wifi: iwlwifi: mld: stop TX during firmware restart

When iwlwifi firmware crashes (e.g., NMI_INTERRUPT_UNKNOWN on Intel
BE201/Wi-Fi 7), iwl_mld_nic_error() sets mld->fw_status.in_hw_restart
to true. However, iwl_mld_tx_from_txq() does not check this flag before
dequeuing frames from mac80211 and pushing them to the transport layer.

Since the firmware is dead, iwl_trans_tx() returns -EIO for each frame,
which then gets freed immediately. Under high-throughput conditions
(e.g., Tailscale UDP traffic or active SSH sessions), this creates a
tight dequeue-send-fail-free loop that wastes CPU cycles and generates
rapid skb allocation churn, leading to memory pressure from slab
fragmentation.

The RX path already has this guard (iwl_mld_rx_mpdu checks
in_hw_restart at rx.c:1906), and so does the TXQ allocation worker
(iwl_mld_add_txqs_wk at tx.c:156). Add the same guard to
iwl_mld_tx_from_txq() to stop all TX during firmware restart.

Frames left in mac80211's TXQs are naturally drained after restart
completes, when queue reallocation triggers iwl_mld_tx_from_txq()
via iwl_mld_add_txq_list(), or when new upper-layer traffic invokes
wake_tx_queue.

Tested on ASUS Zenbook 14 UX3405CA with Intel BE201 (Wi-Fi 7) on
kernel 6.19.5 where the firmware crashes approximately every 10-15
minutes under Tailscale traffic.

Fixes: d1e879e ("wifi: iwlwifi: add iwlmld sub-driver")
Cc: stable@vger.kernel.org
Signed-off-by: Sheroz Juraev <goodmartiandev@gmail.com>
Link: https://patch.msgid.link/20260315081221.2678478-1-goodmartiandev@gmail.com
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>

Author: goodmartian

drivers/net/wireless/intel/iwlwifi/mld/tx.c

@@ -973,6 +973,16 @@ void iwl_mld_tx_from_txq(struct iwl_mld *mld, struct ieee80211_txq *txq)
 	struct sk_buff *skb = NULL;
 	u8 zero_addr[ETH_ALEN] = {};
 
+	/*
+	 * Don't transmit during firmware restart. The firmware is dead,
+	 * so iwl_trans_tx() would return -EIO for each frame. Avoid the
+	 * overhead of dequeuing from mac80211 only to immediately free
+	 * the skbs, and the potential memory pressure from rapid skb
+	 * allocation churn during high-throughput restart scenarios.
+	 */
+	if (unlikely(mld->fw_status.in_hw_restart))
+		return;
+
 	/*
 	 * No need for threads to be pending here, they can leave the first
 	 * taker all the work.