RES IND: pick lchan with least interference

In lchan_select, do not return on the first available lchan, but iterate
through all available lchans. Among those that would match, pick the one
with the lowest interference level as indicated by earlier RES IND.

Lchans that have no interference ratings are picked last.

This feature is off by default, enable per BTS with

 network
  bts N
   channel allocator avoid-interference 1

osmo-bsc still does the same ascending/descending lchan allocation as
before this patch with the default setting of:

   channel allocator avoid-interference 0

Related: SYS#5313
Change-Id: I844494092193811dfd9fa4d52983cbaed0fc9248

1 files changed, 48 insertions, 11 deletions

diff --git a/src/osmo-bsc/lchan_select.c b/src/osmo-bsc/lchan_select.c
index efa2ff29b..3618fb4e8 100644
--- a/src/osmo-bsc/lchan_select.c
+++ b/src/osmo-bsc/lchan_select.c
@@ -30,22 +30,36 @@
 #include <osmocom/bsc/lchan_select.h>
 #include <osmocom/bsc/bts.h>
 
+static struct gsm_lchan *pick_better_lchan(struct gsm_lchan *a, struct gsm_lchan *b)
+{
+	if (!a)
+		return b;
+	if (!b)
+		return a;
+	/* comparing negative dBm values: smaller value means less interference. */
+	if (b->interf_dbm < a->interf_dbm)
+		return b;
+	return a;
+}
+
 static struct gsm_lchan *
 _lc_find_trx(struct gsm_bts_trx *trx, enum gsm_phys_chan_config pchan,
 	     enum gsm_phys_chan_config as_pchan, bool allow_pchan_switch, bool log)
 {
 	struct gsm_lchan *lchan;
+	struct gsm_lchan *found_lchan = NULL;
 	struct gsm_bts_trx_ts *ts;
 	int j, start, stop, dir;
 
-#define LOGPLCHANALLOC(fmt, args...) \
+#define LOGPLCHANALLOC(fmt, args...) do { \
 	if (log) \
 		LOGP(DRLL, LOGL_DEBUG, "looking for lchan %s%s%s%s: " fmt, \
 		     gsm_pchan_name(pchan), \
 		     pchan == as_pchan ? "" : " as ", \
 		     pchan == as_pchan ? "" : gsm_pchan_name(as_pchan), \
 		     ((pchan != as_pchan) && !allow_pchan_switch) ? " without pchan switch" : "", \
-		     ## args)
+		     ## args); \
+	} while (0)
 
 	if (!trx_is_usable(trx)) {
 		LOGPLCHANALLOC("%s trx not usable\n", gsm_trx_name(trx));
@@ -87,20 +101,43 @@ _lc_find_trx(struct gsm_bts_trx *trx, enum gsm_phys_chan_config pchan,
 		/* TS is (going to be) in desired pchan mode. Go ahead and check for an available lchan. */
 		lchans_as_pchan = pchan_subslots(as_pchan);
 		ts_for_n_lchans(lchan, ts, lchans_as_pchan) {
-			if (lchan->fi->state == LCHAN_ST_UNUSED) {
-				LOGPLCHANALLOC("%s ss=%d is available%s\n",
+			struct gsm_lchan *was = found_lchan;
+
+			if (lchan->fi->state != LCHAN_ST_UNUSED) {
+				LOGPLCHANALLOC("%s ss=%d in type=%s,state=%s not suitable\n",
 					       gsm_ts_and_pchan_name(ts), lchan->nr,
-					       ts->pchan_is != as_pchan ? " after dyn PCHAN change" : "");
-				return lchan;
+					       gsm_lchant_name(lchan->type),
+					       osmo_fsm_inst_state_name(lchan->fi));
+				continue;
 			}
-			LOGPLCHANALLOC("%s ss=%d in type=%s,state=%s not suitable\n",
-				       gsm_ts_and_pchan_name(ts), lchan->nr,
-				       gsm_lchant_name(lchan->type),
-				       osmo_fsm_inst_state_name(lchan->fi));
+
+			found_lchan = pick_better_lchan(found_lchan, lchan);
+			if (found_lchan != was)
+				LOGPLCHANALLOC("%s ss=%d interf=%u=%ddBm is %s%s\n",
+					       gsm_ts_and_pchan_name(ts), lchan->nr,
+					       lchan->interf_band, lchan->interf_dbm,
+					       was == NULL ? "available" : "better",
+					       ts->pchan_is != as_pchan ? ", after dyn PCHAN change" : "");
+			else
+				LOGPLCHANALLOC("%s ss=%d interf=%u=%ddBm is also available but not better\n",
+					       gsm_ts_and_pchan_name(ts), lchan->nr,
+					       lchan->interf_band, lchan->interf_dbm);
+
+			/* When picking an lchan with least interference, continue to loop across all lchans. When
+			 * ignoring interference levels, return the first match. */
+			if (found_lchan && !trx->bts->chan_alloc_avoid_interf)
+				return found_lchan;
 		}
 	}
 
-	return NULL;
+	if (found_lchan)
+		LOGPLCHANALLOC("%s ss=%d interf=%ddBm%s is the best pick\n",
+			       gsm_ts_and_pchan_name(found_lchan->ts), found_lchan->nr,
+			       found_lchan->interf_dbm,
+			       found_lchan->ts->pchan_is != as_pchan ? ", after dyn PCHAN change," : "");
+	else
+		LOGPLCHANALLOC("Nothing found");
+	return found_lchan;
 #undef LOGPLCHANALLOC
 }