wcn36xx: implement flush op to speed up connected scan

Without ieee80211_ops->flush implemented to empty HW queues, mac80211 will
do a 100ms dead wait after stopping SW queues, before leaving the operating
channel to resume a software connected scan[1].
(see ieee80211_scan_state_resume)

This wait is correctly included in the calculation for whether or not
we've exceeded max off-channel time, as it occurs after sending the null
frame with PS bit set. Thus, with 125 ms max off-channel time we only
have 25 ms of scan time, which technically isn't even enough to scan one
channel (although mac80211 always scans at least one channel per off-
channel window).

Moreover, for passive probes we end up spending at least 100 ms + 111 ms
(IEEE80211_PASSIVE_CHANNEL_TIME) "off-channel"[2], which exceeds the listen
interval of 200 ms that we provide in our association request frame. That's
technically out-of-spec.

[1]: Until recently, wcn36xx performed software (rather than FW-offloaded)
scanning when 5GHz channels are requested. This apparent limitation is now
resolved -- see commit 1395f8a6a4d5 ("wcn36xx: Enable hardware scan offload
for 5Ghz band").
[2]: in quotes because about 100 ms of it is still on-channel but with PS
set
Signed-off-by: Benjamin Li <benl@squareup.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211027170306.555535-3-benl@squareup.com

drivers/net/wireless/ath/wcn36xx/dxe.c

@@ -834,6 +834,53 @@ int wcn36xx_dxe_tx_frame(struct wcn36xx *wcn,
 	return ret;
 }
 
+static bool _wcn36xx_dxe_tx_channel_is_empty(struct wcn36xx_dxe_ch *ch)
+{
+	unsigned long flags;
+	struct wcn36xx_dxe_ctl *ctl_bd_start, *ctl_skb_start;
+	struct wcn36xx_dxe_ctl *ctl_bd, *ctl_skb;
+	bool ret = true;
+
+	spin_lock_irqsave(&ch->lock, flags);
+
+	/* Loop through ring buffer looking for nonempty entries. */
+	ctl_bd_start = ch->head_blk_ctl;
+	ctl_bd = ctl_bd_start;
+	ctl_skb_start = ctl_bd_start->next;
+	ctl_skb = ctl_skb_start;
+	do {
+		if (ctl_skb->skb) {
+			ret = false;
+			goto unlock;
+		}
+		ctl_bd = ctl_skb->next;
+		ctl_skb = ctl_bd->next;
+	} while (ctl_skb != ctl_skb_start);
+
+unlock:
+	spin_unlock_irqrestore(&ch->lock, flags);
+	return ret;
+}
+
+int wcn36xx_dxe_tx_flush(struct wcn36xx *wcn)
+{
+	int i = 0;
+
+	/* Called with mac80211 queues stopped. Wait for empty HW queues. */
+	do {
+		if (_wcn36xx_dxe_tx_channel_is_empty(&wcn->dxe_tx_l_ch) &&
+		    _wcn36xx_dxe_tx_channel_is_empty(&wcn->dxe_tx_h_ch)) {
+			return 0;
+		}
+		/* This ieee80211_ops callback is specifically allowed to
+		 * sleep.
+		 */
+		usleep_range(1000, 1100);
+	} while (++i < 100);
+
+	return -EBUSY;
+}
+
 int wcn36xx_dxe_init(struct wcn36xx *wcn)
 {
 	int reg_data = 0, ret;

drivers/net/wireless/ath/wcn36xx/dxe.h

@@ -466,5 +466,6 @@ int wcn36xx_dxe_tx_frame(struct wcn36xx *wcn,
 			 struct wcn36xx_tx_bd *bd,
 			 struct sk_buff *skb,
 			 bool is_low);
+int wcn36xx_dxe_tx_flush(struct wcn36xx *wcn);
 void wcn36xx_dxe_tx_ack_ind(struct wcn36xx *wcn, u32 status);
 #endif	/* _DXE_H_ */

drivers/net/wireless/ath/wcn36xx/main.c

@@ -1281,6 +1281,16 @@ static void wcn36xx_ipv6_addr_change(struct ieee80211_hw *hw,
 }
 #endif
 
+static void wcn36xx_flush(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
+			  u32 queues, bool drop)
+{
+	struct wcn36xx *wcn = hw->priv;
+
+	if (wcn36xx_dxe_tx_flush(wcn)) {
+		wcn36xx_err("Failed to flush hardware tx queues\n");
+	}
+}
+
 static const struct ieee80211_ops wcn36xx_ops = {
 	.start			= wcn36xx_start,
 	.stop			= wcn36xx_stop,
@@ -1308,6 +1318,7 @@ static const struct ieee80211_ops wcn36xx_ops = {
 #if IS_ENABLED(CONFIG_IPV6)
 	.ipv6_addr_change	= wcn36xx_ipv6_addr_change,
 #endif
+	.flush			= wcn36xx_flush,
 
 	CFG80211_TESTMODE_CMD(wcn36xx_tm_cmd)
 };