sysmo: emit empty RTP ticks during FACCH stealing on TCH/F

When FACCH stealing occurs and sysmoBTS L1 delivers GsmL1_Sapi_FacchF
instead of GsmL1_Sapi_TchF, that 20 ms unit still needs to be
accounted for in the RTP timestamp cadence, and if we run with
rtp continuous-streaming enabled, then an actual BFI packet needs
to be emitted.  The original code failed to do either; the present
change implements correct behavior for TCH/F.

The present patch only covers the case of TCH/F; handling of TCH/H
is left as a FIXME for other/later developers.

Related: OS#5974
Change-Id: I39d15faade28fb7d670493a99a0e0bdb654e2a4a
---
M src/osmo-bts-sysmo/l1_if.c
M src/osmo-bts-sysmo/l1_if.h
M src/osmo-bts-sysmo/tch.c
3 files changed, 64 insertions(+), 0 deletions(-)

diff --git a/src/osmo-bts-sysmo/l1_if.c b/src/osmo-bts-sysmo/l1_if.c
index 19f0c5a..24a3d1e 100644
--- a/src/osmo-bts-sysmo/l1_if.c
+++ b/src/osmo-bts-sysmo/l1_if.c
@@ -980,6 +980,25 @@
 		return rc;
 	}
 
+	/* If we got FACCH, the RTP clock needs to account for it,
+	 * and if we have rtp continuous-streaming enabled,
+	 * an actual BFI packet will be emitted.
+	 *
+	 * Only the case of TCH/F is currently handled; the task of
+	 * supporting TCH/H is left as a FIXME for other/later
+	 * developers.  The difficulty with supporting FACCH/H is that
+	 * only one GsmL1_Sapi_FacchH message will be received,
+	 * covering 40 ms of time, but two RTP "tick" output calls
+	 * will need to be made, with appropriately adjusted frame
+	 * numbers.  As a further consideration for rtp continuous-streaming
+	 * mode, having the two RTP BFI packets sent directly back to back,
+	 * as opposed to proper 20 ms timing, may be so undesirable
+	 * that some deployments may rather disable TCH/H (use only TCH/F)
+	 * than deal with the extra pain, or use TCH/H only on SDR-based
+	 * BTS with osmo-bts-trx where correct timing is easily achieved. */
+	if (data_ind->sapi == GsmL1_Sapi_FacchF)
+		l1if_tch_rx_facch(trx, chan_nr, l1p_msg);
+
 	/* fill L1SAP header */
 	sap_msg = l1sap_msgb_alloc(data_ind->msgUnitParam.u8Size);
 	l1sap = msgb_l1sap_prim(sap_msg);
diff --git a/src/osmo-bts-sysmo/l1_if.h b/src/osmo-bts-sysmo/l1_if.h
index 8691eef..5b2da04 100644
--- a/src/osmo-bts-sysmo/l1_if.h
+++ b/src/osmo-bts-sysmo/l1_if.h
@@ -129,6 +129,8 @@
 		    const uint8_t *rtp_pl, unsigned int rtp_pl_len, uint32_t fn,
 		    bool use_cache, bool marker);
 int l1if_tch_rx(struct gsm_bts_trx *trx, uint8_t chan_nr, struct msgb *l1p_msg);
+int l1if_tch_rx_facch(struct gsm_bts_trx *trx, uint8_t chan_nr,
+		      struct msgb *l1p_msg);
 int l1if_tch_fill(struct gsm_lchan *lchan, uint8_t *l1_buffer);
 struct msgb *gen_empty_tch_msg(struct gsm_lchan *lchan, uint32_t fn);
 
diff --git a/src/osmo-bts-sysmo/tch.c b/src/osmo-bts-sysmo/tch.c
index 832991f..3e7a1a1 100644
--- a/src/osmo-bts-sysmo/tch.c
+++ b/src/osmo-bts-sysmo/tch.c
@@ -623,6 +623,29 @@
 	return -EINVAL;
 }
 
+/*! \brief provide an RTP empty payload "tick" to upper layers upon FACCH */
+int l1if_tch_rx_facch(struct gsm_bts_trx *trx, uint8_t chan_nr,
+		      struct msgb *l1p_msg)
+{
+	GsmL1_Prim_t *l1p = msgb_l1prim(l1p_msg);
+	GsmL1_PhDataInd_t *data_ind = &l1p->u.phDataInd;
+	struct msgb *rmsg = NULL;
+	struct gsm_lchan *lchan = &trx->ts[L1SAP_CHAN2TS(chan_nr)].lchan[l1sap_chan2ss(chan_nr)];
+
+	if (is_recv_only(lchan->abis_ip.speech_mode))
+		return -EAGAIN;
+
+	LOGPLCFN(lchan, DL1P, LOGL_DEBUG, data_ind->u32Fn, "chan_nr %d Rx FACCH\n", chan_nr);
+	/* Push empty payload to upper layers */
+	rmsg = msgb_alloc_headroom(256, 128, "L1P-to-RTP");
+	return add_l1sap_header(trx, rmsg, lchan, chan_nr, data_ind->u32Fn,
+				data_ind->measParam.fBer * 10000,
+				data_ind->measParam.fLinkQuality * 10,
+				0,	/* suppress RSSI like in osmo-bts-trx */
+				data_ind->measParam.i16BurstTiming * 64,
+				0);
+}
+
 struct msgb *gen_empty_tch_msg(struct gsm_lchan *lchan, uint32_t fn)
 {
 	struct msgb *msg;

