lchan and assignment FSMs: make Channel Mode Modify more sane

The Channel Mode Modify procedure is currently implemented for changing
a TCH lchan from signalling to voice mode. For that, however, it is
re-using (abusing) the channel activation structs and state transitions,
and thus always implies activating a voice stream when the mode
modification is done.

I will add a Channel Mode Modify to enable VAMOS mode soon, so I require
separate structs and state transitions which also work on an lchan that
already has a voice stream established: a struct lchan_modify_info and
LCHAN_EV_REQUEST_MODE_MODIFY, and dedicated assignment FSM state
ASSIGNMENT_ST_WAIT_LCHAN_MODIFIED.

For the part where a Channel Mode Modify enables a voice stream after
switching from signalling to speech mode, still use the channel
activation code path, but only once the mode modification is done.

General improvements:
- To ask for a mode modification, emit an FSM event that ensures a mode
  modify only happens when the lchan state allows it.
- The new lchan_modify_info struct reflects only those parts that have
  an effect during a mode modification (before the lchan_activate_info
  was fully populated, many values not having an effect).
- More accurate logging, indicating "Mode Modify" instead of "Channel
  Activation"

A TTCN3 test for the Channel Mode Modify procedure is added in
Idf4efaed986de0bbd2b663313e837352cc139f0f, and the test passes both
before and after this patch is applied.

Related: SYS#4895
Change-Id: I4986844f839b1c9672c61d916eb3d33d0042d747

3 files changed, 28 insertions, 0 deletions

diff --git a/include/osmocom/bsc/assignment_fsm.h b/include/osmocom/bsc/assignment_fsm.h
index 156da42fa..b4af335d1 100644
--- a/include/osmocom/bsc/assignment_fsm.h
+++ b/include/osmocom/bsc/assignment_fsm.h
@@ -24,11 +24,13 @@ enum assignment_fsm_state {
 	ASSIGNMENT_ST_WAIT_RR_ASS_COMPLETE,
 	ASSIGNMENT_ST_WAIT_LCHAN_ESTABLISHED,
 	ASSIGNMENT_ST_WAIT_MGW_ENDPOINT_TO_MSC,
+	ASSIGNMENT_ST_WAIT_LCHAN_MODIFIED,
 };
 
 enum assignment_fsm_event {
 	ASSIGNMENT_EV_LCHAN_ACTIVE,
 	ASSIGNMENT_EV_LCHAN_ESTABLISHED,
+	ASSIGNMENT_EV_LCHAN_MODIFIED,
 	ASSIGNMENT_EV_LCHAN_ERROR,
 	ASSIGNMENT_EV_MSC_MGW_OK,
 	ASSIGNMENT_EV_MSC_MGW_FAIL,
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index bd83ea004..5a202bf4e 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -564,6 +564,7 @@ enum lchan_activate_for {
 	ACTIVATE_FOR_ASSIGNMENT,
 	ACTIVATE_FOR_HANDOVER,
 	ACTIVATE_FOR_VTY,
+	ACTIVATE_FOR_MODE_MODIFY_RTP,
 };
 
 extern const struct value_string lchan_activate_mode_names[];
@@ -589,6 +590,25 @@ struct lchan_activate_info {
 	uint8_t ta;
 };
 
+enum lchan_modify_for {
+	MODIFY_FOR_NONE,
+	MODIFY_FOR_ASSIGNMENT,
+	MODIFY_FOR_VTY,
+};
+
+extern const struct value_string lchan_modify_for_names[];
+static inline const char *lchan_modify_for_name(enum lchan_modify_for modify_for)
+{ return get_value_string(lchan_modify_for_names, modify_for); }
+
+struct lchan_modify_info {
+	enum lchan_modify_for modify_for;
+	enum gsm48_chan_mode chan_mode;
+	bool requires_voice_stream;
+	uint16_t msc_assigned_cic;
+	/* AMR config */
+	uint16_t s15_s0;
+};
+
 struct gsm_lchan {
 	/* The TS that we're part of */
 	struct gsm_bts_trx_ts *ts;
@@ -614,6 +634,11 @@ struct gsm_lchan {
 	} activate;
 
 	struct {
+		struct lchan_modify_info info;
+		bool concluded;
+	} modify;
+
+	struct {
 		/* If an event to release the lchan comes in while still waiting for responses, just mark this
 		 * flag, so that the lchan will gracefully release at the next sensible junction. */
 		bool requested;
diff --git a/include/osmocom/bsc/lchan_fsm.h b/include/osmocom/bsc/lchan_fsm.h
index 99964a417..74e2a9668 100644
--- a/include/osmocom/bsc/lchan_fsm.h
+++ b/include/osmocom/bsc/lchan_fsm.h
@@ -65,6 +65,7 @@ void lchan_release(struct gsm_lchan *lchan, bool do_rr_release,
 
 void lchan_activate(struct gsm_lchan *lchan, struct lchan_activate_info *info);
 void lchan_ready_to_switch_rtp(struct gsm_lchan *lchan);
+void lchan_mode_modify(struct gsm_lchan *lchan, struct lchan_modify_info *info);
 
 static inline const char *lchan_state_name(struct gsm_lchan *lchan)
 {