[PATCH v1 2/4] wifi: ath11k: enable airtime queue limits

[Julius Bairaktaris <julius@bairaktaris.de>]

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. AQL is not untuned on this driver, it
is absent.

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 against one HE 160 MHz station at MCS 11, taken
from that station's own aql file while it received 280 Mbit/s: its BE queue
depth reaches 6356 us of the 12000 us default limit, and 2972 us when the
limit is lowered to 500/1000 us. 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 returns zero, no airtime is charged, and AQL stays inert, so this
cannot behave worse than the current code.

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.

An AQL enable was proposed once before [1] with a custom .wake_tx_queue and
a tx-completion driven push. That scheduler is not needed:
ieee80211_tx_dequeue() performs the check itself, so the generic handler is
already braked.

ath10k and mt76 set the same feature bit.

Link: https://lore.kernel.org/all/20230501130725.7171-1-quic_tamizhr@quicinc.com/ [1]
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@bairaktaris.de>
---
 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