iwlwifi: pcie: buffer packets to avoid overflowing Tx queues

When the Tx queues are full above a threshold, we
immediately stop the mac80211's queue to stop getting new
packets. This worked until TSO was enabled.
With TSO, one single packet from mac80211 can use many
descriptors since a large send needs to be split into
several segments.
This means that stopping mac80211's queues is not enough
and we also need to ensure that we don't overflow the Tx
queues with one single packet from mac80211.
Add code to transport layer to do just that. Stop
mac80211's queue as soon as the queue is full above the
same threshold as before, and keep pushing the current
packet along with its segments on the queue, but check
that we don't overflow. If that would happen, buffer the
segments, and send them when there is room in the Tx queue
again. Of course, we first need to send the buffered
segments and only then, wake up mac80211's queues.
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/intel/iwlwifi/pcie/internal.h

@@ -280,6 +280,7 @@ struct iwl_txq {
 	bool ampdu;
 	bool block;
 	unsigned long wd_timeout;
+	struct sk_buff_head overflow_q;
 };
 
 static inline dma_addr_t

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

@@ -571,6 +571,7 @@ static int iwl_pcie_txq_init(struct iwl_trans *trans, struct iwl_txq *txq,
 		return ret;
 
 	spin_lock_init(&txq->lock);
+	__skb_queue_head_init(&txq->overflow_q);
 
 	/*
 	 * Tell nic where to find circular buffer of Tx Frame Descriptors for
@@ -621,6 +622,13 @@ static void iwl_pcie_txq_unmap(struct iwl_trans *trans, int txq_id)
 		q->read_ptr = iwl_queue_inc_wrap(q->read_ptr);
 	}
 	txq->active = false;
+
+	while (!skb_queue_empty(&txq->overflow_q)) {
+		struct sk_buff *skb = __skb_dequeue(&txq->overflow_q);
+
+		iwl_op_mode_free_skb(trans->op_mode, skb);
+	}
+
 	spin_unlock_bh(&txq->lock);
 
 	/* just in case - this queue may have been stopped */
@@ -1052,8 +1060,41 @@ void iwl_trans_pcie_reclaim(struct iwl_trans *trans, int txq_id, int ssn,
 
 	iwl_pcie_txq_progress(txq);
 
+	if (iwl_queue_space(&txq->q) > txq->q.low_mark &&
+	    test_bit(txq_id, trans_pcie->queue_stopped)) {
+		struct sk_buff_head skbs;
+
+		__skb_queue_head_init(&skbs);
+		skb_queue_splice_init(&txq->overflow_q, &skbs);
+
+		/*
+		 * This is tricky: we are in reclaim path which is non
+		 * re-entrant, so noone will try to take the access the
+		 * txq data from that path. We stopped tx, so we can't
+		 * have tx as well. Bottom line, we can unlock and re-lock
+		 * later.
+		 */
+		spin_unlock_bh(&txq->lock);
+
+		while (!skb_queue_empty(&skbs)) {
+			struct sk_buff *skb = __skb_dequeue(&skbs);
+			struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+			u8 dev_cmd_idx = IWL_TRANS_FIRST_DRIVER_DATA + 1;
+			struct iwl_device_cmd *dev_cmd =
+				info->driver_data[dev_cmd_idx];
+
+			/*
+			 * Note that we can very well be overflowing again.
+			 * In that case, iwl_queue_space will be small again
+			 * and we won't wake mac80211's queue.
+			 */
+			iwl_trans_pcie_tx(trans, skb, dev_cmd, txq_id);
+		}
+		spin_lock_bh(&txq->lock);
+
 		if (iwl_queue_space(&txq->q) > txq->q.low_mark)
 			iwl_wake_queue(trans, txq);
+	}
 
 	if (q->read_ptr == q->write_ptr) {
 		IWL_DEBUG_RPM(trans, "Q %d - last tx reclaimed\n", q->id);
@@ -2161,6 +2202,8 @@ int iwl_trans_pcie_tx(struct iwl_trans *trans, struct sk_buff *skb,
 
 		csum = skb_checksum(skb, offs, skb->len - offs, 0);
 		*(__sum16 *)(skb->data + csum_offs) = csum_fold(csum);
+
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
 	}
 
 	if (skb_is_nonlinear(skb) &&
@@ -2177,6 +2220,22 @@ int iwl_trans_pcie_tx(struct iwl_trans *trans, struct sk_buff *skb,
 
 	spin_lock(&txq->lock);
 
+	if (iwl_queue_space(q) < q->high_mark) {
+		iwl_stop_queue(trans, txq);
+
+		/* don't put the packet on the ring, if there is no room */
+		if (unlikely(iwl_queue_space(q) < 3)) {
+			struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+
+			info->driver_data[IWL_TRANS_FIRST_DRIVER_DATA + 1] =
+				dev_cmd;
+			__skb_queue_tail(&txq->overflow_q, skb);
+
+			spin_unlock(&txq->lock);
+			return 0;
+		}
+	}
+
 	/* In AGG mode, the index in the ring must correspond to the WiFi
 	 * sequence number. This is a HW requirements to help the SCD to parse
 	 * the BA.
@@ -2281,12 +2340,6 @@ int iwl_trans_pcie_tx(struct iwl_trans *trans, struct sk_buff *skb,
 	 * At this point the frame is "transmitted" successfully
 	 * and we will get a TX status notification eventually.
 	 */
-	if (iwl_queue_space(q) < q->high_mark) {
-		if (wait_write_ptr)
-			iwl_pcie_txq_inc_wr_ptr(trans, txq);
-		else
-			iwl_stop_queue(trans, txq);
-	}
 	spin_unlock(&txq->lock);
 	return 0;
 out_err: