From dad4e7c3f7923be519af7d842f6dba9d94142da2 Mon Sep 17 00:00:00 2001
From: Neels Hofmeyr <neels@hofmeyr.de>
Date: Fri, 13 Nov 2020 04:19:24 +0100
Subject: fix TCH/H allocation: use half occupied dyn TS instead of switching
 more dyn TS

Change-Id: I5a8d943f31774af00664d037550be14e767d312a
---
 src/osmo-bsc/lchan_select.c | 47 +++++++++++++++++++++++++++++----------------
 src/osmo-bsc/timeslot_fsm.c | 10 ++++++++--
 2 files changed, 38 insertions(+), 19 deletions(-)

(limited to 'src/osmo-bsc')

diff --git a/src/osmo-bsc/lchan_select.c b/src/osmo-bsc/lchan_select.c
index 6d3caaced..64f4939e5 100644
--- a/src/osmo-bsc/lchan_select.c
+++ b/src/osmo-bsc/lchan_select.c
@@ -32,17 +32,19 @@
 
 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)
+	     enum gsm_phys_chan_config as_pchan, bool allow_pchan_switch)
 {
 	struct gsm_lchan *lchan;
 	struct gsm_bts_trx_ts *ts;
 	int j, start, stop, dir;
 
 #define LOGPLCHANALLOC(fmt, args...) \
-		LOGP(DRLL, LOGL_DEBUG, "looking for lchan %s%s%s: " fmt, \
+		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), ## args)
+		     pchan == as_pchan ? "" : gsm_pchan_name(as_pchan), \
+		     ((pchan != as_pchan) && !allow_pchan_switch) ? " without pchan switch" : "", \
+		     ## args)
 
 	if (!trx_is_usable(trx)) {
 		LOGPLCHANALLOC("%s trx not usable\n", gsm_trx_name(trx));
@@ -73,9 +75,10 @@ _lc_find_trx(struct gsm_bts_trx *trx, enum gsm_phys_chan_config pchan,
 			continue;
 		}
 		/* Next, is this timeslot in or can it be switched to the pchan we want to use it for? */
-		if (!ts_usable_as_pchan(ts, as_pchan)) {
-			LOGPLCHANALLOC("%s is not usable as %s\n", gsm_ts_and_pchan_name(ts),
-				       gsm_pchan_name(as_pchan));
+		if (!ts_usable_as_pchan(ts, as_pchan, allow_pchan_switch)) {
+			LOGPLCHANALLOC("%s is not usable as %s%s\n", gsm_ts_and_pchan_name(ts),
+				       gsm_pchan_name(as_pchan),
+				       allow_pchan_switch ? "" : " without pchan switch");
 			continue;
 		}
 
@@ -104,18 +107,28 @@ _lc_dyn_find_bts(struct gsm_bts *bts, enum gsm_phys_chan_config pchan,
 {
 	struct gsm_bts_trx *trx;
 	struct gsm_lchan *lc;
+	int allow_pchan_switch;
+	bool try_pchan_switch;
 
-	if (bts->chan_alloc_reverse) {
-		llist_for_each_entry_reverse(trx, &bts->trx_list, list) {
-			lc = _lc_find_trx(trx, pchan, dyn_as_pchan);
-			if (lc)
-				return lc;
-		}
-	} else {
-		llist_for_each_entry(trx, &bts->trx_list, list) {
-			lc = _lc_find_trx(trx, pchan, dyn_as_pchan);
-			if (lc)
-				return lc;
+	/* First find an lchan that needs no change in its timeslot pchan mode.
+	 * In particular, this ensures that handover to a dynamic timeslot in TCH/H favors timeslots that are currently
+	 * using only one of two TCH/H, so that we don't switch more dynamic timeslots to TCH/H than necessary.
+	 * For non-dynamic timeslots, it is not necessary to do a second pass with allow_pchan_switch ==
+	 * true, because they never switch anyway. */
+	try_pchan_switch = (pchan != dyn_as_pchan);
+	for (allow_pchan_switch = 0; allow_pchan_switch <= (try_pchan_switch ? 1 : 0); allow_pchan_switch++) {
+		if (bts->chan_alloc_reverse) {
+			llist_for_each_entry_reverse(trx, &bts->trx_list, list) {
+				lc = _lc_find_trx(trx, pchan, dyn_as_pchan, (bool)allow_pchan_switch);
+				if (lc)
+					return lc;
+			}
+		} else {
+			llist_for_each_entry(trx, &bts->trx_list, list) {
+				lc = _lc_find_trx(trx, pchan, dyn_as_pchan, (bool)allow_pchan_switch);
+				if (lc)
+					return lc;
+			}
 		}
 	}
 
diff --git a/src/osmo-bsc/timeslot_fsm.c b/src/osmo-bsc/timeslot_fsm.c
index 106e6a1fa..d8636a100 100644
--- a/src/osmo-bsc/timeslot_fsm.c
+++ b/src/osmo-bsc/timeslot_fsm.c
@@ -1004,7 +1004,7 @@ bool ts_is_pchan_switching(struct gsm_bts_trx_ts *ts, enum gsm_phys_chan_config
 /* Does the timeslot's *current* state allow use as this PCHAN kind? If the ts is in switchover, return
  * true if the switchover's target PCHAN matches, i.e. an lchan for this pchan kind could be requested
  * and will be served after the switch. (Do not check whether any lchans are actually available.) */
-bool ts_usable_as_pchan(struct gsm_bts_trx_ts *ts, enum gsm_phys_chan_config as_pchan)
+bool ts_usable_as_pchan(struct gsm_bts_trx_ts *ts, enum gsm_phys_chan_config as_pchan, bool allow_pchan_switch)
 {
 	enum gsm_phys_chan_config target_pchan;
 
@@ -1022,5 +1022,11 @@ bool ts_usable_as_pchan(struct gsm_bts_trx_ts *ts, enum gsm_phys_chan_config as_
 	if (ts_is_lchan_waiting_for_pchan(ts, &target_pchan))
 		return target_pchan == as_pchan;
 
-	return ts_is_capable_of_pchan(ts, as_pchan);
+	if (!ts_is_capable_of_pchan(ts, as_pchan))
+		return false;
+
+	if (!allow_pchan_switch && ts->pchan_is != as_pchan)
+		return false;
+
+	return true;
 }
-- 
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