From d128d8482048e39e13631c13e58849799f205077 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Sat, 30 Dec 2023 13:34:57 +1100
Subject: [PATCH] HAL_SITL: fixed unbuffered UART writes in SITL

this was causing PPP to slow down by about 1000x due to duplicate
writes
---
 libraries/AP_HAL_SITL/UARTDriver.cpp | 1 +
 libraries/AP_HAL_SITL/UARTDriver.h   | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/libraries/AP_HAL_SITL/UARTDriver.cpp b/libraries/AP_HAL_SITL/UARTDriver.cpp
index 1fa4d0f9685672..9b7516af6938bb 100644
--- a/libraries/AP_HAL_SITL/UARTDriver.cpp
+++ b/libraries/AP_HAL_SITL/UARTDriver.cpp
@@ -799,6 +799,7 @@ uint16_t UARTDriver::read_from_async_csv(uint8_t *buffer, uint16_t space)
 
 void UARTDriver::handle_writing_from_writebuffer_to_device()
 {
+    WITH_SEMAPHORE(write_mtx);
     if (!_connected) {
         _check_reconnect();
         return;
diff --git a/libraries/AP_HAL_SITL/UARTDriver.h b/libraries/AP_HAL_SITL/UARTDriver.h
index 7555ae596b9a06..70d1fa332f4b27 100644
--- a/libraries/AP_HAL_SITL/UARTDriver.h
+++ b/libraries/AP_HAL_SITL/UARTDriver.h
@@ -109,6 +109,8 @@ class HALSITL::UARTDriver : public AP_HAL::UARTDriver {
     uint32_t last_read_tick_us;
     uint32_t last_write_tick_us;
 
+    HAL_Semaphore write_mtx;
+
     SITL::SerialDevice *_sim_serial_device;
 
     struct {