From 2e15285c519bf070b9c782f68717310cfae4ee67 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Mon, 22 Jan 2018 07:40:55 +1100
Subject: [PATCH] HAL_ChibiOS: refuse unbuffered writes without DMA

we would need much more complex code to handle this
---
 libraries/AP_HAL_ChibiOS/UARTDriver.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index bf6c8c48ff..a906709d4b 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -686,6 +686,10 @@ void UARTDriver::update_rts_line(void)
  */
 bool UARTDriver::set_unbuffered_writes(bool on)
 {
+    if (on && !sdef.dma_tx) {
+        // we can't implement low latemcy writes safely without TX DMA
+        return false;
+    }
     unbuffered_writes = on;
     return true;
 }