Send EUTRAN neighs based on whether Common Id msg contained Last used E-UTRAN PLMN ID

From 3GPP TS 48.008 sec 3.1.30 "Common ID":
"""
If the SCCP connection is established due to CSFB from E-UTRAN and the MSC supports
return to the last used PLMN after CS fallback, then it should send the COMMON ID message
to the BSS including the Last used E-UTRAN PLMN ID information element if available at
the MSC immediately following the successful SCCP connection setup.
"""

Furthermore, 3GPP TS 48.008 version 16.0.0 Release 16 "3.2.1.21 CLEAR COMMAND",
for field CSFB Indication, states:
"""
NOTE: This information element doesn't serve any useful purpose. MSCs should not send the
information element unless it is required by the recipients (due to the need to interwork
with older versions of the protocol). It is expected that in future versions of the present
document, this information element will be deleted from this message.
"""

Hence, build up the EUTRAN neighbor list based on whether we received
the Last E-UTRAN PLMN ID IE during Common Id. In the future, we should
probably filter the list while populating it based on the received IE.

This change will also allow reusing same mechanism for SRVCC
EUTRAN->GERAN support, where te Last E-UTRAN PLMN ID IE can be found
inside "Old BSS to New BSS information" IE in msg HANDOVER REQUEST.

Related: SYS#5337
Change-Id: I5d290ac55eca5adde1c33396422f4c10b83c03d5

3 files changed, 15 insertions, 9 deletions

diff --git a/include/osmocom/bsc/bsc_subscr_conn_fsm.h b/include/osmocom/bsc/bsc_subscr_conn_fsm.h
index 142d535c2..a681bc4aa 100644
--- a/include/osmocom/bsc/bsc_subscr_conn_fsm.h
+++ b/include/osmocom/bsc/bsc_subscr_conn_fsm.h
@@ -2,6 +2,7 @@
 #include <osmocom/gsm/protocol/gsm_08_08.h>
 #include <osmocom/gsm/protocol/gsm_04_08.h>
 #include <osmocom/core/fsm.h>
+#include "osmocom/bsc/gsm_data.h"
 
 #define BSUB_USE_CONN "conn"
 
@@ -48,11 +49,6 @@ enum gscon_fsm_event {
 	GSCON_EV_LCS_LOC_REQ_END,
 };
 
-struct gscon_clear_cmd_data {
-	enum gsm0808_cause cause_0808;
-	bool is_csfb;
-};
-
 struct gsm_subscriber_connection;
 struct gsm_network;
 struct msgb;
@@ -93,3 +89,10 @@ void gscon_forget_mgw_endpoint_ci(struct gsm_subscriber_connection *conn, struct
 
 bool gscon_is_aoip(struct gsm_subscriber_connection *conn);
 bool gscon_is_sccplite(struct gsm_subscriber_connection *conn);
+
+
+static inline const struct osmo_plmn_id *gscon_last_eutran_plmn(const struct gsm_subscriber_connection *conn)
+{
+	return (conn && conn->last_eutran_plmn_valid) ?
+		&conn->last_eutran_plmn : NULL;
+}
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 2a9da26bc..448098b9b 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -334,6 +334,9 @@ struct gsm_subscriber_connection {
 
 	struct gsm48_classmark3 cm3;
 	bool cm3_valid;
+
+	bool last_eutran_plmn_valid;
+	struct osmo_plmn_id last_eutran_plmn;
 };
 
 
@@ -614,6 +617,8 @@ struct gsm_lchan {
 		bool requested;
 		bool do_rr_release;
 		enum gsm48_rr_cause rr_cause;
+		bool last_eutran_plmn_valid;
+		struct osmo_plmn_id last_eutran_plmn;
 
 		/* There is an RSL error cause of value 0, so we need a separate flag. */
 		bool in_error;
@@ -623,9 +628,6 @@ struct gsm_lchan {
 		/* If a release event is being handled, ignore other ricocheting release events until that
 		 * release handling has concluded. */
 		bool in_release_handler;
-
-		/* is this release at the end of a CSFB call? */
-		bool is_csfb;
 	} release;
 
 	/* The logical channel type */
diff --git a/include/osmocom/bsc/lchan_fsm.h b/include/osmocom/bsc/lchan_fsm.h
index 9fe7db107..ded7f5481 100644
--- a/include/osmocom/bsc/lchan_fsm.h
+++ b/include/osmocom/bsc/lchan_fsm.h
@@ -53,7 +53,8 @@ void lchan_fsm_init();
 
 void lchan_fsm_alloc(struct gsm_lchan *lchan);
 void lchan_release(struct gsm_lchan *lchan, bool do_rr_release,
-		   bool err, enum gsm48_rr_cause cause_rr);
+		   bool err, enum gsm48_rr_cause cause_rr,
+		   const struct osmo_plmn_id *last_eutran_plmn);
 
 void lchan_activate(struct gsm_lchan *lchan, struct lchan_activate_info *info);
 void lchan_ready_to_switch_rtp(struct gsm_lchan *lchan);