feat: process RC packets back from CRSF module as trainer input (#5724)

i.e. allow head tracker data to be relayed via backpack!

radio/src/telemetry/crossfire.cpp

@@ -22,9 +22,15 @@
 #include "crossfire.h"
 #include "edgetx.h"
 
+#include "trainer.h"
+
 // clang-format off
 #define CS(id,subId,name,unit,precision) {id,subId,unit,precision,name}
 
+#define CROSSFIRE_CH_BITS           11
+#define CROSSFIRE_CH_MASK           ((1 << CROSSFIRE_CH_BITS) - 1)
+#define CROSSFIRE_CH_CENTER         0x3E0
+
 const CrossfireSensor crossfireSensors[] = {
   CS(LINK_ID,        0, STR_SENSOR_RX_RSSI1,      UNIT_DB,                0),
   CS(LINK_ID,        1, STR_SENSOR_RX_RSSI2,      UNIT_DB,                0),
@@ -194,6 +200,27 @@ void processCrossfireTelemetryFrame(uint8_t module, uint8_t* rxBuffer,
       }
       break;
 
+    case CHANNELS_ID:
+      if (g_model.trainerData.mode == TRAINER_MODE_CRSF) {
+        uint8_t inputbitsavailable = 0;
+        uint32_t inputbits = 0;
+        uint8_t  byteIdx = 3;
+        int16_t *pulses = trainerInput;
+
+        for (int i = 0; i < min(CROSSFIRE_CHANNELS_COUNT, MAX_TRAINER_CHANNELS); i++) {
+          while (inputbitsavailable < CROSSFIRE_CH_BITS) {
+            inputbits |= (uint32_t)(rxBuffer[byteIdx++]) << inputbitsavailable;
+            inputbitsavailable += 8;
+          }
+          *pulses++ = ((int32_t)(inputbits & CROSSFIRE_CH_MASK) - CROSSFIRE_CH_CENTER) * 5 / 8;
+          inputbitsavailable -= CROSSFIRE_CH_BITS;
+          inputbits >>= CROSSFIRE_CH_BITS;
+        }
+
+        trainerResetTimer();
+      }
+      break;
+
     case LINK_RX_ID:
       if (getCrossfireTelemetryValue<1>(4, value, rxBuffer))
         processCrossfireTelemetryValue(RX_RSSI_PERC_INDEX, value);