From 6fd8c28522c7c5143d2384a91a2116b4d0738102 Mon Sep 17 00:00:00 2001
From: overkillfpv <t.hitchins@exemail.com.au>
Date: Sat, 7 Mar 2026 12:48:22 +1100
Subject: [PATCH] Cache preamble SF to avoid redundant setPreambleLength() on
 every TX

---
 src/helpers/radiolib/RadioLibWrappers.cpp | 9 +++++++--
 src/helpers/radiolib/RadioLibWrappers.h   | 3 ++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/helpers/radiolib/RadioLibWrappers.cpp b/src/helpers/radiolib/RadioLibWrappers.cpp
index 9260e6df..883cc623 100644
--- a/src/helpers/radiolib/RadioLibWrappers.cpp
+++ b/src/helpers/radiolib/RadioLibWrappers.cpp
@@ -26,7 +26,8 @@ void setFlag(void) {
 
 void RadioLibWrapper::begin() {
   _radio->setPacketReceivedAction(setFlag);  // this is also SentComplete interrupt
-  _radio->setPreambleLength(getSpreadingFactor() <= 8 ? 32 : 16); // longer preamble for lower SF improves reliability
+  _preamble_sf = getSpreadingFactor();
+  _radio->setPreambleLength(_preamble_sf <= 8 ? 32 : 16); // longer preamble for lower SF improves reliability
   state = STATE_IDLE;
 
   if (_board->getStartupReason() == BD_STARTUP_RX_PACKET) {  // received a LoRa packet (while in deep sleep)
@@ -143,7 +144,11 @@ uint32_t RadioLibWrapper::getEstAirtimeFor(int len_bytes) {
 
 bool RadioLibWrapper::startSendRaw(const uint8_t* bytes, int len) {
   _board->onBeforeTransmit();
-  _radio->setPreambleLength(getSpreadingFactor() <= 8 ? 32 : 16); // keep preamble in sync with current SF
+  uint8_t sf = getSpreadingFactor();
+  if (sf != _preamble_sf) {
+    _preamble_sf = sf;
+    _radio->setPreambleLength(sf <= 8 ? 32 : 16); // update preamble when SF has changed
+  }
   int err = _radio->startTransmit((uint8_t *) bytes, len);
   if (err == RADIOLIB_ERR_NONE) {
     state = STATE_TX_WAIT;
diff --git a/src/helpers/radiolib/RadioLibWrappers.h b/src/helpers/radiolib/RadioLibWrappers.h
index c9791a4b..0dbc8897 100644
--- a/src/helpers/radiolib/RadioLibWrappers.h
+++ b/src/helpers/radiolib/RadioLibWrappers.h
@@ -11,6 +11,7 @@ protected:
   int16_t _noise_floor, _threshold;
   uint16_t _num_floor_samples;
   int32_t _floor_sample_sum;
+  uint8_t _preamble_sf;
 
   void idle();
   void startRecv();
@@ -19,7 +20,7 @@ protected:
   virtual void doResetAGC();
 
 public:
-  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board) { n_recv = n_sent = 0; }
+  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board), _preamble_sf(0) { n_recv = n_sent = 0; }
 
   void begin() override;
   virtual void powerOff() { _radio->sleep(); }