protocol(p25): honor explicit FDMA IDENs in frequency mapping

Author: arancormonk

src/protocol/p25/p25_frequency.c

@@ -66,11 +66,13 @@ process_channel_to_freq(dsd_opts* opts, dsd_state* state, int channel) {
     int type = state->p25_chan_type[iden] & 0xF;
     // OP25-derived slots-per-carrier by channel type
     static const int slots_per_carrier[16] = {1, 1, 1, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2};
-    // Derive division only when TDMA iden is known. This matches OP25, which
-    // only divides the channel by tdma when an IDEN_UP_TDMA has populated the
-    // table. Without that knowledge, treat channel as FDMA (denom=1).
+    // Derive division only when TDMA iden is known. Explicit IDEN hints take
+    // precedence over the system-level TDMA fallback so mixed P1/P2 systems do
+    // not halve explicitly FDMA channel numbers.
     int denom = 1;
-    if ((state->p25_chan_tdma[iden] & 0x1) != 0) {
+    int explicit_hint = state->p25_chan_tdma_explicit[iden];
+    int use_tdma_denom = (explicit_hint == 2 || (explicit_hint == 0 && (state->p25_chan_tdma[iden] & 0x1) != 0));
+    if (use_tdma_denom) {
         if (type < 0 || type > 15) {
             fprintf(stderr, "\n  P25 FREQ: unknown iden type %d (iden %d)", type, iden);
             return 0;
@@ -80,7 +82,7 @@ process_channel_to_freq(dsd_opts* opts, dsd_state* state, int channel) {
             fprintf(stderr, "\n  P25 FREQ: invalid slots/carrier for type %d", type);
             return 0;
         }
-    } else {
+    } else if (explicit_hint != 1) {
         // Fallback: if the system is known to carry Phase 2 (TDMA) voice but
         // we have not yet seen an IDEN_UP_TDMA for this iden, assume 2
         // slots/carrier. This avoids early mis-tunes where a TDMA grant
@@ -150,11 +152,13 @@ p25_format_chan_suffix(const dsd_state* state, uint16_t chan, int slot_hint, cha
     int type = state->p25_chan_type[iden] & 0xF;
     static const int slots_per_carrier[16] = {1, 1, 1, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2};
     int denom = 1;
-    if ((state->p25_chan_tdma[iden] & 0x1) != 0) {
+    int explicit_hint = state->p25_chan_tdma_explicit[iden];
+    int use_tdma_denom = (explicit_hint == 2 || (explicit_hint == 0 && (state->p25_chan_tdma[iden] & 0x1) != 0));
+    if (use_tdma_denom) {
         if (type >= 0 && type <= 15) {
             denom = slots_per_carrier[type];
         }
-    } else if (state->p25_sys_is_tdma == 1) {
+    } else if (explicit_hint != 1 && state->p25_sys_is_tdma == 1) {
         // Conservative fallback when TDMA IDEN not yet learned
         denom = 2;
     }

tests/protocol/p25/test_p25_format_chan_suffix.c

@@ -66,5 +66,18 @@ main(void) {
     p25_format_chan_suffix(&st3, ch, -1, buf, sizeof buf);
     rc |= expect_eq_str("fallback denom=2", buf, " (FDMA 0003 S2)");
 
+    // Case 4: Explicit FDMA must not show a TDMA suffix even if the system has
+    // Phase 2 voice and the legacy TDMA bit is stale.
+    static dsd_state st4;
+    memset(&st4, 0, sizeof st4);
+    st4.p25_sys_is_tdma = 1;
+    st4.p25_chan_tdma[id] = 1;
+    st4.p25_chan_tdma_explicit[id] = 1;
+    st4.p25_chan_type[id] = 3;
+    ch = (uint16_t)((id << 12) | 0x000A);
+    memset(buf, 0, sizeof buf);
+    p25_format_chan_suffix(&st4, ch, -1, buf, sizeof buf);
+    rc |= expect_eq_str("explicit fdma suffix empty", buf, "");
+
     return rc;
 }

tests/protocol/p25/test_p25_freq_fallback_tdma.c

@@ -81,5 +81,22 @@ main(void) {
     long want = 851000000 + 3 * 100 * 125; // 851.0375 MHz
     rc |= expect_eq_long("fallback denom2", f, want);
 
+    // Explicit FDMA must override both the TDMA system hint and any stale TDMA
+    // bit for this IDEN. Otherwise mixed P1/P2 systems can tune halfway between
+    // real FDMA channels.
+    static dsd_state st_fdma;
+    memset(&st_fdma, 0, sizeof st_fdma);
+    st_fdma.p25_sys_is_tdma = 1;
+    st_fdma.p25_chan_tdma[id] = 1;
+    st_fdma.p25_chan_tdma_explicit[id] = 1;
+    st_fdma.p25_chan_type[id] = 3;
+    st_fdma.p25_base_freq[id] = 851000000 / 5;
+    st_fdma.p25_chan_spac[id] = 100;
+
+    chan = (id << 12) | 0x000A;
+    f = process_channel_to_freq(&opts, &st_fdma, chan);
+    want = 851000000 + 10 * 100 * 125; // FDMA denom=1
+    rc |= expect_eq_long("explicit fdma no fallback", f, want);
+
     return rc;
 }