trxcon: do not call l1sched_prim_dequeue() at ul_bid != 0

It may happen that the Tx queue is empty at TDMA Fn corresponding to
ul_bid == 0, and then shortly after something appears at ul_bid != 0.

The lchan Tx handlers call the encoding functions from libosmocoding
only at bid == 0, so dequeueing at ul_bid != 0 makes no sense.

Change-Id: Ic0bbe2ab6c0ccd96c1f8af8aa17ac88adf7f88ed
Related: OS#5500
---
M src/host/trxcon/src/sched_trx.c
1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/src/host/trxcon/src/sched_trx.c b/src/host/trxcon/src/sched_trx.c
index acf060e..bcd24f0 100644
--- a/src/host/trxcon/src/sched_trx.c
+++ b/src/host/trxcon/src/sched_trx.c
@@ -122,14 +122,18 @@
 		return;
 
 	/* If no primitive is being processed, try obtaining one from Tx queue */
-	if (lchan->prim == NULL)
-		lchan->prim = l1sched_lchan_prim_dequeue(lchan, br->fn);
 	if (lchan->prim == NULL) {
-		/* If CBTX (Continuous Burst Transmission) is required */
-		if (l1sched_lchan_desc[chan].flags & L1SCHED_CH_FLAG_CBTX)
-			l1sched_lchan_prim_assign_dummy(lchan);
-		if (lchan->prim == NULL)
+		/* Align to the first burst of a block */
+		if (frame->ul_bid != 0)
 			return;
+		lchan->prim = l1sched_lchan_prim_dequeue(lchan, br->fn);
+		if (lchan->prim == NULL) {
+			/* If CBTX (Continuous Burst Transmission) is required */
+			if (l1sched_lchan_desc[chan].flags & L1SCHED_CH_FLAG_CBTX)
+				l1sched_lchan_prim_assign_dummy(lchan);
+			if (lchan->prim == NULL)
+				return;
+		}
 	}
 
 	/* TODO: report TX buffers health to the higher layers */

