early-IA: use the correct TRXneels/early-ia

In early-Immediate-Assignment, the BSC sends the IMM ASS message
directly after it sent the Channel Activation message, and osmo-bts
should cache it until the Channel Activation is complete.

So far the code had a bug: it assumed that the lchan was on the same TRX
where the IMM ASS is transmitted -- but actually, 'trx' refers to the
BCCH channel's TRX, i.e. always c0.

Instead, look up the correct TRX by the ARFCN in the IMM ASS message.

Now, when frequency hopping is enabled, there will be no ARFCN in the
IMM ASS message, hence this fix does not work with frequency hopping.
Related osmo-bsc patch disallows this combination.

(To also support frequency hopping, osmo-bsc would need to modify the
RSL protocol: send the IMM ASS message as a custom IE directly as part
of the Channel Activation. Then it is always possible to correllate the
IMM ASS with a specific trx and lchan, no matter what information it
contains. However, early-IA is a "bad" feature in itself as it
"promotes" having high latency on Abis. It seems unnecessary to do extra
work to also support this odd use case for frequency hopping.)

Related: osmo-bsc I8d375e5155be7b53034d5c0be5566d2f33af5db0
Related: SYS#6655
Change-Id: Id9a930e5c67122812b229dc27ea2bfe246b67611

1 files changed, 35 insertions, 10 deletions

diff --git a/src/common/rsl.c b/src/common/rsl.c
index deeb255c..9405c6ec 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -371,6 +371,16 @@ static bool chan_nr_is_dchan(uint8_t chan_nr)
 		return true;
 }
 
+static struct gsm_bts_trx *trx_lookup_by_arfcn(struct llist_head *trx_list, uint16_t arfcn)
+{
+	struct gsm_bts_trx *trx;
+	llist_for_each_entry(trx, trx_list, list) {
+		if (trx->arfcn == arfcn)
+			return trx;
+	}
+	return NULL;
+}
+
 static struct gsm_lchan *lchan_lookup(struct gsm_bts_trx *trx, uint8_t chan_nr,
 				      const char *log_name)
 {
@@ -1383,18 +1393,33 @@ static int rsl_rx_imm_ass(struct gsm_bts_trx *trx, struct msgb *msg)
 	 * the channel is active. Hence we still wait for the activation, but don't need the Abis roundtrip of Activ ACK
 	 * -> Immediate Assignment via the BSC.
 	 * If anything is wrong with the sizes or the lchan lookup, behave normally, i.e. do not do the RR IA caching,
-	 * but just send the RR message to the MS as-is. */
+	 * but just send the RR message to the MS as-is.
+	 * 'trx' here is the TRX of the BCCH channel. To find the correct TRX for the IMM ASS target, we need to look up
+	 * the ARFCN that is contained in the IMM ASS message. When frequency hopping is enabled, there will not be an
+	 * ARFCN, so we cannot support early-IA with frequency hopping enabled. */
 	if (msg->len >= sizeof(struct gsm48_imm_ass)) {
 		struct gsm48_imm_ass *rr_ia = (void*)msg->data;
-		struct gsm_lchan *ia_target_lchan = lchan_lookup(trx, rr_ia->chan_desc.chan_nr, "Early IA check: ");
-		if (ia_target_lchan && ia_target_lchan->state != LCHAN_S_ACTIVE) {
-			/* Target lchan is not yet active. Cache the IA.
-			 * If a previous IA is still lingering, free it. */
-			msgb_free(ia_target_lchan->early_rr_ia);
-			ia_target_lchan->early_rr_ia = msg;
-
-			/* return 1 means: don't msgb_free() the msg */
-			return 1;
+		if (rr_ia->chan_desc.h0.h == 0) {
+			/* hopping is disabled. */
+			struct gsm_bts_trx *ia_target_trx;
+			uint16_t arfcn;
+			arfcn = (rr_ia->chan_desc.h0.arfcn_high << 8) + rr_ia->chan_desc.h0.arfcn_low;
+
+			ia_target_trx = trx_lookup_by_arfcn(&trx->bts->trx_list, arfcn);
+			if (ia_target_trx) {
+				/* found the ARFCN's trx */
+				struct gsm_lchan *ia_target_lchan;
+				ia_target_lchan = lchan_lookup(ia_target_trx, rr_ia->chan_desc.chan_nr, "Early IA check: ");
+				if (ia_target_lchan && ia_target_lchan->state != LCHAN_S_ACTIVE) {
+					/* Target lchan is not yet active. Cache the IA.
+					 * If a previous IA is still lingering, free it. */
+					msgb_free(ia_target_lchan->early_rr_ia);
+					ia_target_lchan->early_rr_ia = msg;
+
+					/* return 1 means: don't msgb_free() the msg */
+					return 1;
+				}
+			}
 		}
 	}