because
ieee80211_txq_airtime_check() admits a frame while pending is still
below it. Without this patch the file does not exist and the limits are
never consulted.

The rate the estimator needs is already there: ath11k keeps a per-station
rate_info in arsta->last_txrate and passes it to ieee80211_tx_status_ext(),
which stores it as tx_stats.last_rate_info, and that is what
ieee80211_calc_expected_tx_airtime() reads. Where no rate is known yet the
estimator falls back to its 4 us floor, so a station's queue is charged the
minimum until the first HTT PPDU stats arrive and the per-AC limits of 5000
and 12000 us stay loose over that window. The current code charges nothing
and consults no limit at all, so that window changes nothing. A rate that
has gone stale low overcharges a station that has since sped up, bounded
by the same lag.

The estimate is only as fresh as arsta->last_txrate, which is updated from
the HTT PPDU stats when a descriptor is evicted from ar->ppdu_stats_info,
so the rate behind it can lag a rate change by up to
HTT_PPDU_DESC_MAX_DEPTH PPDUs.

ieee80211_sta_update_pending_airtime() returns the airtime but does not
reschedule the txq, and ath11k has no completion-side push, so a queue held
back by AQL is re-poked only by the next enqueue or queue wake.

The bit alone brakes the queue but does not refill it: the check at the top
of ieee80211_tx_dequeue() stops the drain, and nothing restarts it until
mac80211 next calls the wake_tx_queue callback. Later patches in this
series give the driver a scheduling round and run it from tx completion,
which is what makes a binding limit affordable.

ath10k and mt76 set the same feature bit.

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.9.0.1-02146-QCAHKSWPL_SILICONZ-1

Assisted-by: Claude:claude-opus-5
Signed-off-by: Julius Bairaktaris <julius@xxxxxxxxxxxxxx>
---
 drivers/net/wireless/ath/ath11k/mac.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/wireless/ath/ath11k/mac.c b/drivers/net/wireless/ath/ath11k/mac.c
index 2d55cdc4d165..38f641bbc53c 100644
--- a/drivers/net/wireless/ath/ath11k/mac.c
+++ b/drivers/net/wireless/ath/ath11k/mac.c
@@ -10592,6 +10592,7 @@ static int __ath11k_mac_register(struct ath11k *ar)
 	ar->hw->sta_data_size = sizeof(struct ath11k_sta);
 
 	wiphy_ext_feature_set(ar->hw->wiphy, NL80211_EXT_FEATURE_CQM_RSSI_LIST);
+	wiphy_ext_feature_set(ar->hw->wiphy, NL80211_EXT_FEATURE_AQL);
 	wiphy_ext_feature_set(ar->hw->wiphy, NL80211_EXT_FEATURE_STA_TX_PWR);
 	if (test_bit(WMI_TLV_SERVICE_BSS_COLOR_OFFLOAD,
 		     ar->ab->wmi_ab.svc_map)) {
-- 
2.53.0