From 252d6ccfd350e616c3353856c1390d3a7913e3ac Mon Sep 17 00:00:00 2001
From: "Dr.-Ing. Amilcar do Carmo Lucas" <amilcar.lucas@iav.de>
Date: Thu, 8 Jul 2021 15:25:00 +0200
Subject: [PATCH] AP_HAL_ChibiOS: get the last time when the tx write buffer
 got emptied

---
 libraries/AP_HAL_ChibiOS/UARTDriver.cpp | 5 +++++
 libraries/AP_HAL_ChibiOS/UARTDriver.h   | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index a04537fdbddaf..9a247e5d82782 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -146,6 +146,11 @@ void UARTDriver::uart_thread()
         if (_tx_initialised && ((mask & EVT_TRANSMIT_DATA_READY) || need_tick || (hd_tx_active && (mask & EVT_TRANSMIT_END)))) {
             _tx_timer_tick();
         }
+
+        // record the time when a half-duplex TX buffer got emptied
+        if (_tx_initialised && hd_tx_active && (mask & EVT_TRANSMIT_END) && _writebuf.is_empty()) {
+            _last_write_completed_us = now;
+        }
     }
 }
 #pragma GCC diagnostic pop
diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.h b/libraries/AP_HAL_ChibiOS/UARTDriver.h
index 1a045fa9e7e81..e88e626b8300d 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.h
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.h
@@ -131,6 +131,13 @@ class ChibiOS::UARTDriver : public AP_HAL::UARTDriver {
      */
     bool is_dma_enabled() const override { return rx_dma_enabled && tx_dma_enabled; }
 
+    // get the last time when the tx write buffer got emptied
+#if CH_CFG_USE_EVENTS == TRUE
+    uint32_t get_last_tx_empty_us() const override { return ((tx_dma_enabled || half_duplex) && _writebuf.is_empty()) ? _last_write_completed_us : 0; }
+#else
+    uint32_t get_last_tx_empty_us() const override { return (tx_dma_enabled && _writebuf.is_empty()) ? _last_write_completed_us : 0; }
+#endif
+
 private:
     const SerialDef &sdef;
     bool rx_dma_enabled;