From 47a5d78397865caa4fbbfcd3699522ce3333a018 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Tue, 16 Jun 2020 18:28:40 +1000
Subject: [PATCH] HAL_ChibiOS: disable DMA on high contention UARTs

switch to interrupt driven when in high contention
---
 libraries/AP_HAL_ChibiOS/UARTDriver.cpp | 12 ++++++++++++
 libraries/AP_HAL_ChibiOS/UARTDriver.h   |  1 +
 2 files changed, 13 insertions(+)

diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 1be107da63..d0bf1b7967 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -901,6 +901,16 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
         chEvtGetAndClearEvents(EVT_TRANSMIT_DMA_COMPLETE);
 
         if (dma_handle->has_contention()) {
+            if (_baudrate <= 115200) {
+                contention_counter += 3;
+                if (contention_counter > 1000) {
+                    // more than 25% of attempts to use this DMA
+                    // channel are getting contention and we have a
+                    // low baudrate. Switch off DMA for future
+                    // transmits on this low baudrate UART
+                    tx_dma_enabled = false;
+                }
+            }
             /*
             someone else is using this same DMA channel. To reduce
             latency we will drop the TX size with DMA on this UART to
@@ -911,6 +921,8 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
             if (tx_len > max_tx_bytes) {
                 tx_len = max_tx_bytes;
             }
+        } else if (contention_counter > 0) {
+            contention_counter--;
         }
 
         chSysLock();
diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.h b/libraries/AP_HAL_ChibiOS/UARTDriver.h
index 4d555ea875..c2c1348c41 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.h
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.h
@@ -178,6 +178,7 @@ private:
     volatile uint8_t rx_bounce_idx;
     uint8_t *rx_bounce_buf[2];
     uint8_t *tx_bounce_buf;
+    uint16_t contention_counter;
 #endif
     ByteBuffer _readbuf{0};
     ByteBuffer _writebuf{0};