Bluetooth: Prioritize SCO traffic

When scheduling TX packets, send all SCO/eSCO packets first, check for
pending SCO/eSCO packets after every ACL/LE packet and send them if any
are pending.  This is done to make sure that we can meet SCO deadlines
on slow interfaces like UART.

If we were to queue up multiple ACL packets without checking for a SCO
packet, we might miss the SCO timing. For example:

The time it takes to send a maximum size ACL packet (1024 bytes):
t = 10/8 * 1024 bytes * 8 bits/byte * 1 packet / baudrate
        where 10/8 is uart overhead due to start/stop bits per byte

Replace t = 3.75ms (SCO deadline), which gives us a baudrate of 2730666.

At a baudrate of 3000000, if we didn't check for SCO packets within 1024
bytes, we would miss the 3.75ms timing window.
Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/hci_core.c

@@ -4240,6 +4240,54 @@ static void __check_timeout(struct hci_dev *hdev, unsigned int cnt)
 	}
 }
 
+/* Schedule SCO */
+static void hci_sched_sco(struct hci_dev *hdev)
+{
+	struct hci_conn *conn;
+	struct sk_buff *skb;
+	int quote;
+
+	BT_DBG("%s", hdev->name);
+
+	if (!hci_conn_num(hdev, SCO_LINK))
+		return;
+
+	while (hdev->sco_cnt && (conn = hci_low_sent(hdev, SCO_LINK, &quote))) {
+		while (quote-- && (skb = skb_dequeue(&conn->data_q))) {
+			BT_DBG("skb %p len %d", skb, skb->len);
+			hci_send_frame(hdev, skb);
+
+			conn->sent++;
+			if (conn->sent == ~0)
+				conn->sent = 0;
+		}
+	}
+}
+
+static void hci_sched_esco(struct hci_dev *hdev)
+{
+	struct hci_conn *conn;
+	struct sk_buff *skb;
+	int quote;
+
+	BT_DBG("%s", hdev->name);
+
+	if (!hci_conn_num(hdev, ESCO_LINK))
+		return;
+
+	while (hdev->sco_cnt && (conn = hci_low_sent(hdev, ESCO_LINK,
+						     &quote))) {
+		while (quote-- && (skb = skb_dequeue(&conn->data_q))) {
+			BT_DBG("skb %p len %d", skb, skb->len);
+			hci_send_frame(hdev, skb);
+
+			conn->sent++;
+			if (conn->sent == ~0)
+				conn->sent = 0;
+		}
+	}
+}
+
 static void hci_sched_acl_pkt(struct hci_dev *hdev)
 {
 	unsigned int cnt = hdev->acl_cnt;
@@ -4271,6 +4319,10 @@ static void hci_sched_acl_pkt(struct hci_dev *hdev)
 			hdev->acl_cnt--;
 			chan->sent++;
 			chan->conn->sent++;
+
+			/* Send pending SCO packets right away */
+			hci_sched_sco(hdev);
+			hci_sched_esco(hdev);
 		}
 	}
 
@@ -4355,54 +4407,6 @@ static void hci_sched_acl(struct hci_dev *hdev)
 	}
 }
 
-/* Schedule SCO */
-static void hci_sched_sco(struct hci_dev *hdev)
-{
-	struct hci_conn *conn;
-	struct sk_buff *skb;
-	int quote;
-
-	BT_DBG("%s", hdev->name);
-
-	if (!hci_conn_num(hdev, SCO_LINK))
-		return;
-
-	while (hdev->sco_cnt && (conn = hci_low_sent(hdev, SCO_LINK, &quote))) {
-		while (quote-- && (skb = skb_dequeue(&conn->data_q))) {
-			BT_DBG("skb %p len %d", skb, skb->len);
-			hci_send_frame(hdev, skb);
-
-			conn->sent++;
-			if (conn->sent == ~0)
-				conn->sent = 0;
-		}
-	}
-}
-
-static void hci_sched_esco(struct hci_dev *hdev)
-{
-	struct hci_conn *conn;
-	struct sk_buff *skb;
-	int quote;
-
-	BT_DBG("%s", hdev->name);
-
-	if (!hci_conn_num(hdev, ESCO_LINK))
-		return;
-
-	while (hdev->sco_cnt && (conn = hci_low_sent(hdev, ESCO_LINK,
-						     &quote))) {
-		while (quote-- && (skb = skb_dequeue(&conn->data_q))) {
-			BT_DBG("skb %p len %d", skb, skb->len);
-			hci_send_frame(hdev, skb);
-
-			conn->sent++;
-			if (conn->sent == ~0)
-				conn->sent = 0;
-		}
-	}
-}
-
 static void hci_sched_le(struct hci_dev *hdev)
 {
 	struct hci_chan *chan;
@@ -4437,6 +4441,10 @@ static void hci_sched_le(struct hci_dev *hdev)
 			cnt--;
 			chan->sent++;
 			chan->conn->sent++;
+
+			/* Send pending SCO packets right away */
+			hci_sched_sco(hdev);
+			hci_sched_esco(hdev);
 		}
 	}
 
@@ -4459,9 +4467,9 @@ static void hci_tx_work(struct work_struct *work)
 
 	if (!hci_dev_test_flag(hdev, HCI_USER_CHANNEL)) {
 		/* Schedule queues and send stuff to HCI driver */
-		hci_sched_acl(hdev);
 		hci_sched_sco(hdev);
 		hci_sched_esco(hdev);
+		hci_sched_acl(hdev);
 		hci_sched_le(hdev);
 	}