Bluetooth: Use tx window from config response for ack timing

This change addresses an L2CAP ERTM throughput problem when a remote
device does not fully utilize the available transmit window.

The L2CAP ERTM transmit window size determines the maximum number of
unacked frames that may be outstanding at any time. It is configured
separately for each direction of an ERTM connection. Each side sends a
configuration request with a tx_win field indicating how many unacked
frames it is capable of receiving before sending an ack. The
configuration response's tx_win field shows how many frames the
transmitter will actually send before waiting for an ack.

It's important to trace both the actual transmit window (to check for
validity of incoming frames) and the number of frames that the
transmitter will send before waiting (to send acks at the appropriate
time). Now there are separate tx_win and ack_win values. ack_win is
updated based on configuration responses, and is used to determine
when acks are sent.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

include/net/bluetooth/l2cap.h

@@ -464,6 +464,7 @@ struct l2cap_chan {
 
 	__u16		tx_win;
 	__u16		tx_win_max;
+	__u16		ack_win;
 	__u8		max_tx;
 	__u16		retrans_timeout;
 	__u16		monitor_timeout;

net/bluetooth/l2cap_core.c

@@ -431,6 +431,7 @@ void l2cap_chan_set_defaults(struct l2cap_chan *chan)
 	chan->max_tx = L2CAP_DEFAULT_MAX_TX;
 	chan->tx_win = L2CAP_DEFAULT_TX_WINDOW;
 	chan->tx_win_max = L2CAP_DEFAULT_TX_WINDOW;
+	chan->ack_win = L2CAP_DEFAULT_TX_WINDOW;
 	chan->sec_level = BT_SECURITY_LOW;
 
 	set_bit(FLAG_FORCE_ACTIVE, &chan->flags);
@@ -1877,10 +1878,10 @@ static void l2cap_send_ack(struct l2cap_chan *chan)
 				frames_to_ack = 0;
 		}
 
-		/* Ack now if the tx window is 3/4ths full.
+		/* Ack now if the window is 3/4ths full.
 		 * Calculate without mul or div
 		 */
-		threshold = chan->tx_win;
+		threshold = chan->ack_win;
 		threshold += threshold << 1;
 		threshold >>= 2;
 
@@ -2786,6 +2787,7 @@ static inline void l2cap_txwin_setup(struct l2cap_chan *chan)
 						L2CAP_DEFAULT_TX_WINDOW);
 		chan->tx_win_max = L2CAP_DEFAULT_TX_WINDOW;
 	}
+	chan->ack_win = chan->tx_win;
 }
 
 static int l2cap_build_conf_req(struct l2cap_chan *chan, void *data)
@@ -3175,10 +3177,9 @@ static int l2cap_parse_conf_rsp(struct l2cap_chan *chan, void *rsp, int len, voi
 			break;
 
 		case L2CAP_CONF_EWS:
-			chan->tx_win = min_t(u16, val,
-						L2CAP_DEFAULT_EXT_WINDOW);
+			chan->ack_win = min_t(u16, val, chan->ack_win);
 			l2cap_add_conf_opt(&ptr, L2CAP_CONF_EWS, 2,
-							chan->tx_win);
+					   chan->tx_win);
 			break;
 
 		case L2CAP_CONF_EFS:
@@ -3207,6 +3208,9 @@ static int l2cap_parse_conf_rsp(struct l2cap_chan *chan, void *rsp, int len, voi
 			chan->retrans_timeout = le16_to_cpu(rfc.retrans_timeout);
 			chan->monitor_timeout = le16_to_cpu(rfc.monitor_timeout);
 			chan->mps    = le16_to_cpu(rfc.max_pdu_size);
+			if (!test_bit(FLAG_EXT_CTRL, &chan->flags))
+				chan->ack_win = min_t(u16, chan->ack_win,
+						      rfc.txwin_size);
 
 			if (test_bit(FLAG_EFS_ENABLE, &chan->flags)) {
 				chan->local_msdu = le16_to_cpu(efs.msdu);
@@ -3268,7 +3272,17 @@ static void l2cap_conf_rfc_get(struct l2cap_chan *chan, void *rsp, int len)
 {
 	int type, olen;
 	unsigned long val;
-	struct l2cap_conf_rfc rfc;
+	/* Use sane default values in case a misbehaving remote device
+	 * did not send an RFC or extended window size option.
+	 */
+	u16 txwin_ext = chan->ack_win;
+	struct l2cap_conf_rfc rfc = {
+		.mode = chan->mode,
+		.retrans_timeout = __constant_cpu_to_le16(L2CAP_DEFAULT_RETRANS_TO),
+		.monitor_timeout = __constant_cpu_to_le16(L2CAP_DEFAULT_MONITOR_TO),
+		.max_pdu_size = cpu_to_le16(chan->imtu),
+		.txwin_size = min_t(u16, chan->ack_win, L2CAP_DEFAULT_TX_WINDOW),
+	};
 
 	BT_DBG("chan %p, rsp %p, len %d", chan, rsp, len);
 
@@ -3278,32 +3292,27 @@ static void l2cap_conf_rfc_get(struct l2cap_chan *chan, void *rsp, int len)
 	while (len >= L2CAP_CONF_OPT_SIZE) {
 		len -= l2cap_get_conf_opt(&rsp, &type, &olen, &val);
 
-		if (type != L2CAP_CONF_RFC)
-			continue;
-
-		if (olen != sizeof(rfc))
+		switch (type) {
+		case L2CAP_CONF_RFC:
+			if (olen == sizeof(rfc))
+				memcpy(&rfc, (void *)val, olen);
 			break;
-
-		memcpy(&rfc, (void *)val, olen);
-		goto done;
+		case L2CAP_CONF_EWS:
+			txwin_ext = val;
+			break;
+		}
 	}
 
-	/* Use sane default values in case a misbehaving remote device
-	 * did not send an RFC option.
-	 */
-	rfc.mode = chan->mode;
-	rfc.retrans_timeout = __constant_cpu_to_le16(L2CAP_DEFAULT_RETRANS_TO);
-	rfc.monitor_timeout = __constant_cpu_to_le16(L2CAP_DEFAULT_MONITOR_TO);
-	rfc.max_pdu_size = cpu_to_le16(chan->imtu);
-
-	BT_ERR("Expected RFC option was not found, using defaults");
-
-done:
 	switch (rfc.mode) {
 	case L2CAP_MODE_ERTM:
 		chan->retrans_timeout = le16_to_cpu(rfc.retrans_timeout);
 		chan->monitor_timeout = le16_to_cpu(rfc.monitor_timeout);
-		chan->mps    = le16_to_cpu(rfc.max_pdu_size);
+		chan->mps = le16_to_cpu(rfc.max_pdu_size);
+		if (test_bit(FLAG_EXT_CTRL, &chan->flags))
+			chan->ack_win = min_t(u16, chan->ack_win, txwin_ext);
+		else
+			chan->ack_win = min_t(u16, chan->ack_win,
+					      rfc.txwin_size);
 		break;
 	case L2CAP_MODE_STREAMING:
 		chan->mps    = le16_to_cpu(rfc.max_pdu_size);