Re: [PATCH v2 1/1] Bluetooth: Prioritize SCO traffic

[Abhishek Pandit-Subedi <abhishekpandit@chromium.org>]

On Mon, Mar 23, 2020 at 11:58 AM Marcel Holtmann <marcel@holtmann.org> wrote:
>
> 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.

Done.

>
> > 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.

Done

>
> >               }
> >       }
> >
> > @@ -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.

Then we would send at least 1 ACL/LE packet before SCO even if there
were SCO pending when we entered this function. I think it is still
better to keep SCO/eSCO at the top.

>
> Regards
>
> Marcel
>