wifi: mac80211: don't WARN for late channel/color switch

There's really no value in the WARN stack trace etc., the reason
for this happening isn't directly related to the calling function
anyway. Also, syzbot has been observing it constantly, and there's
no way we can resolve it there - those systems are just slow.

Instead print an error message (once) and add a comment about what
really causes this message.

Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Link: https://patch.msgid.link/20250617104902.146e10919be1.I85f352ca4a2dce6f556e5ff45ceaa5f3769cb5ce@changeid
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/debug.h

···
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Portions
- * Copyright (C) 2022 - 2024 Intel Corporation
+ * Copyright (C) 2022 - 2025 Intel Corporation
  */
 #ifndef __MAC80211_DEBUG_H
 #define __MAC80211_DEBUG_H
+#include <linux/once_lite.h>
 #include <net/cfg80211.h>
 
 #ifdef CONFIG_MAC80211_OCB_DEBUG
···
 		else							\
 			_sdata_err((link)->sdata, fmt, ##__VA_ARGS__);	\
 	} while (0)
+#define link_err_once(link, fmt, ...)					\
+	DO_ONCE_LITE(link_err, link, fmt, ##__VA_ARGS__)
 #define link_id_info(sdata, link_id, fmt, ...)				\
 	do {								\
 		if (ieee80211_vif_is_mld(&sdata->vif))			\

net/mac80211/tx.c

···
  * Copyright 2006-2007	Jiri Benc <[e-mail]>
  * Copyright 2007	Johannes Berg <[e-mail]>
  * Copyright 2013-2014  Intel Mobile Communications GmbH
- * Copyright (C) 2018-2024 Intel Corporation
+ * Copyright (C) 2018-2025 Intel Corporation
  *
  * Transmit and frame generation functions.
  */
···
 	}
 }
 
-static u8 __ieee80211_beacon_update_cntdwn(struct beacon_data *beacon)
+static u8 __ieee80211_beacon_update_cntdwn(struct ieee80211_link_data *link,
+					   struct beacon_data *beacon)
 {
-	beacon->cntdwn_current_counter--;
+	if (beacon->cntdwn_current_counter == 1) {
+		/*
+		 * Channel switch handling is done by a worker thread while
+		 * beacons get pulled from hardware timers. It's therefore
+		 * possible that software threads are slow enough to not be
+		 * able to complete CSA handling in a single beacon interval,
+		 * in which case we get here. There isn't much to do about
+		 * it, other than letting the user know that the AP isn't
+		 * behaving correctly.
+		 */
+		link_err_once(link,
+			      "beacon TX faster than countdown (channel/color switch) completion\n");
+		return 0;
+	}
 
-	/* the counter should never reach 0 */
-	WARN_ON_ONCE(!beacon->cntdwn_current_counter);
+	beacon->cntdwn_current_counter--;
 
 	return beacon->cntdwn_current_counter;
 }
···
 	if (!beacon)
 		goto unlock;
 
-	count = __ieee80211_beacon_update_cntdwn(beacon);
+	count = __ieee80211_beacon_update_cntdwn(link, beacon);
 
 unlock:
 	rcu_read_unlock();
···
 
 		if (beacon->cntdwn_counter_offsets[0]) {
 			if (!is_template)
-				__ieee80211_beacon_update_cntdwn(beacon);
+				__ieee80211_beacon_update_cntdwn(link, beacon);
 
 			ieee80211_set_beacon_cntdwn(sdata, beacon, link);
 		}
···
 				 * for now we leave it consistent with overall
 				 * mac80211's behavior.
 				 */
-				__ieee80211_beacon_update_cntdwn(beacon);
+				__ieee80211_beacon_update_cntdwn(link, beacon);
 
 			ieee80211_set_beacon_cntdwn(sdata, beacon, link);
 		}