Bluetooth: Controller: Fix flash sync start from starving connection

Fix flash sync start from repeatedly forcing connection
event be skipped. Space new flash operation to be placed
in the past so that it does not force itself on to an
overlapping connection event.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

Author: cvinayak

subsys/bluetooth/controller/flash/soc_flash_nrf_ticker.c

@@ -26,14 +26,13 @@
 #define FLASH_SYNC_SWITCHING_TIME (FLASH_RADIO_ABORT_DELAY_US +\
 				   FLASH_RADIO_WORK_DELAY_US)
 
-struct ticker_sync_context {
-	uint32_t interval;    /* timeslot interval. */
-	uint32_t slot;        /* timeslot length. */
-	uint32_t ticks_begin; /* timeslot begin timestamp */
+static struct {
+	uint32_t interval;       /* timeslot interval. */
+	uint32_t slot;           /* timeslot length. */
+	uint32_t ticks_begin;    /* timeslot begin timestamp */
+	uint32_t ticks_previous; /* timeslot previous reference */
 	int result;
-};
-
-static struct ticker_sync_context _ticker_sync_context;
+} _ticker_sync_context;
 
 /* semaphore for synchronization of flash operations */
 static struct k_sem sem_sync;
@@ -151,6 +150,7 @@ static void time_slot_callback_abort(uint32_t ticks_at_expire,
 				     void *context)
 {
 	ll_radio_state_abort();
+
 	time_slot_delay(ticks_at_expire,
 			HAL_TICKER_US_TO_TICKS(FLASH_RADIO_WORK_DELAY_US),
 			time_slot_callback_work,
@@ -163,6 +163,18 @@ static void time_slot_callback_prepare(uint32_t ticks_at_expire,
 				       uint16_t lazy, uint8_t force,
 				       void *context)
 {
+	uint32_t ticks_elapsed;
+	uint32_t ticks_now;
+
+	/* Skip flash operation if in the past, this can be when requested in the past due to
+	 * flash operations requested too often.
+	 */
+	ticks_now = ticker_ticks_now_get();
+	ticks_elapsed = ticker_ticks_diff_get(ticks_now, ticks_at_expire);
+	if (ticks_elapsed > HAL_TICKER_US_TO_TICKS(FLASH_RADIO_WORK_DELAY_US)) {
+		return;
+	}
+
 #if defined(CONFIG_BT_CTLR_LOW_LAT)
 	time_slot_callback_abort(ticks_at_expire, ticks_drift, remainder, lazy,
 				 force, context);
@@ -178,6 +190,7 @@ static void time_slot_callback_prepare(uint32_t ticks_at_expire,
 int nrf_flash_sync_init(void)
 {
 	k_sem_init(&sem_sync, 0, 1);
+
 	return 0;
 }
 
@@ -193,10 +206,26 @@ void nrf_flash_sync_set_context(uint32_t duration)
 int nrf_flash_sync_exe(struct flash_op_desc *op_desc)
 {
 	uint8_t instance_index;
+	uint32_t ticks_elapsed;
+	uint32_t ticks_slot;
+	uint32_t ticks_now;
 	uint8_t ticker_id;
 	uint32_t ret;
 	int result;
 
+	/* Check if flash operation request too often that can starve connection events.
+	 * We will request to start ticker in the past so that it does not `force` itself onto
+	 * scheduling by causing connection events to be skipped.
+	 */
+	ticks_now = ticker_ticks_now_get();
+	ticks_elapsed = ticker_ticks_diff_get(ticks_now, _ticker_sync_context.ticks_previous);
+	_ticker_sync_context.ticks_previous = ticks_now;
+	ticks_slot = HAL_TICKER_US_TO_TICKS(_ticker_sync_context.slot);
+	if (ticks_elapsed < ticks_slot) {
+		/* Set ticker start reference one flash slot interval in the past */
+		ticks_now = ticker_ticks_diff_get(ticks_now, ticks_slot);
+	}
+
 	/* Get the ticker instance and ticker id for flash operations */
 	ll_timeslice_ticker_id_get(&instance_index, &ticker_id);
 
@@ -205,7 +234,7 @@ int nrf_flash_sync_exe(struct flash_op_desc *op_desc)
 			   3, /* user id for thread mode */
 			      /* (MAYFLY_CALL_ID_PROGRAM) */
 			   ticker_id, /* flash ticker id */
-			   ticker_ticks_now_get(), /* current tick */
+			   ticks_now, /* current tick */
 			   0, /* first int. immediately */
 			   /* period */
 			   HAL_TICKER_US_TO_TICKS(