Bluetooth: ISO: support setting send Time_Stamp and LE Read ISO TX Sync

For ISO packets with HW TX timestamping enabled, retrieve ISO send timestamps via LE Read ISO TX Sync. Specification allows retrieving the timestamp only for the "latest" sent packet.

Report SDU Reference time as the timestamp, as this is the value relevant for userspace send synchronization.

Due to overall poor description of the timing features in ISO-over-HCI transport in Core Specification v6.2, it is ambiguous what packet the command actually reports, so timestamps may not be assigned to the exactly correct packets, user applications have to deal with it.

Signed-off-by Pauli Virtanen <[email protected]>

---
 include/net/bluetooth/hci.h      |  2 +-
 include/net/bluetooth/hci_core.h |  3 +-
 net/bluetooth/hci_conn.c         | 53 +++++++++++++++++++--
 net/bluetooth/hci_event.c        | 79 +++++++++++++++++++++++++++++++-
 4 files changed, 131 insertions(+), 6 deletions(-)

include/net/bluetooth/hci.h +1 -1

index 89ad9470fa71..b536587a909f 100644
@@ -2138,7 +2138,7 @@ struct hci_rp_le_read_iso_tx_sync {
        __u8    status;
        __le16  handle;
        __le16  seq;
-       __le32  imestamp;
+       __le32  timestamp;
        __u8    offset[3];
 } __packed;
 

include/net/bluetooth/hci_core.h +2 -1

index c82cec45a9a1..5d530349c2a4 100644
@@ -272,6 +272,7 @@ struct adv_info {
 
 struct tx_queue {
        struct sk_buff_head queue;
+       struct sk_buff *iso_last_tx;
        unsigned int extra;
        unsigned int tracked;
 };

@@ -1630,7 +1631,7 @@ void hci_conn_failed(struct hci_conn *conn, u8 status);
 u8 hci_conn_set_handle(struct hci_conn *conn, u16 handle);
 
 void hci_conn_tx_queue(struct hci_conn *conn, struct sk_buff *skb);
-void hci_conn_tx_dequeue(struct hci_conn *conn);
+void hci_conn_tx_dequeue(struct hci_conn *conn, bool last);
 void hci_setup_tx_timestamp(struct sk_buff *skb, size_t key_offset,
                            const struct sockcm_cookie *sockc);
 

net/bluetooth/hci_conn.c +50 -3

index a47f5daffdbf..342747a7c225 100644
@@ -1232,6 +1232,9 @@ void hci_conn_del(struct hci_conn *conn)
        skb_queue_purge(&conn->data_q);
        skb_queue_purge(&conn->tx_q.queue);
 
+       kfree_skb(conn->tx_q.iso_last_tx);
+       conn->tx_q.iso_last_tx = NULL;
+
        /* Remove the connection from the list and cleanup its remaining
         * state. This is a separate function since for some cases like
         * BT_CONNECT_SCAN we *only* want the cleanup part without the

@@ -2388,6 +2391,20 @@ struct hci_conn *hci_connect_cis(struct hci_dev *hdev, bdaddr_t *dst,
        return cis;
 }
 
+static int hci_conn_le_read_iso_tx_sync(struct hci_conn *conn)
+{
+       struct hci_cp_le_read_iso_tx_sync cp;
+
+       if (!(conn->hdev->commands[41] & 0x40))
+               return -EOPNOTSUPP;
+
+       memset(&cp, 0, sizeof(cp));
+       cp.handle = cpu_to_le16(conn->handle);
+
+       return hci_send_cmd(conn->hdev, HCI_OP_LE_READ_ISO_TX_SYNC,
+                           sizeof(cp), &cp);
+}
+
 /* Check link security requirement */
 int hci_conn_check_link_mode(struct hci_conn *conn)
 {

@@ -3156,6 +3173,18 @@ void hci_conn_tx_queue(struct hci_conn *conn, struct sk_buff *skb)
        struct tx_queue *comp = &conn->tx_q;
        bool track = false;
 
+       /* Check for HW timestamping */
+       switch (conn->type) {
+       case CIS_LINK:
+       case BIS_LINK:
+               if (skb->sk &&
+                   (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP_NOBPF)) {
+                       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
+                       track = true;
+               }
+               break;
+       }
+
        /* Emit SND now, ie. just before sending to driver */
        if (skb_shinfo(skb)->tx_flags & SKBTX_SW_TSTAMP)
                __skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SND);

@@ -3214,7 +3243,7 @@ void hci_conn_tx_queue(struct hci_conn *conn, struct sk_buff *skb)
        skb_queue_purge(&comp->queue);
 }
 
-void hci_conn_tx_dequeue(struct hci_conn *conn)
+void hci_conn_tx_dequeue(struct hci_conn *conn, bool last)
 {
        struct tx_queue *comp = &conn->tx_q;
        struct sk_buff *skb;

@@ -3234,8 +3263,24 @@ void hci_conn_tx_dequeue(struct hci_conn *conn)
 
        if (skb->sk) {
                comp->tracked--;
-               __skb_tstamp_tx(skb, NULL, NULL, skb->sk,
-                               SCM_TSTAMP_COMPLETION);
+
+               if (skb_shinfo(skb)->tx_flags & SKBTX_COMPLETION_TSTAMP)
+                       __skb_tstamp_tx(skb, NULL, NULL, skb->sk,
+                                       SCM_TSTAMP_COMPLETION);
+
+               switch (conn->type) {
+               case CIS_LINK:
+               case BIS_LINK:
+                       if (!last)
+                               break;
+                       if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP_NOBPF) {
+                               kfree_skb(conn->tx_q.iso_last_tx);
+                               conn->tx_q.iso_last_tx = skb;
+                               hci_conn_le_read_iso_tx_sync(conn);
+                               return;
+                       }
+                       break;
+               }
        }
 
        kfree_skb(skb);

@@ -3283,6 +3328,8 @@ int hci_ethtool_ts_info(unsigned int index, int sk_proto,
 
        switch (sk_proto) {
        case BTPROTO_ISO:
+               info->so_timestamping |= SOF_TIMESTAMPING_TX_HARDWARE;
+               fallthrough;
        case BTPROTO_L2CAP:
                info->so_timestamping |= SOF_TIMESTAMPING_TX_SOFTWARE;
                info->so_timestamping |= SOF_TIMESTAMPING_TX_COMPLETION;

net/bluetooth/hci_event.c +36 -0

index 286529d2e554..f2aade36a39e 100644
@@ -3777,6 +3777,81 @@ static inline void handle_cmd_cnt_and_timer(struct hci_dev *hdev, u8 ncmd)
        rcu_read_unlock();
 }
 
+static u8 hci_cc_le_read_iso_tx_sync(struct hci_dev *hdev, void *data,
+                                    struct sk_buff *skb)
+{
+       struct hci_rp_le_read_iso_tx_sync *rp = data;
+       struct skb_shared_hwtstamps ssh;
+       struct hci_conn *conn;
+       u32 offset;
+
+       bt_dev_dbg(hdev, "status 0x%2.2x", rp->status);
+
+       if (rp->status)
+               return rp->status;
+
+       offset = rp->offset[0] | (rp->offset[1] << 8) | (rp->offset[2] << 16);
+       bt_dev_dbg(hdev, "ts %u to %u", __le32_to_cpu(rp->timestamp), offset);
+
+       hci_dev_lock(hdev);
+
+       conn = hci_conn_hash_lookup_handle(hdev, __le16_to_cpu(rp->handle));
+       if (!conn)
+               goto unlock;
+
+       if (!conn->tx_q.iso_last_tx)
+               goto unlock;
+
+       /* Report SDU synchronization reference as the timestamp. This value is
+        * most useful for userspace, as it is the Time_Stamp field in sent
+        * packets. See Fig. 3.1 and 3.2 in Core v6.2 Vol 6 Part G Sec. 3.
+        *
+        * Unfortunately, the ISO-over-HCI transport is poorly described in the
+        * specification.
+        *
+        * The HCI specification contradicts itself which packet timestamp is
+        * reported by Le Read ISO TX Sync: "scheduled for transmission" in Core
+        * v6.2 Vol 4 Part E Sec 4.7 vs. "transmitted" in Sec. 7.8.96.
+        * The specification is also unclear on what these words mean.

net/bluetooth/hci_event.c +32 -2

+        *
+        * Userspace application that handle these timestamps must then be
+        * prepared that the ISO timestamp is reported on the wrong packet
+        * depending on controller. Actual packet TX must rely on backpressure
+        * (monitoring packet completion via COMPLETION tstamp), and the
+        * timestamps are useful only for aligning the playback of multiple ISO
+        * streams.
+        */
+
+       memset(&ssh, 0, sizeof(ssh));
+
+       /* SDU Reference = BIG/CIG Reference - Time_Offset */
+       ssh.hwtstamp = us_to_ktime(__le32_to_cpu(rp->timestamp) - offset);
+
+       /* The value may be 0, but skb_tstamp_tx() does not allow reporting zero
+        * HW timestamp. Offset by 1 ns as a workaround.
+        */
+       ssh.hwtstamp += 1;
+
+       skb_tstamp_tx(conn->tx_q.iso_last_tx, &ssh);
+
+       kfree_skb(conn->tx_q.iso_last_tx);
+       conn->tx_q.iso_last_tx = NULL;
+
+unlock:
+       hci_dev_unlock(hdev);
+       return rp->status;
+}
+
 static u8 hci_cc_le_read_buffer_size_v2(struct hci_dev *hdev, void *data,
                                        struct sk_buff *skb)
 {

@@ -4224,6 +4299,8 @@ static const struct hci_cc {
        HCI_CC(HCI_OP_LE_READ_TRANSMIT_POWER, hci_cc_le_read_transmit_power,
               sizeof(struct hci_rp_le_read_transmit_power)),
        HCI_CC_STATUS(HCI_OP_LE_SET_PRIVACY_MODE, hci_cc_le_set_privacy_mode),
+       HCI_CC(HCI_OP_LE_READ_ISO_TX_SYNC, hci_cc_le_read_iso_tx_sync,
+              sizeof(struct hci_rp_le_read_iso_tx_sync)),
        HCI_CC(HCI_OP_LE_READ_BUFFER_SIZE_V2, hci_cc_le_read_buffer_size_v2,
               sizeof(struct hci_rp_le_read_buffer_size_v2)),
        HCI_CC_VL(HCI_OP_LE_SET_CIG_PARAMS, hci_cc_le_set_cig_params,

@@ -4509,7 +4586,7 @@ static void hci_num_comp_pkts_evt(struct hci_dev *hdev, void *data,
                }
 
                for (i = 0; i < count; ++i)
-                       hci_conn_tx_dequeue(conn);
+                       hci_conn_tx_dequeue(conn, (i == count - 1));
 
                switch (conn->type) {
                case ACL_LINK:
--

2.53.0

Hi Pauli,

Quoted 244 lines · Pauli Virtanen

For ISO packets with HW TX timestamping enabled, retrieve ISO send timestamps via LE Read ISO TX Sync. Specification allows retrieving the
timestamp only for the "latest" sent packet.

Report SDU Reference time as the timestamp, as this is the value relevant for userspace send synchronization.

Due to overall poor description of the timing features in ISO-over- HCI
transport in Core Specification v6.2, it is ambiguous what packet the
command actually reports, so timestamps may not be assigned to the exactly correct packets, user applications have to deal with it.

Signed-off-by Pauli Virtanen <[email protected]>

---
 include/net/bluetooth/hci.h      |  2 +- include/net/bluetooth/hci_core.h |  3 +- net/bluetooth/hci_conn.c         | 53 +++++++++++++++++++-- net/bluetooth/hci_event.c        | 79
+++++++++++++++++++++++++++++++-

 4 files changed, 131 insertions(+), 6 deletions(-)

include/net/bluetooth/hci.h +1 -1

index 89ad9470fa71..b536587a909f 100644
@@ -2138,7 +2138,7 @@ struct hci_rp_le_read_iso_tx_sync {
         __u8    status;
         __le16  handle;
         __le16  seq;
-       __le32  imestamp;
+       __le32  timestamp;
         __u8    offset[3];
  } __packed;
 

include/net/bluetooth/hci_core.h +1 -0

index c82cec45a9a1..5d530349c2a4 100644
@@ -272,6 +272,7 @@ struct adv_info {
 
  struct tx_queue {
         struct sk_buff_head queue;
+       struct sk_buff *iso_last_tx;
         unsigned int extra;
         unsigned int tracked;
  };

@@ -1630,7 +1631,7 @@ void hci_conn_failed(struct hci_conn *conn,
 u8 status);
  u8 hci_conn_set_handle(struct hci_conn *conn, u16 handle);
 

 void hci_conn_tx_queue(struct hci_conn *conn, struct sk_buff *skb);

include/net/bluetooth/hci_core.h +1 -1

-void hci_conn_tx_dequeue(struct hci_conn *conn);
+void hci_conn_tx_dequeue(struct hci_conn *conn, bool last);

 void hci_setup_tx_timestamp(struct sk_buff *skb, size_t key_offset,
                            const struct sockcm_cookie *sockc);

net/bluetooth/hci_conn.c +3 -0

index a47f5daffdbf..342747a7c225 100644
@@ -1232,6 +1232,9 @@ void hci_conn_del(struct hci_conn *conn)
         skb_queue_purge(&conn->data_q);
         skb_queue_purge(&conn->tx_q.queue);
 
+       kfree_skb(conn->tx_q.iso_last_tx);
+       conn->tx_q.iso_last_tx = NULL;
+
         /* Remove the connection from the list and cleanup its

remaining
         * state. This is a separate function since for some cases like
         * BT_CONNECT_SCAN we *only* want the cleanup part without the

net/bluetooth/hci_conn.c +14 -0

@@ -2388,6 +2391,20 @@ struct hci_conn *hci_connect_cis(struct
 hci_dev *hdev, bdaddr_t *dst,
         return cis;
  }
 
+static int hci_conn_le_read_iso_tx_sync(struct hci_conn *conn)
+{
+       struct hci_cp_le_read_iso_tx_sync cp;
+
+       if (!(conn->hdev->commands[41] & 0x40))
+               return -EOPNOTSUPP;
+
+       memset(&cp, 0, sizeof(cp));
+       cp.handle = cpu_to_le16(conn->handle);
+
+       return hci_send_cmd(conn->hdev, HCI_OP_LE_READ_ISO_TX_SYNC,
+                           sizeof(cp), &cp);
+}
+
  /* Check link security requirement */
  int hci_conn_check_link_mode(struct hci_conn *conn)

 {

net/bluetooth/hci_conn.c +12 -0

@@ -3156,6 +3173,18 @@ void hci_conn_tx_queue(struct hci_conn
 *conn, struct sk_buff *skb)
         struct tx_queue *comp = &conn->tx_q;
         bool track = false;
 
+       /* Check for HW timestamping */
+       switch (conn->type) {
+       case CIS_LINK:
+       case BIS_LINK:
+               if (skb->sk &&
+                   (skb_shinfo(skb)->tx_flags &
 SKBTX_HW_TSTAMP_NOBPF)) {
+                       skb_shinfo(skb)->tx_flags |=
 SKBTX_IN_PROGRESS;
+                       track = true;
+               }
+               break;
+       }
+

        /* Emit SND now, ie. just before sending to driver */ if (skb_shinfo(skb)->tx_flags & SKBTX_SW_TSTAMP) __skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SND);

net/bluetooth/hci_conn.c +1 -1

@@ -3214,7 +3243,7 @@ void hci_conn_tx_queue(struct hci_conn *conn,
 struct sk_buff *skb)
         skb_queue_purge(&comp->queue);
  }
 
-void hci_conn_tx_dequeue(struct hci_conn *conn)
+void hci_conn_tx_dequeue(struct hci_conn *conn, bool last)
  {
         struct tx_queue *comp = &conn->tx_q;

        struct sk_buff *skb;

net/bluetooth/hci_conn.c +18 -2

@@ -3234,8 +3263,24 @@ void hci_conn_tx_dequeue(struct hci_conn
 *conn)
 
         if (skb->sk) {
                 comp->tracked--;
-               __skb_tstamp_tx(skb, NULL, NULL, skb->sk,
-                               SCM_TSTAMP_COMPLETION);
+
+               if (skb_shinfo(skb)->tx_flags &
 SKBTX_COMPLETION_TSTAMP)
+                       __skb_tstamp_tx(skb, NULL, NULL, skb->sk,
+                                       SCM_TSTAMP_COMPLETION);
+
+               switch (conn->type) {
+               case CIS_LINK:
+               case BIS_LINK:
+                       if (!last)
+                               break;
+                       if (skb_shinfo(skb)->tx_flags &
 SKBTX_HW_TSTAMP_NOBPF) {
+                               kfree_skb(conn->tx_q.iso_last_tx);
+                               conn->tx_q.iso_last_tx = skb;
+                               hci_conn_le_read_iso_tx_sync(conn);
+                               return;
+                       }
+                       break;
+               }

        }

        kfree_skb(skb);

net/bluetooth/hci_conn.c +0 -0

@@ -3283,6 +3328,8 @@ int hci_ethtool_ts_info(unsigned int index,
 int sk_proto,
 
         switch (sk_proto) {

        case BTPROTO_ISO:
+               info->so_timestamping |=
SOF_TIMESTAMPING_TX_HARDWARE;
+               fallthrough;
        case BTPROTO_L2CAP:
                info->so_timestamping |=
SOF_TIMESTAMPING_TX_SOFTWARE;
                info->so_timestamping |=
SOF_TIMESTAMPING_TX_COMPLETION;

net/bluetooth/hci_event.c +15 -0

index 286529d2e554..f2aade36a39e 100644
@@ -3777,6 +3777,81 @@ static inline void
 handle_cmd_cnt_and_timer(struct hci_dev *hdev, u8 ncmd)
         rcu_read_unlock();
  }
 
+static u8 hci_cc_le_read_iso_tx_sync(struct hci_dev *hdev, void
 *data,
+                                    struct sk_buff *skb)
+{
+       struct hci_rp_le_read_iso_tx_sync *rp = data;
+       struct skb_shared_hwtstamps ssh;
+       struct hci_conn *conn;
+       u32 offset;
+
+       bt_dev_dbg(hdev, "status 0x%2.2x", rp->status);
+
+       if (rp->status)
+               return rp->status;
+
+       offset = rp->offset[0] | (rp->offset[1] << 8) | (rp-
 >offset[2] << 16);
+       bt_dev_dbg(hdev, "ts %u to %u", __le32_to_cpu(rp-

Quoted 1 line

timestamp), offset);

net/bluetooth/hci_event.c +4 -0

+
+       hci_dev_lock(hdev);
+
+       conn = hci_conn_hash_lookup_handle(hdev, __le16_to_cpu(rp-

Quoted 1 line

handle));

+       if (!conn)
+               goto unlock;

net/bluetooth/hci_event.c +5 -0

+
+       if (!conn->tx_q.iso_last_tx)
+               goto unlock;
+
+       /* Report SDU synchronization reference as the timestamp.

This value is
+        * most useful for userspace, as it is the Time_Stamp field in sent
+        * packets. See Fig. 3.1 and 3.2 in Core v6.2 Vol 6 Part G Sec. 3.
+        *
+        * Unfortunately, the ISO-over-HCI transport is poorly described in the
+        * specification.
+        *
+        * The HCI specification contradicts itself which packet timestamp is
+        * reported by Le Read ISO TX Sync: "scheduled for transmission" in Core
+        * v6.2 Vol 4 Part E Sec 4.7 vs. "transmitted" in Sec. 7.8.96.
+        * The specification is also unclear on what these words mean.
+        *
+        * We assume here it means "transmitted", and additionally that
+        * "transmitted" means the last packet reported in Number of Completed
+        * Packets.
+        *
+        * The specification is also unclear what sequence number

Quoted 8 lines

is returned by
+        * this command. We assume it is a sequence number assigned by the
+        * controller. These are equivalent to the controller clock, so they are
+        * useless for generic Host (which we are) with no direct access to
+        * controller clock.

Interesting analysis. I thought the sequence number for NOCP must match the sequence number set by the host when sending the packet, and
its sole purpose was to be able to fetch the hardware time. Did you observe something different? If the sequence number had to synchronize
with a clock, the host would need to generate empty packets even when

there is nothing to transmit. This would consume power and memory solely to keep the sequence number in sync with the clock. I don't actually understand why NOCP was even used with ISO if the intention was to use sequence numbers like above, at very least NOCP should have
been updated to carry the hardware clock and other timing information directly rather than using another command that consumes even more system power and memory.

Quoted 69 lines

+        *
+        * Userspace application that handle these timestamps must then be
+        * prepared that the ISO timestamp is reported on the wrong packet
+        * depending on controller. Actual packet TX must rely on backpressure
+        * (monitoring packet completion via COMPLETION tstamp), and the
+        * timestamps are useful only for aligning the playback of multiple ISO
+        * streams.
+        */

net/bluetooth/hci_event.c +5 -0

+
+       memset(&ssh, 0, sizeof(ssh));
+
+       /* SDU Reference = BIG/CIG Reference - Time_Offset */
+       ssh.hwtstamp = us_to_ktime(__le32_to_cpu(rp->timestamp) -

offset);
+
+       /* The value may be 0, but skb_tstamp_tx() does not allow reporting zero
+        * HW timestamp. Offset by 1 ns as a workaround. +        */
+       ssh.hwtstamp += 1;

net/bluetooth/hci_event.c +11 -0

+
+       skb_tstamp_tx(conn->tx_q.iso_last_tx, &ssh);
+
+       kfree_skb(conn->tx_q.iso_last_tx);
+       conn->tx_q.iso_last_tx = NULL;
+
+unlock:
+       hci_dev_unlock(hdev);
+       return rp->status;
+}
+

 static u8 hci_cc_le_read_buffer_size_v2(struct hci_dev *hdev, void *data,
                                        struct sk_buff *skb) {

net/bluetooth/hci_event.c +2 -0

@@ -4224,6 +4299,8 @@ static const struct hci_cc {
         HCI_CC(HCI_OP_LE_READ_TRANSMIT_POWER,
 hci_cc_le_read_transmit_power,
                sizeof(struct hci_rp_le_read_transmit_power)),
         HCI_CC_STATUS(HCI_OP_LE_SET_PRIVACY_MODE,
 hci_cc_le_set_privacy_mode),
+       HCI_CC(HCI_OP_LE_READ_ISO_TX_SYNC,
 hci_cc_le_read_iso_tx_sync,
+              sizeof(struct hci_rp_le_read_iso_tx_sync)),

        HCI_CC(HCI_OP_LE_READ_BUFFER_SIZE_V2, hci_cc_le_read_buffer_size_v2,
               sizeof(struct hci_rp_le_read_buffer_size_v2)), HCI_CC_VL(HCI_OP_LE_SET_CIG_PARAMS, hci_cc_le_set_cig_params,

net/bluetooth/hci_event.c +1 -1

@@ -4509,7 +4586,7 @@ static void hci_num_comp_pkts_evt(struct
 hci_dev *hdev, void *data,
                 }
 
                 for (i = 0; i < count; ++i)
-                       hci_conn_tx_dequeue(conn);
+                       hci_conn_tx_dequeue(conn, (i == count -
 1));
 

                switch (conn->type) {
                case ACL_LINK:
--
2.53.0

Hi Luiz,

Quoted 7 lines

Interesting analysis. I thought the sequence number for NOCP must match the sequence number set by the host when sending the packet, and
its sole purpose was to be able to fetch the hardware time. Did you observe something different? If the sequence number had to synchronize
with a clock, the host would need to generate empty packets even when

Quoted 8 lines

there is nothing to transmit. This would consume power and memory solely to keep the sequence number in sync with the clock. I don't actually understand why NOCP was even used with ISO if the intention was to use sequence numbers like above, at very least NOCP should have
been updated to carry the hardware clock and other timing information directly rather than using another command that consumes even more system power and memory.

There's no relationship to NOCP specified in Core Spec, it's pretty vague. However, I don't see any other way for host to get clock sync for TX than via NOCP (at least for <1 packet accuracy).

It would indeed make most sense that sequence number is what the host sent in the ISO packet, but it is not specified. Core Specification v6.2 Vol 6 Part G Sec 2 also says:

"""
SDUs sent to the upper layer by the ISOAL shall be given a sequence number which is initialized to 0 when the CIS or BIS is created. SDUs received by the ISOAL from the upper layer shall be given a sequence number which is initialized to 0 when the CIS or BIS is created. The upper layer may synchronize its sequence number with the sequence number in the ISOAL once the Datapath is configured and the link is established.
"""

so one gets the impression controller shall assign new sequence numbers and use those, and host shall "synchronize" to it in some unspecified way.

I saw some related discussions by the Zephyr people, I think the outcome was along the lines that the spec is so vague that these features are unusable.

Some clarifications would be useful here, as I don't get what was the way the spec intends that these are used.

***

IIRC, in the controllers I tested (Intel AX210, Realtek 8761CU) the returned timestamp and sequence number are always zero, so they haven't actually implemented the command. I'd have to check what Mediatek MT7925 does, it was at least setting RX timestamps properly.

In Zephyr IIRC controller-assigned sequence number is used, but I'd have to double check this.

Quoted 70 lines

Quoted 69 lines

+        *
+        * Userspace application that handle these timestamps must then be
+        * prepared that the ISO timestamp is reported on the wrong packet
+        * depending on controller. Actual packet TX must rely on backpressure
+        * (monitoring packet completion via COMPLETION tstamp), and the
+        * timestamps are useful only for aligning the playback of multiple ISO
+        * streams.
+        */

net/bluetooth/hci_event.c +5 -0

+
+       memset(&ssh, 0, sizeof(ssh));
+
+       /* SDU Reference = BIG/CIG Reference - Time_Offset */
+       ssh.hwtstamp = us_to_ktime(__le32_to_cpu(rp->timestamp) -

offset);
+
+       /* The value may be 0, but skb_tstamp_tx() does not allow reporting zero
+        * HW timestamp. Offset by 1 ns as a workaround. +        */
+       ssh.hwtstamp += 1;

net/bluetooth/hci_event.c +11 -0

+
+       skb_tstamp_tx(conn->tx_q.iso_last_tx, &ssh);
+
+       kfree_skb(conn->tx_q.iso_last_tx);
+       conn->tx_q.iso_last_tx = NULL;
+
+unlock:
+       hci_dev_unlock(hdev);
+       return rp->status;
+}
+

 static u8 hci_cc_le_read_buffer_size_v2(struct hci_dev *hdev, void *data,
                                        struct sk_buff *skb) {

(quoted diff) +2 -0

@@ -4224,6 +4299,8 @@ static const struct hci_cc {
         HCI_CC(HCI_OP_LE_READ_TRANSMIT_POWER,
 hci_cc_le_read_transmit_power,
                sizeof(struct hci_rp_le_read_transmit_power)),
         HCI_CC_STATUS(HCI_OP_LE_SET_PRIVACY_MODE,
 hci_cc_le_set_privacy_mode),
+       HCI_CC(HCI_OP_LE_READ_ISO_TX_SYNC,
 hci_cc_le_read_iso_tx_sync,
+              sizeof(struct hci_rp_le_read_iso_tx_sync)),

        HCI_CC(HCI_OP_LE_READ_BUFFER_SIZE_V2, hci_cc_le_read_buffer_size_v2,
               sizeof(struct hci_rp_le_read_buffer_size_v2)), HCI_CC_VL(HCI_OP_LE_SET_CIG_PARAMS, hci_cc_le_set_cig_params,

net/bluetooth/hci_event.c +1 -1

@@ -4509,7 +4586,7 @@ static void hci_num_comp_pkts_evt(struct
 hci_dev *hdev, void *data,
                 }
 
                 for (i = 0; i < count; ++i)
-                       hci_conn_tx_dequeue(conn);
+                       hci_conn_tx_dequeue(conn, (i == count -
 1));
 

                switch (conn->type) {
                case ACL_LINK:
--
2.53.0