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

1 files changed, 7 insertions, 0 deletions

diff --git a/src/osmo-bsc/gsm_data.c b/src/osmo-bsc/gsm_data.c
index d5a9d6b17..6c0822907 100644
--- a/src/osmo-bsc/gsm_data.c
+++ b/src/osmo-bsc/gsm_data.c
@@ -916,6 +916,13 @@ const struct value_string lchan_activate_mode_names[] = {
 	{}
 };
 
+const struct value_string lchan_modify_for_names[] = {
+	OSMO_VALUE_STRING(MODIFY_FOR_NONE),
+	OSMO_VALUE_STRING(MODIFY_FOR_ASSIGNMENT),
+	OSMO_VALUE_STRING(MODIFY_FOR_VTY),
+	{}
+};
+
 /* This may be specific to RR Channel Release, and the mappings were chosen by pure naive guessing without a proper
  * specification available. */
 enum gsm48_rr_cause bsc_gsm48_rr_cause_from_gsm0808_cause(enum gsm0808_cause c)