host/trxcon/scheduler: implement SCH decoding

Change-Id: Ic5e7416271d647752cd1aaf90de51fa48286798e

2 files changed, 73 insertions, 2 deletions

diff --git a/src/host/trxcon/sched_lchan_desc.c b/src/host/trxcon/sched_lchan_desc.c
index 93927a63..1ff6e144 100644
--- a/src/host/trxcon/sched_lchan_desc.c
+++ b/src/host/trxcon/sched_lchan_desc.c
@@ -44,6 +44,10 @@ int rx_data_fn(struct trx_instance *trx, struct trx_ts *ts,
 	uint32_t fn, enum trx_lchan_type chan, uint8_t bid,
 	sbit_t *bits, uint16_t nbits, int8_t rssi, float toa);
 
+int rx_sch_fn(struct trx_instance *trx, struct trx_ts *ts,
+	uint32_t fn, enum trx_lchan_type chan, uint8_t bid,
+	sbit_t *bits, uint16_t nbits, int8_t rssi, float toa);
+
 const struct trx_lchan_desc trx_lchan_desc[_TRX_CHAN_MAX] = {
 	{
 		TRXC_IDLE,		"IDLE",
@@ -69,8 +73,11 @@ const struct trx_lchan_desc trx_lchan_desc[_TRX_CHAN_MAX] = {
 		0x00,			LID_DEDIC,
 		0x00,			0x00,
 
-		/* We already have clock indications from TRX */
-		NULL,			NULL,
+		/**
+		 * We already have clock indications from TRX,
+		 * but we also need BSIC (BCC / NCC) value.
+		 */
+		rx_sch_fn,		NULL,
 	},
 	{
 		TRXC_BCCH,		"BCCH",
diff --git a/src/host/trxcon/sched_lchan_handlers.c b/src/host/trxcon/sched_lchan_handlers.c
index 2c2cb6a7..41e60d8b 100644
--- a/src/host/trxcon/sched_lchan_handlers.c
+++ b/src/host/trxcon/sched_lchan_handlers.c
@@ -34,6 +34,7 @@
 #include <osmocom/core/msgb.h>
 #include <osmocom/core/fsm.h>
 
+#include <osmocom/gsm/gsm_utils.h>
 #include <osmocom/coding/gsm0503_coding.h>
 
 #include "l1ctl_proto.h"
@@ -159,3 +160,66 @@ int rx_data_fn(struct trx_instance *trx, struct trx_ts *ts,
 	/* TODO: AGC, TA loops */
 	return 0;
 }
+
+static void decode_sb(struct gsm_time *time, uint8_t *bsic, uint8_t *sb_info)
+{
+	uint8_t t3p;
+	uint32_t sb;
+
+	sb = (sb_info[3] << 24)
+	   | (sb_info[2] << 16)
+	   | (sb_info[1] << 8)
+	   | sb_info[0];
+
+	*bsic = (sb >> 2) & 0x3f;
+
+	/* TS 05.02 Chapter 3.3.2.2.1 SCH Frame Numbers */
+	time->t1 = ((sb >> 23) & 0x01)
+		 | ((sb >> 7) & 0x1fe)
+		 | ((sb << 9) & 0x600);
+
+	time->t2 = (sb >> 18) & 0x1f;
+
+	t3p = ((sb >> 24) & 0x01) | ((sb >> 15) & 0x06);
+	time->t3 = t3p * 10 + 1;
+
+	/* TS 05.02 Chapter 4.3.3 TDMA frame number */
+	time->fn = gsm_gsmtime2fn(time);
+}
+
+int rx_sch_fn(struct trx_instance *trx, struct trx_ts *ts,
+	uint32_t fn, enum trx_lchan_type chan, uint8_t bid,
+	sbit_t *bits, uint16_t nbits, int8_t rssi, float toa)
+{
+	sbit_t payload[2 * 39];
+	struct gsm_time time;
+	uint8_t sb_info[4];
+	uint8_t bsic;
+	int rc;
+
+	/* Obtain payload from burst */
+	memcpy(payload, bits + 3, 39);
+	memcpy(payload + 39, bits + 3 + 39 + 64, 39);
+
+	/* Attempt to decode */
+	rc = gsm0503_sch_decode(sb_info, payload);
+	if (rc) {
+		LOGP(DSCH, LOGL_DEBUG, "Received bad SCH burst at fn=%u\n", fn);
+		return rc;
+	}
+
+	/* Decode BSIC and TDMA frame number */
+	decode_sb(&time, &bsic, sb_info);
+
+	LOGP(DSCH, LOGL_DEBUG, "Received SCH: bsic=%u, fn=%u, sched_fn=%u\n",
+		bsic, time.fn, trx->sched.fn_counter_proc);
+
+	/* Check if decoded frame number matches */
+	if (time.fn != fn) {
+		LOGP(DSCH, LOGL_ERROR, "Decoded fn=%u does not match "
+			"fn=%u provided by scheduler\n", time.fn, fn);
+		return -EINVAL;
+	}
+
+	return 0;
+}