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Date: Wed, 18 Mar 2020 18:11:51 +0100
From: Marcel Holtmann <marcel@...tmann.org>
To: Abhishek Pandit-Subedi <abhishekpandit@...omium.org>
Cc: Bluez mailing list <linux-bluetooth@...r.kernel.org>,
ChromeOS Bluetooth Upstreaming
<chromeos-bluetooth-upstreaming@...omium.org>,
"David S. Miller" <davem@...emloft.net>,
Johan Hedberg <johan.hedberg@...il.com>,
netdev <netdev@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 1/1] Bluetooth: Prioritize SCO traffic on slow interfaces
Hi Abhishek,
>>> When scheduling TX packets, send all SCO/eSCO packets first and then
>>> send only 1 ACL/LE packet in a loop while checking that there are no SCO
>>> packets 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
>>> and is pretty close to a common baudrate of 3000000 used for BT. At this
>>> baudrate, if we sent two 1024 byte ACL packets, we would miss the 3.75ms
>>> timing window.
>>>
>>> Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@...omium.org>
>>> ---
>>>
>>> include/net/bluetooth/hci_core.h | 1 +
>>> net/bluetooth/hci_core.c | 91 +++++++++++++++++++++++++-------
>>> 2 files changed, 73 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/include/net/bluetooth/hci_core.h b/include/net/bluetooth/hci_core.h
>>> index d4e28773d378..f636c89f1fe1 100644
>>> --- a/include/net/bluetooth/hci_core.h
>>> +++ b/include/net/bluetooth/hci_core.h
>>> @@ -315,6 +315,7 @@ struct hci_dev {
>>> __u8 ssp_debug_mode;
>>> __u8 hw_error_code;
>>> __u32 clock;
>>> + __u8 sched_limit;
>>
>> why do you need this parameter?
>
> This is really only necessary on systems where the data transfer rate
> to the controller is low. I want the driver to set whether we should
> aggressively schedule SCO packets. A quirk might actually be better
> than a variable (wasn't sure what is preferable).
or maybe we try without driver choice first. I would assume what is required for UART, will not harm USB or SDIO transports either.
>>> __u16 devid_source;
>>> __u16 devid_vendor;
>>> diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
>>> index dbd2ad3a26ed..00a72265cd96 100644
>>> --- a/net/bluetooth/hci_core.c
>>> +++ b/net/bluetooth/hci_core.c
>>> @@ -4239,18 +4239,32 @@ static void __check_timeout(struct hci_dev *hdev, unsigned int cnt)
>>> }
>>> }
>>>
>>> -static void hci_sched_acl_pkt(struct hci_dev *hdev)
>>> +/* Limit packets in flight when SCO/eSCO links are active. */
>>> +static bool hci_sched_limit(struct hci_dev *hdev)
>>> +{
>>> + return hdev->sched_limit && hci_conn_num(hdev, SCO_LINK);
>>> +}
>>> +
>>> +static bool hci_sched_acl_pkt(struct hci_dev *hdev)
>>> {
>>> unsigned int cnt = hdev->acl_cnt;
>>> struct hci_chan *chan;
>>> struct sk_buff *skb;
>>> int quote;
>>> + bool sched_limit = hci_sched_limit(hdev);
>>> + bool resched = false;
>>>
>>> __check_timeout(hdev, cnt);
>>>
>>> while (hdev->acl_cnt &&
>>> (chan = hci_chan_sent(hdev, ACL_LINK, "e))) {
>>> u32 priority = (skb_peek(&chan->data_q))->priority;
>>> +
>>> + if (sched_limit && quote > 0) {
>>> + resched = true;
>>> + quote = 1;
>>> + }
>>> +
>>> while (quote-- && (skb = skb_peek(&chan->data_q))) {
>>> BT_DBG("chan %p skb %p len %d priority %u", chan, skb,
>>> skb->len, skb->priority);
>>> @@ -4271,19 +4285,26 @@ static void hci_sched_acl_pkt(struct hci_dev *hdev)
>>> chan->sent++;
>>> chan->conn->sent++;
>>> }
>>> +
>>> + if (resched && cnt != hdev->acl_cnt)
>>> + break;
>>> }
>>>
>>> - if (cnt != hdev->acl_cnt)
>>> + if (hdev->acl_cnt == 0 && cnt != hdev->acl_cnt)
>>> hci_prio_recalculate(hdev, ACL_LINK);
>>> +
>>> + return resched;
>>> }
>>>
>>> -static void hci_sched_acl_blk(struct hci_dev *hdev)
>>> +static bool hci_sched_acl_blk(struct hci_dev *hdev)
>>> {
>>> unsigned int cnt = hdev->block_cnt;
>>> struct hci_chan *chan;
>>> struct sk_buff *skb;
>>> int quote;
>>> u8 type;
>>> + bool sched_limit = hci_sched_limit(hdev);
>>> + bool resched = false;
>>>
>>> __check_timeout(hdev, cnt);
>>>
>>> @@ -4297,6 +4318,12 @@ static void hci_sched_acl_blk(struct hci_dev *hdev)
>>> while (hdev->block_cnt > 0 &&
>>> (chan = hci_chan_sent(hdev, type, "e))) {
>>> u32 priority = (skb_peek(&chan->data_q))->priority;
>>> +
>>> + if (sched_limit && quote > 0) {
>>> + resched = true;
>>> + quote = 1;
>>> + }
>>> +
>>> while (quote > 0 && (skb = skb_peek(&chan->data_q))) {
>>> int blocks;
>>>
>>> @@ -4311,7 +4338,7 @@ static void hci_sched_acl_blk(struct hci_dev *hdev)
>>>
>>> blocks = __get_blocks(hdev, skb);
>>> if (blocks > hdev->block_cnt)
>>> - return;
>>> + return false;
>>>
>>> hci_conn_enter_active_mode(chan->conn,
>>> bt_cb(skb)->force_active);
>>> @@ -4325,33 +4352,39 @@ static void hci_sched_acl_blk(struct hci_dev *hdev)
>>> chan->sent += blocks;
>>> chan->conn->sent += blocks;
>>> }
>>> +
>>> + if (resched && cnt != hdev->block_cnt)
>>> + break;
>>> }
>>>
>>> - if (cnt != hdev->block_cnt)
>>> + if (hdev->block_cnt == 0 && cnt != hdev->block_cnt)
>>> hci_prio_recalculate(hdev, type);
>>> +
>>> + return resched;
>>> }
>>>
>>> -static void hci_sched_acl(struct hci_dev *hdev)
>>> +static bool hci_sched_acl(struct hci_dev *hdev)
>>> {
>>> BT_DBG("%s", hdev->name);
>>>
>>> /* No ACL link over BR/EDR controller */
>>> if (!hci_conn_num(hdev, ACL_LINK) && hdev->dev_type == HCI_PRIMARY)
>>> - return;
>>> + goto done;
>>
>> Style wise the goto done is overkill. Just return false.
>
> Will do.
>
>>
>>>
>>> /* No AMP link over AMP controller */
>>> if (!hci_conn_num(hdev, AMP_LINK) && hdev->dev_type == HCI_AMP)
>>> - return;
>>> + goto done;
>>>
>>> switch (hdev->flow_ctl_mode) {
>>> case HCI_FLOW_CTL_MODE_PACKET_BASED:
>>> - hci_sched_acl_pkt(hdev);
>>> - break;
>>> + return hci_sched_acl_pkt(hdev);
>>>
>>> case HCI_FLOW_CTL_MODE_BLOCK_BASED:
>>> - hci_sched_acl_blk(hdev);
>>> - break;
>>> + return hci_sched_acl_blk(hdev);
>>
>> So the block based mode is for AMP controllers and not used on BR/EDR controllers. Since AMP controllers only transport ACL packet and no SCO/eSCO packets, we can ignore this here.
>
> Ok, I'll remove it there.
>
>>
>>> }
>>> +
>>> +done:
>>> + return false;
>>> }
>>>
>>> /* Schedule SCO */
>>> @@ -4402,16 +4435,18 @@ static void hci_sched_esco(struct hci_dev *hdev)
>>> }
>>> }
>>>
>>> -static void hci_sched_le(struct hci_dev *hdev)
>>> +static bool hci_sched_le(struct hci_dev *hdev)
>>> {
>>> struct hci_chan *chan;
>>> struct sk_buff *skb;
>>> int quote, cnt, tmp;
>>> + bool sched_limit = hci_sched_limit(hdev);
>>> + bool resched = false;
>>>
>>> BT_DBG("%s", hdev->name);
>>>
>>> if (!hci_conn_num(hdev, LE_LINK))
>>> - return;
>>> + return resched;
>>>
>>> cnt = hdev->le_pkts ? hdev->le_cnt : hdev->acl_cnt;
>>>
>>> @@ -4420,6 +4455,12 @@ static void hci_sched_le(struct hci_dev *hdev)
>>> tmp = cnt;
>>> while (cnt && (chan = hci_chan_sent(hdev, LE_LINK, "e))) {
>>> u32 priority = (skb_peek(&chan->data_q))->priority;
>>> +
>>> + if (sched_limit && quote > 0) {
>>> + resched = true;
>>> + quote = 1;
>>> + }
>>> +
>>> while (quote-- && (skb = skb_peek(&chan->data_q))) {
>>> BT_DBG("chan %p skb %p len %d priority %u", chan, skb,
>>> skb->len, skb->priority);
>>> @@ -4437,6 +4478,9 @@ static void hci_sched_le(struct hci_dev *hdev)
>>> chan->sent++;
>>> chan->conn->sent++;
>>> }
>>> +
>>> + if (resched && cnt != tmp)
>>> + break;
>>> }
>>>
>>> if (hdev->le_pkts)
>>> @@ -4444,24 +4488,33 @@ static void hci_sched_le(struct hci_dev *hdev)
>>> else
>>> hdev->acl_cnt = cnt;
>>>
>>> - if (cnt != tmp)
>>> + if (cnt == 0 && cnt != tmp)
>>> hci_prio_recalculate(hdev, LE_LINK);
>>> +
>>> + return resched;
>>> }
>>>
>>> static void hci_tx_work(struct work_struct *work)
>>> {
>>> struct hci_dev *hdev = container_of(work, struct hci_dev, tx_work);
>>> struct sk_buff *skb;
>>> + bool resched;
>>>
>>> BT_DBG("%s acl %d sco %d le %d", hdev->name, hdev->acl_cnt,
>>> hdev->sco_cnt, hdev->le_cnt);
>>>
>>> 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_le(hdev);
>>> + do {
>>> + /* SCO and eSCO send all packets until emptied */
>>> + hci_sched_sco(hdev);
>>> + hci_sched_esco(hdev);
>>> +
>>> + /* Acl and Le send based on quota (priority on ACL per
>>> + * loop)
>>> + */
>>> + resched = hci_sched_acl(hdev) || hci_sched_le(hdev);
>>> + } while (resched);
>>> }
>>
>> I am not in favor of this busy loop. We might want to re-think the whole scheduling by connection type and really only focus on scheduling ACL (BR/EDR and LE) and audio packets (SCO/eSCO and ISO).
>
> I think the busy loop is the simplest solution if we want to solve the
> problem: don't send 2 ACL packets without checking if there is a SCO
> packet scheduled (which is the worst case I'm worried about on UART
> interfaces).
>
> If we get rid of the connection type scheduling and only do audio and
> ACL, we would still need some mechanism to guarantee that you don't
> send >~1100 bytes without checking if SCO is queued (assuming 3000000
> baudrate and 3.75ms latency requirement).
Why don’t we just say that if SCO is queued up, then after each ACL packet we should send a SCO packet.
>
>>
>> In addition, we also need to check that SCO scheduling and A2DP media channel ACL packets do work together. I think that generally it would be best to have a clear rate at which SCO packets are require to pushed down to the hardware. So you really reserve bandwidth and not blindly prioritize them via a busy loop.
>>
> I am less worried about bandwidth and more about latency. If I start
> sending really large ACL packets through UART, it could take multiple
> milliseconds. It really has to be reserved bandwidth per small
> timeslice (like 3.75ms) so I can guarantee that if a SCO packet is
> seen within that time slice, it will be transferred. There will still
> have to be a busy loop though because the amount of data you can send
> in the time slice will probably be less than the data that can be
> in-flight to the controller (i.e. acl_max_pkts).
Right now we kinda let the SCO socket application provide the correct timing. I was thinking that the kernel might need to enforce this.
Regards
Marcel
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