Bluetooth: Controller: Add margin to ISOALs time offset

To ensure payloads are delivered in time for the first subevent
in framed BIS, ISOAL now enforces a (configurable) margin of
the calculated time offset

Without this margin, it has been observed that a broadcaster
can end up consistently missing the first subevent in every third
event in a 7.5 ms ISO with a 10 ms SDU interval

The margin is a conservative 2 ms by default, but can likely be
set a lot lower for most implementations and HWs

Signed-off-by: Troels Nilsson <[email protected]>

subsys/bluetooth/controller/Kconfig.ll_sw_split

@@ -616,6 +616,25 @@ config BT_CTLR_ISOAL_PSN_IGNORE
 	help
 	  Ignore the use of Tx ISO Data Packet Sequence Number.
 
+config BT_CTLR_ISOAL_FRAMED_BIS_OFFSET_MARGIN
+	int "Margin (in microseconds) to be used in framed time offset for BIS"
+	depends on BT_CTLR_ADV_ISO || BT_CTLR_CONN_ISO
+	default 2000
+	range 0 10000
+	help
+	  Needed margin for reliable delivery of payloads will vary, but should
+	  generally be EVENT_OVERHEAD_START_US + a small margin to cover ISOAL
+	  processing overhead
+
+config BT_CTLR_ISOAL_FRAMED_CIS_OFFSET_MARGIN
+	int "Margin (in microseconds) to be used in framed time offset for CIS"
+	depends on BT_CTLR_ADV_ISO || BT_CTLR_CONN_ISO
+	default 0
+	range 0 10000
+	help
+	  Note: Usually no margin is needed for CIS as null PDUs can be used if a payload
+	  is too late for the first subevent
+
 config BT_CTLR_ZLI
 	bool "Use Zero Latency IRQs"
 	depends on ZERO_LATENCY_IRQS

subsys/bluetooth/controller/ll_sw/isoal.c

@@ -1519,7 +1519,7 @@ static isoal_status_t isoal_check_source_hdl_valid(isoal_source_handle_t hdl)
  * @param pdu_release[in]       Callback of PDU deallocator
  * @param hdl[out]              Handle to new source
  *
- * @return ISOAL_STATUS_OK if we could create a new sink; otherwise ISOAL_STATUS_ERR_SOURCE_ALLOC
+ * @return ISOAL_STATUS_OK if we could create a new source; otherwise ISOAL_STATUS_ERR_SOURCE_ALLOC
  */
 isoal_status_t isoal_source_create(
 	uint16_t                    handle,
@@ -1550,6 +1550,7 @@ isoal_status_t isoal_source_create(
 
 	session->handle = handle;
 	session->framed = framed;
+	session->bis = role == ISOAL_ROLE_BROADCAST_SOURCE;
 	session->burst_number = burst_number;
 	session->iso_interval = iso_interval;
 	session->sdu_interval = sdu_interval;
@@ -2343,6 +2344,9 @@ static uint16_t isoal_tx_framed_find_correct_tx_event(const struct isoal_source
 		const bool time_stamp_is_valid = isoal_is_time_stamp_valid(source_ctx,
 									   tx_sdu->cntr_time_stamp,
 									   tx_sdu->time_stamp);
+		const uint16_t offset_margin = session->bis ?
+						    CONFIG_BT_CTLR_ISOAL_FRAMED_BIS_OFFSET_MARGIN :
+						    CONFIG_BT_CTLR_ISOAL_FRAMED_CIS_OFFSET_MARGIN;
 
 		/* Adjust payload number */
 		if (pp->initialized) {
@@ -2437,7 +2441,7 @@ static uint16_t isoal_tx_framed_find_correct_tx_event(const struct isoal_source
 		 * The Time_Offset shall be a positive value.
 		 */
 		while (!isoal_get_time_diff(time_stamp_selected, actual_grp_ref_point, &time_diff)
-			|| time_diff == 0) {
+			|| time_diff <= offset_margin) {
 			/* Advance target to next event */
 			actual_event++;
 			actual_grp_ref_point = isoal_get_wrapped_time_us(actual_grp_ref_point,
@@ -2525,7 +2529,7 @@ static isoal_status_t isoal_tx_framed_produce(isoal_source_handle_t source_hdl,
 		uint64_t next_payload_number;
 		uint16_t sdus_skipped;
 		bool time_diff_valid;
-		uint32_t time_diff;
+		uint32_t time_diff = 0U;
 
 		/* Start of a new SDU */
 		time_diff_valid = isoal_get_time_diff(session->last_input_time_stamp,

subsys/bluetooth/controller/ll_sw/isoal.h

@@ -376,7 +376,8 @@ struct isoal_source_session {
 	uint32_t                   sdu_interval;
 	uint16_t                   handle;
 	uint16_t                   iso_interval;
-	uint8_t                    framed;
+	uint8_t                    framed: 1;
+	uint8_t                    bis: 1;
 	uint8_t                    burst_number;
 	uint8_t                    pdus_per_sdu;
 	uint8_t                    max_pdu_size;

tests/bluetooth/controller/ctrl_isoal/Kconfig

@@ -57,6 +57,25 @@ config BT_CTLR_ISOAL_SN_STRICT
 	depends on BT_CTLR_ADV_ISO || BT_CTLR_CONN_ISO
 	default y
 
+config BT_CTLR_ISOAL_FRAMED_BIS_OFFSET_MARGIN
+	int "Margin (in microseconds) to be used in framed time offset for BIS"
+	depends on BT_CTLR_ADV_ISO || BT_CTLR_CONN_ISO
+	default 2000
+	range 0 10000
+	help
+	  Needed margin for reliable delivery of payloads will vary, but should
+	  generally be EVENT_OVERHEAD_START_US + a small margin to cover ISOAL
+	  processing overhead
+
+config BT_CTLR_ISOAL_FRAMED_CIS_OFFSET_MARGIN
+	int "Margin (in microseconds) to be used in framed time offset for CIS"
+	depends on BT_CTLR_ADV_ISO || BT_CTLR_CONN_ISO
+	default 0
+	range 0 10000
+	help
+	  Note: Usually no margin is needed for CIS as Null PDUs can be used if a payload
+	  is too late for the first subevent
+
 source "tests/bluetooth/controller/common/Kconfig"
 
 source "Kconfig.zephyr"