early IMM ASS 3/n: implement 'pre-ts-ack'

Add experimental 'pre-ts-ack' to the 'immediate-assignment' options:
send the IMM ASS even before a dynamic timeslot is switched. This
possibly saves an Abis roundtrip, but may be racy.

When pre-ts-ack is chosen, already do the IMM ASS before the dyn TS
pchan switch is ACKed.

In Immediate Assignment, in case the dyn TS is not ready yet, get the
pchan kind from lchan->type, which already reflects the target type, and
not from ts->pchan_is, which still reflects the previous pchan type.

Related test is in I2ae28cd92910d4bc341a88571599347a64a18fe5

Related: SYS#5559
Change-Id: I19e6a3d614aa5ae24d64eed96caf53e6f0e8bb74

3 files changed, 26 insertions, 4 deletions

diff --git a/src/osmo-bsc/abis_rsl.c b/src/osmo-bsc/abis_rsl.c
index df319821d..26231ac9d 100644
--- a/src/osmo-bsc/abis_rsl.c
+++ b/src/osmo-bsc/abis_rsl.c
@@ -2072,6 +2072,7 @@ int rsl_tx_imm_assignment(struct gsm_lchan *lchan)
 	struct gsm_bts *bts = lchan->ts->trx->bts;
 	uint8_t buf[GSM_MACBLOCK_LEN];
 	struct gsm48_imm_ass *ia = (struct gsm48_imm_ass *) buf;
+	enum gsm_phys_chan_config pchan;
 
 	/* create IMMEDIATE ASSIGN 04.08 message */
 	memset(ia, 0, sizeof(*ia));
@@ -2079,7 +2080,15 @@ int rsl_tx_imm_assignment(struct gsm_lchan *lchan)
 	ia->proto_discr = GSM48_PDISC_RR;
 	ia->msg_type = GSM48_MT_RR_IMM_ASS;
 	ia->page_mode = GSM48_PM_SAME;
-	rc = gsm48_lchan2chan_desc(&ia->chan_desc, lchan, lchan->tsc, true);
+
+	/* In case the dyn TS is not ready yet, ts->pchan_is still reflects the previous pchan type; so get the pchan
+	 * kind from lchan->type, which already reflects the target type. This only happens for dynamic timeslots.
+	 * gsm_pchan_by_lchan_type() isn't always exact, which is fine for dyn TS with their limited pchan kinds. */
+	if (lchan_state_is(lchan, LCHAN_ST_WAIT_TS_READY))
+		pchan = gsm_pchan_by_lchan_type(lchan->type);
+	else
+		pchan = lchan->ts->pchan_is;
+	rc = gsm48_lchan_and_pchan2chan_desc(&ia->chan_desc, lchan, pchan, lchan->tsc, true);
 	if (rc) {
 		LOG_LCHAN(lchan, LOGL_ERROR, "Error encoding Channel Number\n");
 		return rc;
diff --git a/src/osmo-bsc/bts_vty.c b/src/osmo-bsc/bts_vty.c
index cba0bb0db..577feeca7 100644
--- a/src/osmo-bsc/bts_vty.c
+++ b/src/osmo-bsc/bts_vty.c
@@ -2807,16 +2807,20 @@ DEFUN_ATTR(cfg_bts_srvcc_fast_return, cfg_bts_srvcc_fast_return_cmd,
 }
 
 DEFUN_ATTR(cfg_bts_immediate_assignment, cfg_bts_immediate_assignment_cmd,
-	   "immediate-assignment (post-chan-ack|pre-chan-ack)",
+	   "immediate-assignment (post-chan-ack|pre-chan-ack|pre-ts-ack)",
 	   "Configure time of Immediate Assignment after ChanRqd RACH (Abis optimization)\n"
 	   "Send the Immediate Assignment after the Channel Activation ACK (normal sequence)\n"
 	   "Send the Immediate Assignment directly after Channel Activation (early), without waiting for the ACK;"
-	   " This may help with double allocations on high latency Abis links\n",
+	   " This may help with double allocations on high latency Abis links\n"
+	   "EXPERIMENTAL: If a dynamic timeslot switch is necessary, send the Immediate Assignment even before the"
+	   " timeslot is switched, i.e. even before the Channel Activation is sent (very early)\n",
 	   CMD_ATTR_IMMEDIATE)
 {
 	struct gsm_bts *bts = vty->index;
 
-	if (!strcmp(argv[0], "pre-chan-ack"))
+	if (!strcmp(argv[0], "pre-ts-ack"))
+		bts->imm_ass_time = IMM_ASS_TIME_PRE_TS_ACK;
+	else if (!strcmp(argv[0], "pre-chan-ack"))
 		bts->imm_ass_time = IMM_ASS_TIME_PRE_CHAN_ACK;
 	else
 		bts->imm_ass_time = IMM_ASS_TIME_POST_CHAN_ACK;
@@ -4005,6 +4009,9 @@ static void config_write_bts_single(struct vty *vty, struct gsm_bts *bts)
 	case IMM_ASS_TIME_PRE_CHAN_ACK:
 		vty_out(vty, "  immediate-assignment pre-chan-ack%s", VTY_NEWLINE);
 		break;
+	case IMM_ASS_TIME_PRE_TS_ACK:
+		vty_out(vty, "  immediate-assignment pre-ts-ack%s", VTY_NEWLINE);
+		break;
 	}
 
 	/* BS/MS Power Control parameters */
diff --git a/src/osmo-bsc/lchan_fsm.c b/src/osmo-bsc/lchan_fsm.c
index 1eb9505a8..def70de00 100644
--- a/src/osmo-bsc/lchan_fsm.c
+++ b/src/osmo-bsc/lchan_fsm.c
@@ -739,6 +739,12 @@ static void lchan_fsm_wait_ts_ready_onenter(struct osmo_fsm_inst *fi, uint32_t p
 	/* Prepare an MGW endpoint CI if appropriate. */
 	if (lchan->activate.info.requires_voice_stream)
 		lchan_rtp_fsm_start(lchan);
+
+	if (lchan->activate.info.imm_ass_time == IMM_ASS_TIME_PRE_TS_ACK) {
+		/* Send the Immediate Assignment even before the timeslot is ready, saving a dyn TS timeslot roundtrip
+		 * on Abis (experimental). */
+		lchan_send_imm_ass(fi);
+	}
 }
 
 static void lchan_fsm_wait_ts_ready(struct osmo_fsm_inst *fi, uint32_t event, void *data)