DTX DL: use FSM for AMR

Use dedicated FSM to handle all DTX DL related events:
- add explicit checks if DTX DL is enabled (fixes regression for non-DTX
setup introduced in 654175f33bd412671e3ef8cdd65c0689d10f278c)
- fix handling of AMR CMI for SPEECH frames
- add FSM for DTX DL
- sync with corresponding changes in OpenBSC's
- handle FACCH-related DTX ONSET events

This affects both lc15 and sysmobts and requires corresponding change in
OpenBSC (Change-Id: Idac8609faf9b5ced818fde899ccfc6ed0c42e8fd).

Change-Id: I74a0b42cb34d525b8a70d264135e82994ca70d31

5 files changed, 430 insertions, 56 deletions

diff --git a/src/common/Makefile.am b/src/common/Makefile.am
index 856f7412..dd368d03 100644
--- a/src/common/Makefile.am
+++ b/src/common/Makefile.am
@@ -7,6 +7,7 @@ libbts_a_SOURCES = gsm_data_shared.c sysinfo.c logging.c abis.c oml.c bts.c \
 		   rsl.c vty.c paging.c measurement.c amr.c lchan.c \
 		   load_indication.c pcu_sock.c handover.c msg_utils.c \
 		   tx_power.c bts_ctrl_commands.c bts_ctrl_lookup.c \
-		   l1sap.c cbch.c power_control.c main.c phy_link.c
+		   l1sap.c cbch.c power_control.c main.c phy_link.c \
+		   dtx_dl_amr_fsm.c
 
 libl1sched_a_SOURCES = scheduler.c
diff --git a/src/common/dtx_dl_amr_fsm.c b/src/common/dtx_dl_amr_fsm.c
new file mode 100644
index 00000000..b110cf21
--- /dev/null
+++ b/src/common/dtx_dl_amr_fsm.c
@@ -0,0 +1,320 @@
+/* DTX DL AMR FSM */
+
+/* (C) 2016 by sysmocom s.f.m.c. GmbH
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <osmo-bts/dtx_dl_amr_fsm.h>
+#include <osmo-bts/logging.h>
+
+void dtx_fsm_voice(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_VOICE:
+	case E_FACCH:
+		break;
+	case E_SID_F:
+	case E_SID_U:
+		osmo_fsm_inst_state_chg(fi, ST_SID_F1, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Inexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_sid_f1(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_SID_F:
+/* FIXME: what shall we do if we get SID-FIRST _again_ (twice in a row)?
+   Was observed during testing, let's just ignore it for now */
+		break;
+	case E_SID_U:
+		osmo_fsm_inst_state_chg(fi, ST_SID_U, 0, 0);
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_F, 0, 0);
+		break;
+	case E_COMPL:
+		osmo_fsm_inst_state_chg(fi, ST_SID_F2, 0, 0);
+		break;
+	case E_INHIB:
+		osmo_fsm_inst_state_chg(fi, ST_F1_INH, 0, 0);
+		break;
+	case E_ONSET:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_V, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_sid_f2(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_SID_U:
+		osmo_fsm_inst_state_chg(fi, ST_SID_U, 0, 0);
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_F, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_f1_inh(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_V, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_F, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_u_inh(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_V, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_F, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_sid_upd(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_F, 0, 0);
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_INHIB:
+		osmo_fsm_inst_state_chg(fi, ST_U_INH, 0, 0);
+		break;
+	case E_SID_U:
+	case E_SID_F:
+/* FIXME: what shall we do if we get SID-FIRST _after_ sending SID-UPDATE?
+   Was observed during testing, let's just ignore it for now */
+		break;
+	case E_ONSET:
+		osmo_fsm_inst_state_chg(fi, ST_ONSET_V, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_onset_v(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_SID_U:
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_FACCH_V, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_onset_f(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_SID_U:
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_FACCH:
+		osmo_fsm_inst_state_chg(fi, ST_FACCH, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_facch_v(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_FACCH:
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_SID_U:
+		break;
+	case E_SID_F:
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+void dtx_fsm_facch(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case E_FACCH:
+		break;
+	case E_VOICE:
+		osmo_fsm_inst_state_chg(fi, ST_VOICE, 0, 0);
+		break;
+	case E_SID_U:
+	case E_SID_F:
+		osmo_fsm_inst_state_chg(fi, ST_SID_F1, 0, 0);
+		break;
+	default:
+		LOGP(DL1P, LOGL_ERROR, "Unexpected event %d\n", event);
+		OSMO_ASSERT(0);
+		break;
+	}
+}
+
+static struct osmo_fsm_state dtx_dl_amr_fsm_states[] = {
+	/* default state for non-DTX and DTX when SPEECH is in progress */
+	[ST_VOICE] = {
+		.in_event_mask = X(E_SID_F) | X(E_SID_U) | X(E_VOICE) | X(E_FACCH),
+		.out_state_mask = X(ST_SID_F1),
+		.name = "Voice",
+		.action = dtx_fsm_voice,
+	},
+	/* SID-FIRST or SID-FIRST-P1 in case of AMR HR:
+	   start of silence period (might be interrupted in case of AMR HR) */
+	[ST_SID_F1]= {
+		.in_event_mask = X(E_SID_F) | X(E_SID_U) | X(E_VOICE) | X(E_FACCH) | X(E_COMPL) | X(E_INHIB) | X(E_ONSET),
+		.out_state_mask = X(ST_SID_U) | X(ST_VOICE) | X(ST_ONSET_F) | X(ST_SID_F2) | X(ST_F1_INH) | X(ST_ONSET_V),
+		.name = "SID-FIRST (P1)",
+		.action = dtx_fsm_sid_f1,
+	},
+	/* SID-FIRST P2 (only for AMR HR):
+	   actual start of silence period in case of AMR HR*/
+	[ST_SID_F2]= {
+		.in_event_mask = X(E_SID_U) | X(E_VOICE) | X(E_FACCH),
+		.out_state_mask = X(ST_SID_U) | X(ST_VOICE) | X(ST_ONSET_F),
+		.name = "SID-FIRST (P2)",
+		.action = dtx_fsm_sid_f2,
+	},
+	/* SID-FIRST Inhibited:
+	   incoming SPEECH or FACCH (only for AMR HR) */
+	[ST_F1_INH]= {
+		.in_event_mask = X(E_VOICE) | X(E_FACCH),
+		.out_state_mask = X(ST_VOICE) | X(ST_FACCH_V),
+		.name = "SID-FIRST (Inh)",
+		.action = dtx_fsm_f1_inh,
+	},
+	/* SID-UPDATE Inhibited:
+	   incoming SPEECH or FACCH (only for AMR HR) */
+	[ST_U_INH]= {
+		.in_event_mask = X(E_VOICE) | X(E_FACCH),
+		.out_state_mask = X(ST_VOICE) | X(ST_FACCH_V),
+		.name = "SID-UPDATE (Inh)",
+		.action = dtx_fsm_u_inh,
+	},
+	/* Silence period with periodic comfort noise data updates */
+	[ST_SID_U]= {
+		.in_event_mask = X(E_FACCH) | X(E_VOICE) | X(E_INHIB) | X(E_SID_U) | X(E_SID_F) | X(E_ONSET),
+		.out_state_mask = X(ST_ONSET_F) | X(ST_VOICE) | X(ST_U_INH) | X(ST_SID_U) | X(ST_ONSET_V),
+		.name = "SID-UPDATE",
+		.action = dtx_fsm_sid_upd,
+	},
+	/* ONSET - end of silent period due to incoming SPEECH frame */
+	[ST_ONSET_V]= {
+		.in_event_mask = X(E_VOICE) | X(E_FACCH),
+		.out_state_mask = X(ST_VOICE) | X(ST_FACCH_V),
+		.name = "ONSET (SPEECH)",
+		.action = dtx_fsm_onset_v,
+	},
+	/* ONSET - end of silent period due to incoming FACCH frame */
+	[ST_ONSET_F]= {
+		.in_event_mask = X(E_VOICE) | X(E_FACCH) | X(E_SID_U),
+		.out_state_mask = X(ST_VOICE) | X(ST_FACCH),
+		.name = "ONSET (FACCH)",
+		.action = dtx_fsm_onset_f,
+	},
+	/* FACCH sending state: SPEECH was observed before so once we're done
+	   FSM should get back to VOICE state */
+	[ST_FACCH_V]= {
+		.in_event_mask = X(E_FACCH) | X(E_VOICE) | X(E_SID_U) | X(E_SID_F),
+		.out_state_mask = X(ST_FACCH_V) | X(ST_VOICE) | X(ST_SID_F1),
+		.name = "FACCH (SPEECH)",
+		.action = dtx_fsm_facch_v,
+	},
+	/* FACCH sending state: no SPEECH was observed before so once we're done
+	   FSM should get back to silent period via SID-FIRST */
+	[ST_FACCH]= {
+		.in_event_mask = X(E_FACCH) | X(E_VOICE) | X(E_SID_U) | X(E_SID_F),
+		.out_state_mask = X(ST_FACCH_V) | X(ST_VOICE) | X(ST_SID_F1),
+		.name = "FACCH",
+		.action = dtx_fsm_facch,
+	},
+};
+
+const struct value_string dtx_dl_amr_fsm_event_names[] = {
+	{ E_VOICE,	"Voice" },
+	{ E_FACCH,	"FACCH" },
+	{ E_COMPL,	"Complete P1 -> P2" },
+	{ E_INHIB,	"Inhibit" },
+	{ E_SID_F,	"SID-FIRST" },
+	{ E_SID_U,	"SID-UPDATE" },
+	{ 0, 		NULL }
+};
+
+struct osmo_fsm dtx_dl_amr_fsm = {
+	.name = "DTX DL AMR FSM",
+	.states = dtx_dl_amr_fsm_states,
+	.num_states = ARRAY_SIZE(dtx_dl_amr_fsm_states),
+	.event_names = dtx_dl_amr_fsm_event_names,
+	.log_subsys = DL1C,
+};
diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index dd234eec..f3e620ee 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -39,6 +39,7 @@
 #include <osmo-bts/logging.h>
 #include <osmo-bts/gsm_data.h>
 #include <osmo-bts/l1sap.h>
+#include <osmo-bts/dtx_dl_amr_fsm.h>
 #include <osmo-bts/pcu_if.h>
 #include <osmo-bts/measurement.h>
 #include <osmo-bts/bts.h>
@@ -524,6 +525,7 @@ static int l1sap_ph_rts_ind(struct gsm_bts_trx *trx,
 	uint8_t *p, *si;
 	struct lapdm_entity *le;
 	struct osmo_phsap_prim pp;
+	bool dtxd_facch = false;
 	int rc;
 
 	chan_nr = rts_ind->chan_nr;
@@ -578,8 +580,11 @@ static int l1sap_ph_rts_ind(struct gsm_bts_trx *trx,
 			p[0] = lchan->ms_power_ctrl.current;
 			p[1] = lchan->rqd_ta;
 			le = &lchan->lapdm_ch.lapdm_acch;
-		} else
+		} else {
+			if (lchan->ts->trx->bts->dtxd)
+				dtxd_facch = true;
 			le = &lchan->lapdm_ch.lapdm_dcch;
+		}
 		rc = lapdm_phsap_dequeue_prim(le, &pp);
 		if (rc < 0) {
 			if (L1SAP_IS_LINK_SACCH(link_id)) {
@@ -605,6 +610,10 @@ static int l1sap_ph_rts_ind(struct gsm_bts_trx *trx,
 				memcpy(p, pp.oph.msg->data, GSM_MACBLOCK_LEN);
 				/* check if it is a RR CIPH MODE CMD. if yes, enable RX ciphering */
 				check_for_ciph_cmd(pp.oph.msg, lchan, chan_nr);
+				if (dtxd_facch && lchan->tch.dtx.dl_amr_fsm)
+					osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm,
+							       E_FACCH,
+							       (void *)lchan);
 			}
 			msgb_free(pp.oph.msg);
 		}
@@ -1132,14 +1141,27 @@ int l1sap_chan_act(struct gsm_bts_trx *trx, uint8_t chan_nr, struct tlv_parsed *
 	rc = l1sap_chan_act_dact_modify(trx, chan_nr, PRIM_INFO_ACTIVATE, 0);
 	if (rc)
 		return -RSL_ERR_EQUIPMENT_FAIL;
+
+	/* Init DTX DL FSM if necessary */
+	//FIXME: only do it for AMR TCH/*
+	osmo_fsm_register(&dtx_dl_amr_fsm);
+	lchan->tch.dtx.dl_amr_fsm = osmo_fsm_inst_alloc(&dtx_dl_amr_fsm,
+							tall_bts_ctx, lchan,
+							LOGL_DEBUG, lchan->name);
 	return 0;
 }
 
 int l1sap_chan_rel(struct gsm_bts_trx *trx, uint8_t chan_nr)
 {
+	struct gsm_lchan *lchan = get_lchan_by_chan_nr(trx, chan_nr);
 	LOGP(DL1P, LOGL_INFO, "deactivating channel chan_nr=0x%02x trx=%d\n",
 		chan_nr, trx->nr);
 
+	if (lchan->tch.dtx.dl_amr_fsm) {
+		osmo_fsm_inst_free(lchan->tch.dtx.dl_amr_fsm);
+		lchan->tch.dtx.dl_amr_fsm = NULL;
+	}
+
 	return l1sap_chan_act_dact_modify(trx, chan_nr, PRIM_INFO_DEACTIVATE,
 		0);
 }
diff --git a/src/common/msg_utils.c b/src/common/msg_utils.c
index 99a211fd..f9853c57 100644
--- a/src/common/msg_utils.c
+++ b/src/common/msg_utils.c
@@ -17,6 +17,7 @@
  *
  */
 
+#include <osmo-bts/dtx_dl_amr_fsm.h>
 #include <osmo-bts/msg_utils.h>
 #include <osmo-bts/logging.h>
 #include <osmo-bts/oml.h>
@@ -26,6 +27,7 @@
 #include <osmocom/gsm/protocol/gsm_12_21.h>
 #include <osmocom/gsm/abis_nm.h>
 #include <osmocom/core/msgb.h>
+#include <osmocom/core/fsm.h>
 #include <osmocom/trau/osmo_ortp.h>
 
 #include <arpa/inet.h>
@@ -92,90 +94,119 @@ static int check_manuf(struct msgb *msg, struct abis_om_hdr *omh, size_t msg_siz
 void lchan_set_marker(bool t, struct gsm_lchan *lchan)
 {
 	if (t)
-		lchan->tch.ul_sid = true;
-	else if (lchan->tch.ul_sid) {
-		lchan->tch.ul_sid = false;
+		lchan->tch.dtx.ul_sid = true;
+	else if (lchan->tch.dtx.ul_sid) {
+		lchan->tch.dtx.ul_sid = false;
 		lchan->rtp_tx_marker = true;
 	}
 }
 
-
 /*! \brief Store the last SID frame in lchan context
  *  \param[in] lchan Logical channel on which we check scheduling
  *  \param[in] l1_payload buffer with SID data
  *  \param[in] length length of l1_payload
  *  \param[in] fn Frame Number for which we check scheduling
  *  \param[in] update 0 if SID_FIRST, 1 if SID_UPDATE, -1 if not AMR SID
- *  \param[in] cmr Codec Mode Request (AMR-specific, ignored otherwise)
- *  \param[in] cmi Codec Mode Indication (AMR-specific, ignored otherwise)
  */
-void save_last_sid(struct gsm_lchan *lchan, const uint8_t *l1_payload,
-		   size_t length, uint32_t fn, int update, uint8_t cmr,
-		   int8_t cmi)
+void dtx_cache_payload(struct gsm_lchan *lchan, const uint8_t *l1_payload,
+		       size_t length, uint32_t fn, int update)
 {
 	size_t amr = (update < 0) ? 0 : 2,
-	    copy_len = OSMO_MIN(length + 1, ARRAY_SIZE(lchan->tch.last_sid.buf));
+	    copy_len = OSMO_MIN(length + 1, ARRAY_SIZE(lchan->tch.dtx.cache));
 
-	lchan->tch.last_sid.len = copy_len + amr;
-	lchan->tch.last_sid.fn = fn;
-	lchan->tch.last_sid.is_update = update;
+	lchan->tch.dtx.len = copy_len + amr;
+	lchan->tch.dtx.fn = fn;
+	lchan->tch.dtx.is_update = update;
 
-	if (amr)
-		amr_set_mode_pref(lchan->tch.last_sid.buf, &lchan->tch.amr_mr,
-				  cmi, cmr);
-	memcpy(lchan->tch.last_sid.buf + amr, l1_payload, copy_len);
+	memcpy(lchan->tch.dtx.cache + amr, l1_payload, copy_len);
 }
 
-/*! \brief Check current and cached SID to decide if talkspurt takes place
+/*! \brief Check current state of DTX DL AMR FSM and dispatch necessary events
  *  \param[in] lchan Logical channel on which we check scheduling
  *  \param[in] rtp_pl buffer with RTP data
  *  \param[in] rtp_pl_len length of rtp_pl
  *  \param[in] fn Frame Number for which we check scheduling
  *  \param[in] l1_payload buffer where CMR and CMI prefix should be added
+ *  \param[out] len Length of expected L1 payload
  *  \param[out] ft_out Frame Type to be populated after decoding
- *  \returns 0 if frame should be send immediately (2 byte CMR,CMI prefix added:
- *           caller must adjust length as necessary),
- *           1 if ONSET event is detected
- *           negative if no sending is necessary (either error or cached SID
- *           UPDATE)
+ *  \returns 0 in case of success; negative on error
  */
-int dtx_amr_check_onset(struct gsm_lchan *lchan, const uint8_t *rtp_pl,
+int dtx_dl_amr_fsm_step(struct gsm_lchan *lchan, const uint8_t *rtp_pl,
 			size_t rtp_pl_len, uint32_t fn, uint8_t *l1_payload,
-			uint8_t *ft_out)
+			bool marker, uint8_t *len, uint8_t *ft_out)
 {
 	uint8_t cmr;
 	enum osmo_amr_type ft;
 	enum osmo_amr_quality bfi;
 	int8_t sti, cmi;
-	osmo_amr_rtp_dec(rtp_pl, rtp_pl_len, &cmr, &cmi, &ft, &bfi, &sti);
-	*ft_out = ft; /* only needed for old sysmo firmware */
+	int rc;
 
-	if (ft == AMR_SID) {
-		save_last_sid(lchan, rtp_pl, rtp_pl_len, fn, sti, cmr, cmi);
-		if (sti) /* SID_UPDATE should be cached and send later */
-			return -EAGAIN;
-		else { /* SID_FIRST - cached and send right away */
-			amr_set_mode_pref(l1_payload, &lchan->tch.amr_mr, cmi,
-					  cmr);
-			return 0;
-		}
+	if (rtp_pl == NULL) { /* SID-FIRST P1 -> P2 */
+		*len = 3;
+		memcpy(l1_payload, lchan->tch.dtx.cache, 2);
+		osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm, E_COMPL,
+				       (void *)lchan);
+		return 0;
 	}
 
-	if (ft != AMR_NO_DATA && !osmo_amr_is_speech(ft)) {
-		LOGP(DRTP, LOGL_ERROR, "unsupported AMR FT 0x%02x\n", ft);
-		return -ENOTSUP;
+	rc = osmo_amr_rtp_dec(rtp_pl, rtp_pl_len, &cmr, &cmi, &ft, &bfi, &sti);
+	if (rc < 0) {
+		LOGP(DRTP, LOGL_ERROR, "failed to decode AMR RTP (length %zu)\n",
+		     rtp_pl_len);
+		return rc;
 	}
 
+	/* only needed for old sysmo firmware: */
+	*ft_out = ft;
+
+	/* CMI in downlink tells the L1 encoder which encoding function
+	 * it will use, so we have to use the frame type */
+	if (osmo_amr_is_speech(ft))
+		cmi = ft;
+
+	/* populate L1 payload with CMR/CMI - might be ignored by caller: */
+	amr_set_mode_pref(l1_payload, &lchan->tch.amr_mr, cmi, cmr);
+
+	/* populate DTX cache with CMR/CMI - overwrite cache which will be
+	   either updated or invalidated by caller anyway: */
+	amr_set_mode_pref(lchan->tch.dtx.cache, &lchan->tch.amr_mr, cmi, cmr);
+	*len = 3 + rtp_pl_len;
+
+	/* DTX DL is not enabled, move along */
+	if (!lchan->ts->trx->bts->dtxd)
+		return 0;
+
 	if (osmo_amr_is_speech(ft)) {
-		if (lchan->tch.last_sid.len) { /* force ONSET */
-			lchan->tch.last_sid.len = 0;
-			return 1;
-		}
-		/* We received AMR SPEECH frame - invalidate saved SID */
-		lchan->tch.last_sid.len = 0;
-		amr_set_mode_pref(l1_payload, &lchan->tch.amr_mr, cmi, cmr);
+		if (lchan->tch.dtx.dl_amr_fsm->state == ST_SID_F1 ||
+		    lchan->tch.dtx.dl_amr_fsm->state == ST_SID_U) /* AMR HR */
+			if (lchan->type == GSM_LCHAN_TCH_H && marker)
+				return osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm,
+							      E_INHIB,
+							      (void *)lchan);
+		/* AMR FR */
+		if (marker && lchan->tch.dtx.dl_amr_fsm->state == ST_SID_U)
+			return osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm,
+						      E_ONSET, (void *)lchan);
+		return osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm, E_VOICE,
+					      (void *)lchan);
+	}
+
+	if (ft == AMR_SID) {
+		dtx_cache_payload(lchan, rtp_pl, rtp_pl_len, fn, sti);
+		return osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm,
+					      sti ? E_SID_U : E_SID_F,
+					      (void *)lchan);
+	}
+
+	if (ft != AMR_NO_DATA) {
+		LOGP(DRTP, LOGL_ERROR, "unsupported AMR FT 0x%02x\n", ft);
+		return -ENOTSUP;
 	}
 
+	if (marker)
+		osmo_fsm_inst_dispatch(lchan->tch.dtx.dl_amr_fsm, E_VOICE,
+				       (void *)lchan);
+	*len = 0;
 	return 0;
 }
 
@@ -188,10 +219,10 @@ static inline bool dtx_amr_sid_optional(const struct gsm_lchan *lchan,
 					uint32_t fn)
 {
 	/* Compute approx. time delta based on Fn duration */
-	uint32_t delta = GSM_FN_TO_MS(fn - lchan->tch.last_sid.fn);
+	uint32_t delta = GSM_FN_TO_MS(fn - lchan->tch.dtx.fn);
 
 	/* according to 3GPP TS 26.093 A.5.1.1: */
-	if (lchan->tch.last_sid.is_update) {
+	if (lchan->tch.dtx.is_update) {
 		/* SID UPDATE should be repeated every 8th RTP frame */
 		if (delta < GSM_RTP_FRAME_DURATION_MS * 8)
 			return true;
@@ -252,10 +283,10 @@ uint8_t repeat_last_sid(struct gsm_lchan *lchan, uint8_t *dst, uint32_t fn)
 		if (dtx_amr_sid_optional(lchan, fn))
 			return 0;
 
-	if (lchan->tch.last_sid.len) {
-		memcpy(dst, lchan->tch.last_sid.buf, lchan->tch.last_sid.len);
-		lchan->tch.last_sid.fn = fn;
-		return lchan->tch.last_sid.len + 1;
+	if (lchan->tch.dtx.len) {
+		memcpy(dst, lchan->tch.dtx.cache, lchan->tch.dtx.len);
+		lchan->tch.dtx.fn = fn;
+		return lchan->tch.dtx.len + 1;
 	}
 
 	LOGP(DL1C, LOGL_DEBUG, "Have to send %s frame on TCH but SID buffer "
diff --git a/src/common/rsl.c b/src/common/rsl.c
index d2fea120..6c8f5cc2 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -2199,9 +2199,9 @@ static int rsl_tx_meas_res(struct gsm_lchan *lchan, uint8_t *l3, int l3_len)
 
 	msgb_tv_put(msg, RSL_IE_MEAS_RES_NR, lchan->meas.res_nr++);
 	size_t ie_len = gsm0858_rsl_ul_meas_enc(&lchan->meas.ul_res,
-						lchan->tch.dtxd_active,
+						lchan->tch.dtx.dl_active,
 						meas_res);
-	lchan->tch.dtxd_active = false;
+	lchan->tch.dtx.dl_active = false;
 	if (ie_len >= 3) {
 		msgb_tlv_put(msg, RSL_IE_UPLINK_MEAS, ie_len, meas_res);
 		lchan->meas.flags &= ~LC_UL_M_F_RES_VALID;