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Subject: Re: [PATCH v2 1/1] Bluetooth: Prioritize SCO traffic
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <20200320161922.v2.1.I17e2220fd0c0822c76a15ef89b882fb4cfe3fe89@changeid>
Date:   Mon, 23 Mar 2020 19:58:27 +0100
Cc:     Bluetooth Kernel Mailing List <linux-bluetooth@vger.kernel.org>,
        chromeos-bluetooth-upstreaming@chromium.org,
        "David S. Miller" <davem@davemloft.net>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jakub Kicinski <kuba@kernel.org>
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Message-Id: <C09DCA09-A2C9-4675-B17B-05CE0B5DE172@holtmann.org>
References: <20200320231928.137720-1-abhishekpandit@chromium.org>
 <20200320161922.v2.1.I17e2220fd0c0822c76a15ef89b882fb4cfe3fe89@changeid>
To:     Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Sender: linux-bluetooth-owner@vger.kernel.org
Precedence: bulk

Hi Abhishek,

> 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>
> ---
> 
> Changes in v2:
> * Refactor to check for SCO/eSCO after each ACL/LE packet sent
> * Enabled SCO priority all the time and removed the sched_limit variable
> 
> net/bluetooth/hci_core.c | 111 +++++++++++++++++++++------------------
> 1 file changed, 61 insertions(+), 50 deletions(-)
> 
> diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
> index dbd2ad3a26ed..a29177e1a9d0 100644
> --- a/net/bluetooth/hci_core.c
> +++ b/net/bluetooth/hci_core.c
> @@ -4239,6 +4239,60 @@ 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_sync(struct hci_dev *hdev)
> +{
> +	hci_sched_sco(hdev);
> +	hci_sched_esco(hdev);
> +}
> +

scrap this function. It has almost zero benefit.

> static void hci_sched_acl_pkt(struct hci_dev *hdev)
> {
> 	unsigned int cnt = hdev->acl_cnt;
> @@ -4270,6 +4324,9 @@ 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_sync(hdev);

			hci_sched_esco();
			hci_sched_sco();

> 		}
> 	}
> 
> @@ -4354,54 +4411,6 @@ 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;
> @@ -4436,6 +4445,9 @@ static void hci_sched_le(struct hci_dev *hdev)
> 			cnt--;
> 			chan->sent++;
> 			chan->conn->sent++;
> +
> +			/* Send pending SCO packets right away */
> +			hci_sched_sync(hdev);

Same as above. Just call the two functions.

> 		}
> 	}
> 
> @@ -4458,9 +4470,8 @@ 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_sync(hdev);
> 		hci_sched_acl(hdev);
> -		hci_sched_sco(hdev);
> -		hci_sched_esco(hdev);
> 		hci_sched_le(hdev);

I would actually just move _le up after _acl and then keep _sco and _esco at the bottom. The calls here are just for the case there are no ACL nor LE packets.

Regards

Marcel