osmo-bts-trx: rx_{tchh,tchf}_fn(): use AMR CMI lookup tables

3GPP TS 45.009 defines that Codec Mode Indications shall be sent
with speech frames having specific TDMA frame numbers of their
*first* bursts, which are defined in tables 3.2.1.3-{1,2,3,4}.

Performance-wise it's batter to have these tables implemented as
arrays, rather then using the 'switch' statement.  We can simplify
things even further and have TDMA frame numbers corresponding to
the *last* bursts in them.  This eliminates the need of doing an
additional last-to-first mapping, so that bi->fn can be used.

Change-Id: I46def864729c8f9063af201750456771ea5558d5

src/osmo-bts-trx/sched_lchan_tchf.c

@@ -131,8 +131,7 @@ int rx_tchf_fn(struct l1sched_ts *l1ts, const struct trx_ul_burst_ind *bi)
 		 * the first FN 4,13,21 defines that CMR is included in frame.
 		 * NOTE: A frame ends 7 FN after start.
 		 */
-		fn_begin = trx_sched_lookup_fn(chan_state, 8);
-		amr_is_cmr = !ul_amr_fn_is_cmi(fn_begin);
+		amr_is_cmr = !sched_tchf_ul_amr_cmi_map[bi->fn % 26];
 
 		/* The AFS_ONSET frame itself does not result into an RTP frame
 		 * since it only contains a recognition pattern that marks the

src/osmo-bts-trx/sched_lchan_tchh.c

@@ -165,16 +165,7 @@ int rx_tchh_fn(struct l1sched_ts *l1ts, const struct trx_ul_burst_ind *bi)
 			break;
 		}
 
-		/* Calculate the frame number where the block begins.  Note that
-		 * we need to traverse the measurement histort back by 6 bursts,
-		 * not by 4 bursts.  The reason for this is that the burst shift
-		 * buffer we use for decoding is 6 bursts wide (one SACCH block) but
-		 * TCH/H blocks are only 4 bursts wide.  The decoder functions look
-		 * at the beginning of the buffer while we shift into it at the end,
-		 * this means that TCH/H blocks always decoded delayed by two frame
-		 * number positions late. */
-		fn_begin = trx_sched_lookup_fn(chan_state, 6);
-		fn_is_cmi = ul_amr_fn_is_cmi(fn_begin);
+		fn_is_cmi = sched_tchh_ul_amr_cmi_map[bi->fn % 26];
 
 		/* See comment in function rx_tchf_fn() */
 		amr = 2;

src/osmo-bts-trx/sched_utils.h

@@ -37,6 +37,24 @@ static inline uint16_t compute_ber10k(int n_bits_total, int n_errors)
 		return 10000 * n_errors / n_bits_total;
 }
 
+/* 3GPP TS 45.009, table 3.2.1.3-{1,3}: AMR on Uplink TCH/F */
+static const uint8_t sched_tchf_ul_amr_cmi_map[26] = {
+	[7]  = 1, /* TCH/F: first=0  / last=7 */
+	[16] = 1, /* TCH/F: first=8  / last=16 */
+	[24] = 1, /* TCH/F: first=17 / last=24 */
+};
+
+/* 3GPP TS 45.009, table 3.2.1.3-{2,4}: AMR on Uplink TCH/H */
+static const uint8_t sched_tchh_ul_amr_cmi_map[26] = {
+	[6]  = 1, /* TCH/H(0): first=0  / last=6 */
+	[15] = 1, /* TCH/H(0): first=8  / last=15 */
+	[23] = 1, /* TCH/H(0): first=17 / last=23 */
+
+	[7]  = 1, /* TCH/H(1): first=1  / last=7 */
+	[16] = 1, /* TCH/H(1): first=9  / last=16 */
+	[24] = 1, /* TCH/H(1): first=18 / last=24 */
+};
+
 /*! determine whether an uplink AMR block is CMI according to 3GPP TS 45.009.
  *  \param[in] fn_begin frame number of the beginning of the block.
  *  \returns true in case of CMI; false otherwise. */