iwlwifi: mvm: don't let NDPs mess the packet tracking

We need to track the next packet that we will reclaim in
order to know when the Tx queues are empty. This is useful
when we open or tear down an A-MPDU session which requires
to switch queue.
The next packet being reclaimed is identified by its WiFi
sequence number and this is relevant only when we use QoS.
QoS NDPs do have a TID but have a meaningless sequence
number. The spec mandates the receiver to ignore the
sequence number in this case, allowing the transmitter to
put any sequence number. Our implementation leaves it 0.
When we reclaim a QoS NDP, we can't update the next_relcaim
counter since the sequence number of the QoS NDP itself is
invalid.
We used to update the next_reclaim based on the sequence
number of the QoS NDP which reset it to 1 (0 + 1) and
because of this, we never knew when the queue got empty.
This had to sad consequence to stuck the A-MPDU state
machine in a transient state.
To fix this, don't update next_reclaim when we reclaim
a QoS NDP.

Alesya saw this bug when testing u-APSD. Because the
A-MPDU state machine was stuck in EMPTYING_DELBA, we
updated mac80211 that we still have frames for that
station when it got back to sleep. mac80211 then wrongly
set the TIM bit in the beacon and requested to release
non-existent frames from the A-MPDU queue. This led to
a situation where the client was trying to poll frames
but we had no frames to send.
Reported-by: Alesya Shapira <alesya.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

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

@@ -7,6 +7,7 @@
  *
  * Copyright(c) 2012 - 2014 Intel Corporation. All rights reserved.
  * Copyright(c) 2013 - 2015 Intel Mobile Communications GmbH
+ * Copyright(c) 2016        Intel Deutschland GmbH
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of version 2 of the GNU General Public License as
@@ -963,6 +964,7 @@ static void iwl_mvm_rx_tx_cmd_single(struct iwl_mvm *mvm,
 	struct sk_buff_head skbs;
 	u8 skb_freed = 0;
 	u16 next_reclaimed, seq_ctl;
+	bool is_ndp = false;
 
 	__skb_queue_head_init(&skbs);
 
@@ -1016,6 +1018,20 @@ static void iwl_mvm_rx_tx_cmd_single(struct iwl_mvm *mvm,
 			seq_ctl = le16_to_cpu(hdr->seq_ctrl);
 		}
 
+		if (unlikely(!seq_ctl)) {
+			struct ieee80211_hdr *hdr = (void *)skb->data;
+
+			/*
+			 * If it is an NDP, we can't update next_reclaim since
+			 * its sequence control is 0. Note that for that same
+			 * reason, NDPs are never sent to A-MPDU'able queues
+			 * so that we can never have more than one freed frame
+			 * for a single Tx resonse (see WARN_ON below).
+			 */
+			if (ieee80211_is_qos_nullfunc(hdr->frame_control))
+				is_ndp = true;
+		}
+
 		/*
 		 * TODO: this is not accurate if we are freeing more than one
 		 * packet.
@@ -1079,9 +1095,16 @@ static void iwl_mvm_rx_tx_cmd_single(struct iwl_mvm *mvm,
 			bool send_eosp_ndp = false;
 
 			spin_lock_bh(&mvmsta->lock);
+			if (!is_ndp) {
 				tid_data->next_reclaimed = next_reclaimed;
-			IWL_DEBUG_TX_REPLY(mvm, "Next reclaimed packet:%d\n",
+				IWL_DEBUG_TX_REPLY(mvm,
+						   "Next reclaimed packet:%d\n",
 						   next_reclaimed);
+			} else {
+				IWL_DEBUG_TX_REPLY(mvm,
+						   "NDP - don't update next_reclaimed\n");
+			}
+
 			iwl_mvm_check_ratid_empty(mvm, sta, tid);
 
 			if (mvmsta->sleep_tx_count) {