ecu: add is_dtx_pause() method

The ECU API of libosmocodec is unfortunately a peculiar non-3GPP
entity: an ECU by itself, severed from Rx DTX handler functions,
which is a logical/conceptual function with no place in the standard
3GPP architecture.  The closest thing that exists in the standard
architecture is the TFO spec (TS 28.062 section C.3.2.1.1) calling
for an ECU application, but not comfort noise generation, in the case
of destination leg doing DTXd - but even then it is not a totally
"pure" ECU like libosmocodec API, it is an ECU plus a SID preener,
and the SID preening transform is not possible within the constraints
of existing libosmocodec ECU API.  Hence truly correct handling of
corner cases, particularly invalid SID, is sadly impossible in the
current libosmocodec ECU framework.

The only current user of this API is the UL path in osmo-bts-trx;
however, as described in OS#6040, we would like to move that ECU call
from osmo-bts-trx model-specific code to the common layer of osmo-bts.
The current osmo-bts-trx incarnation avoids the SID handling problem
by suppressing the call to ECU frame_out() after any SID (valid or
invalid) was received on the air, thus pausing the RTP stream instead
of emitting ECU output during DTXu pauses.  We would like to retain
the same behavior when we move this ECU call to the common layer,
into its proper place _after_ the link quality check in l1sap - but
the current method of flagging post-SID state in osmo-bts-trx will
no longer work on the other side of that link quality check.

As a workaround, have the ECU remember via a separate Boolean flag
whether it is in post-SID state or not (was the most recent frame_in()
any kind of SID or not), and provide is_dtx_pause() method to
retrieve this flag - the relocated ECU call in osmo-bts UL path
will use this new is_dtx_pause() method call to decide if it should
call frame_out() or switch to pausing RTP output.

Related: OS#6040
Change-Id: I3857be84bba12aaca0c2cca91458b7e13c5a642a

include/osmocom/codec/ecu.h

@@ -62,12 +62,16 @@ int osmo_ecu_frame_in(struct osmo_ecu_state *st, bool bfi,
 /* generate output data for a substitute/erroneous frame */
 int osmo_ecu_frame_out(struct osmo_ecu_state *st, uint8_t *frame_out);
 
+/* is the stream handled by this ECU currently in a DTX pause? */
+bool osmo_ecu_is_dtx_pause(struct osmo_ecu_state *st);
+
 struct osmo_ecu_ops {
 	struct osmo_ecu_state * (*init)(void *ctx, enum osmo_ecu_codec codec);
 	void (*destroy)(struct osmo_ecu_state *);
 	int (*frame_in)(struct osmo_ecu_state *st, bool bfi,
 			const uint8_t *frame, unsigned int frame_bytes);
 	int (*frame_out)(struct osmo_ecu_state *st, uint8_t *frame_out);
+	bool (*is_dtx_pause)(struct osmo_ecu_state *st);
 };
 
 int osmo_ecu_register(const struct osmo_ecu_ops *ops, enum osmo_ecu_codec codec);

src/codec/ecu.c

@@ -96,6 +96,20 @@ int osmo_ecu_frame_out(struct osmo_ecu_state *st, uint8_t *frame_out)
 	return g_ecu_ops[st->codec]->frame_out(st, frame_out);
 }
 
+/*! check if the current state of this ECU is a DTX pause.
+ *  \param[in] st ECU state/instance on which to operate
+ *  \return true if DTX pause, false otherwise */
+bool osmo_ecu_is_dtx_pause(struct osmo_ecu_state *st)
+{
+	if (st->codec >= ARRAY_SIZE(g_ecu_ops))
+		return false;
+	if (!g_ecu_ops[st->codec])
+		return false;
+	if (!g_ecu_ops[st->codec]->is_dtx_pause)
+		return false;
+	return g_ecu_ops[st->codec]->is_dtx_pause(st);
+}
+
 /***********************************************************************
  * low-level API for ECU implementations
  ***********************************************************************/

src/codec/ecu_fr.c

@@ -95,6 +95,7 @@ struct fr_ecu_state {
 	uint8_t sid_xmaxc;
 	uint8_t sid_reemit_count;
 	struct osmo_prbs_state prng;
+	bool last_input_was_sid;
 };
 
 /* This function is the frame input to the ECU - all inputs to this
@@ -110,6 +111,7 @@ static void fr_ecu_input(struct fr_ecu_state *fr, const uint8_t *frame)
 	case OSMO_GSM631_SID_CLASS_SPEECH:
 		memcpy(fr->speech_frame, frame, GSM_FR_BYTES);
 		fr->pr_state = STATE_SPEECH;
+		fr->last_input_was_sid = false;
 		return;
 	case OSMO_GSM631_SID_CLASS_INVALID:
 		/* GSM 06.31 section 6.1.2 says: "an invalid SID frame
@@ -136,6 +138,7 @@ static void fr_ecu_input(struct fr_ecu_state *fr, const uint8_t *frame)
 		 * GSM 06.31 an invalid SID is still an accepted SID frame
 		 * for the purpose of "lost SID" logic. */
 		fr->sid_reemit_count = 0;
+		fr->last_input_was_sid = true;
 		return;
 	case OSMO_GSM631_SID_CLASS_VALID:
 		/* save LARc part */
@@ -144,6 +147,7 @@ static void fr_ecu_input(struct fr_ecu_state *fr, const uint8_t *frame)
 		fr->sid_xmaxc = ((frame[27] & 0x1F) << 1) | (frame[28] >> 7);
 		fr->pr_state = STATE_SID;
 		fr->sid_reemit_count = 0;
+		fr->last_input_was_sid = true;
 		return;
 	default:
 		/* There are only 3 possible SID classifications per GSM 06.31
@@ -320,10 +324,18 @@ static int ecu_fr_frame_out(struct osmo_ecu_state *st, uint8_t *frame_out)
 	return GSM_FR_BYTES;
 }
 
+static bool ecu_fr_is_dtx_pause(struct osmo_ecu_state *st)
+{
+	struct fr_ecu_state *fr = (struct fr_ecu_state *) &st->data;
+
+	return fr->last_input_was_sid;
+}
+
 static const struct osmo_ecu_ops osmo_ecu_ops_fr = {
 	.init = ecu_fr_init,
 	.frame_in = ecu_fr_frame_in,
 	.frame_out = ecu_fr_frame_out,
+	.is_dtx_pause = ecu_fr_is_dtx_pause,
 };
 
 static __attribute__((constructor)) void on_dso_load_ecu_fr(void)