Avoid switching dyn ts to sdcch8 if it starves later TCH

In case an MS requests a channel to establish a voice call, we usuually
try to assign an SDCCH to negotiate the call and finally make the MS
switch to a TCH. However, it doesn't make much sense to provoke a switch
of a dynamic TS into SDCCH8 if that would mean we end up with no TS
available for TCH at the next step close in time. In that case, we are
better assigning the TCH directly so that the full call can be
established successfully.

Related: SYS#5309
Change-Id: I3b32968949a7bdcbebf5a823359295bac51d8e08

1 files changed, 63 insertions, 3 deletions

diff --git a/src/osmo-bsc/abis_rsl.c b/src/osmo-bsc/abis_rsl.c
index f941d7e2b..b41fe7932 100644
--- a/src/osmo-bsc/abis_rsl.c
+++ b/src/osmo-bsc/abis_rsl.c
@@ -1912,6 +1912,62 @@ static bool force_free_lchan_for_emergency(struct chan_rqd *rqd)
 	return true;
 }
 
+struct gsm_lchan *_select_sdcch_for_call(struct gsm_bts *bts, const struct chan_rqd *rqd, enum gsm_chan_t lctype)
+{
+	struct gsm_lchan *lchan = NULL;
+	int free_tchf, free_tcch;
+	bool needs_dyn_switch;
+
+
+	lchan = lchan_avail_by_type(bts, GSM_LCHAN_SDCCH, false);
+	if (!lchan)
+		return NULL;
+
+	needs_dyn_switch = lchan->ts->pchan_on_init == GSM_PCHAN_OSMO_DYN &&
+					lchan->ts->pchan_is != GSM_PCHAN_SDCCH8_SACCH8C;
+
+	free_tchf = bts_count_free_ts(bts, GSM_PCHAN_TCH_F);
+	free_tcch = bts_count_free_ts(bts, GSM_PCHAN_TCH_H);
+	if (free_tchf == 0 && free_tcch == 0) {
+		LOG_BTS(bts, DRSL, LOGL_INFO,
+			"CHAN RQD: 0x%x Requesting %s reason=call but no TCH available\n",
+			rqd->ref.ra, gsm_lchant_name(lctype));
+		return NULL;
+	}
+
+	/* There's a TCH available and we'll not switch any of them, so we are fine */
+	if (!needs_dyn_switch)
+		goto select_lchan;
+
+	/* We need to switch, but there's at least 2 TCH TS available so we are fine: */
+	if (free_tchf > 1 || free_tcch > 2)
+		goto select_lchan;
+
+	/* At this point (needs_dyn_switch==true), following cases are possible:
+	 * [A] H=0, F=1
+	 * [B] H=1, F=0
+	 * [B] H=1, F=1
+	 * [C] H=2, F=1
+	 * If condition [C] is met, it means there's 1 dynamic TS and it's the
+	 * same as the dynamic TS available for SDCCH requiring switch, so selecting
+	 * it would basically leave us without free TCH, so avoid selecting it.
+	 * Regarding the other conditions, it basically results in them being
+	 * different TS than the one we want to switch, so we are fine selecting
+	 * the TS for SDCCH */
+	if (free_tchf == 1 && free_tcch == 2) {
+		LOG_BTS(bts, DRSL, LOGL_INFO,
+			"CHAN RQD: 0x%x Requesting %s reason=call but dyn TS switch to "
+			"SDCCH would starve the single available TCH timeslot\n",
+			rqd->ref.ra, gsm_lchant_name(lctype));
+		return NULL;
+	}
+
+select_lchan:
+	/* FIXME: we already have lchan, simply do lchan->type = GSM_LCHAN_SDCCH? Split lchan_select_by_type in 2 functions? */
+	lchan = lchan_select_by_type(bts, GSM_LCHAN_SDCCH);
+	return lchan;
+}
+
 void abis_rsl_chan_rqd_queue_poll(struct gsm_bts *bts)
 {
 	struct lchan_activate_info info;
@@ -1948,10 +2004,14 @@ void abis_rsl_chan_rqd_queue_poll(struct gsm_bts *bts)
 	 *
 	 */
 
-	/* Emergency calls will be put on a free TCH/H or TCH/F directly in the code below, all other channel requests
-	 * will get an SDCCH first (if possible). */
-	if (rqd->reason != GSM_CHREQ_REASON_EMERG)
+	if (GSM_CHREQ_REASON_CALL) {
+		 lchan = _select_sdcch_for_call(bts, rqd, lctype);
+	} else if (rqd->reason != GSM_CHREQ_REASON_EMERG) {
 		lchan = lchan_select_by_type(bts, GSM_LCHAN_SDCCH);
+	}
+	/* else: Emergency calls will be put on a free TCH/H or TCH/F directly
+	 * in the code below, all other channel requests will get an SDCCH first
+	 * (if possible). */
 
 	if (!lchan) {
 		LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD: no resources for %s 0x%x, retrying with %s\n",