AP_RCTelemetry: warn if using CRSF on non-DMA channel

don't produce rf_mode messages when not using CRSF for RC control

libraries/AP_RCTelemetry/AP_CRSF_Telem.cpp

@@ -148,8 +148,19 @@ void AP_CRSF_Telem::process_rf_mode_changes()
 {
     const AP_RCProtocol_CRSF::RFMode current_rf_mode = get_rf_mode();
     uint32_t now = AP_HAL::millis();
+
+    // the presence of a uart indicates that we are using CRSF for RC control
+    AP_RCProtocol_CRSF* crsf = AP::crsf();
+    AP_HAL::UARTDriver* uart = nullptr;
+    if (crsf != nullptr) {
+        uart = crsf->get_UART();
+    }
+    // warn the user if their setup is sub-optimal
+    if (_telem_last_report_ms == 0 && uart != nullptr && !uart->is_dma_enabled()) {
+        gcs().send_text(MAV_SEVERITY_WARNING, "CRSF: running on non-DMA serial port");
+    }
     // report a change in RF mode or a chnage of more than 10Hz if we haven't done so in the last 5s
-    if ((now - _telem_last_report_ms > 5000) &&
+    if ((now - _telem_last_report_ms > 5000) && uart != nullptr &&
         (_telem_rf_mode != current_rf_mode || abs(int16_t(_telem_last_avg_rate) - int16_t(_scheduler.avg_packet_rate)) > 25)) {
         gcs().send_text(MAV_SEVERITY_INFO, "CRSF: RF mode %d, rate is %dHz", (uint8_t)current_rf_mode, _scheduler.avg_packet_rate);
         update_custom_telemetry_rates(current_rf_mode);