GSMTAP: Don't log fill frames via GSMTAP

There's very little point in sending fill frames (such as empty PAGING)
or dummy UI frames via GSMTAP all the time.  They serve no purpose other
than to bloat the log files and make it more difficult for users to find
the interesting bits among all this noise.

Change-Id: Icd18dafb235933c9e6aa9d98ddd8fac1522cc9ac

1 files changed, 27 insertions, 0 deletions

diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index 4bf24a5d..97cbac19 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -317,6 +317,28 @@ static int gsmtap_ph_rach(struct osmo_phsap_prim *l1sap, uint8_t *chan_type,
 	return 0;
 }
 
+/* Paging Request 1 with "no identity" content, i.e. empty/dummy paging */
+static const uint8_t paging_fill[GSM_MACBLOCK_LEN] = {
+	0x15, 0x06, 0x21, 0x00, 0x01, 0xf0, 0x2b, 0x2b, 0x2b, 0x2b,
+	0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
+	0x2b, 0x2b, 0x2b };
+
+static bool is_fill_frame(uint8_t chan_type, const uint8_t *data, unsigned int len)
+{
+	switch (chan_type) {
+	case GSMTAP_CHANNEL_AGCH:
+		if (!memcmp(data, fill_frame, GSM_MACBLOCK_LEN))
+			return true;
+		break;
+	case GSMTAP_CHANNEL_PCH:
+		if (!memcmp(data, paging_fill, GSM_MACBLOCK_LEN))
+			return true;
+		break;
+	default:
+		return false;
+	}
+}
+
 static int to_gsmtap(struct gsm_bts_trx *trx, struct osmo_phsap_prim *l1sap)
 {
 	uint8_t *data;
@@ -364,6 +386,11 @@ static int to_gsmtap(struct gsm_bts_trx *trx, struct osmo_phsap_prim *l1sap)
 			return 0;
 	}
 
+	/* don't log fill frames via GSMTAP; they serve no purpose other than
+	 * to clog up your logs */
+	if (is_fill_frame(chan_type, data, len))
+		return 0;
+
 	gsmtap_send(gsmtap, trx->arfcn | uplink, tn, chan_type, ss, fn, 0, 0,
 		data, len);