From e9a8dc7c6ecf9d3b642cb968c757b9c102bc7422 Mon Sep 17 00:00:00 2001
From: Jussi Kivilinna <jussi.kivilinna@haltian.com>
Date: Tue, 11 Apr 2017 06:39:27 -0600
Subject: [PATCH] STM32F7: serial: disallow broken configuration combination of
 CONFIG_STM32F7_FLOWCONTROL_BROKEN=y and CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS
 not set.

---
 arch/arm/src/stm32f7/Kconfig        |  2 +-
 arch/arm/src/stm32f7/stm32_serial.c | 63 +++++++++++++++++------------
 2 files changed, 39 insertions(+), 26 deletions(-)

diff --git a/arch/arm/src/stm32f7/Kconfig b/arch/arm/src/stm32f7/Kconfig
index d8eabdcd4b..2c21fadf61 100644
--- a/arch/arm/src/stm32f7/Kconfig
+++ b/arch/arm/src/stm32f7/Kconfig
@@ -1634,7 +1634,7 @@ config SERIAL_DISABLE_REORDERING
 
 config STM32F7_FLOWCONTROL_BROKEN
 	bool "Use Software UART RTS flow control"
-	depends on STM32F7_USART
+	depends on STM32F7_USART && SERIAL_IFLOWCONTROL_WATERMARKS
 	default n
 	---help---
 		Enable UART RTS flow control using Software. Because STM
diff --git a/arch/arm/src/stm32f7/stm32_serial.c b/arch/arm/src/stm32f7/stm32_serial.c
index ec97ec7096..ea67ddb4db 100644
--- a/arch/arm/src/stm32f7/stm32_serial.c
+++ b/arch/arm/src/stm32f7/stm32_serial.c
@@ -220,43 +220,56 @@
 
 /* Warnings for potentially unsafe configuration combinations. */
 
+#if defined(CONFIG_STM32F7_FLOWCONTROL_BROKEN) && \
+    !defined(CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS)
+#  error "CONFIG_STM32F7_FLOWCONTROL_BROKEN requires \
+          CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS to be enabled."
+#endif
+
+#ifndef CONFIG_STM32F7_FLOWCONTROL_BROKEN
 /* Combination of RXDMA + IFLOWCONTROL does not work as one might expect.
  * Since RXDMA uses circular DMA-buffer, DMA will always keep reading new
  * data from USART peripheral even if DMA buffer underruns. Thus this
  * combination only does following: RTS is asserted on USART setup and
  * deasserted on shutdown and does not perform actual RTS flow-control.
+ *
+ * With SW flow-control, RTS is asserted before UART receive buffer fully
+ * fills, thus preventing data loss if application is slow to process data
+ * from serial device node. However, if RxDMA interrupt is blocked for too
+ * long, data loss is still possible as SW flow-control would also be
+ * blocked.
  */
 
-#if defined(CONFIG_USART1_RXDMA) && defined(CONFIG_USART1_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for USART1. \
-            This combination can lead to data loss."
-#endif
+# if defined(CONFIG_USART1_RXDMA) && defined(CONFIG_USART1_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for USART1. \
+              This combination can lead to data loss."
+#  endif
 
-#if defined(CONFIG_USART2_RXDMA) && defined(CONFIG_USART2_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for USART2. \
-            This combination can lead to data loss."
-#endif
+#  if defined(CONFIG_USART2_RXDMA) && defined(CONFIG_USART2_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for USART2. \
+              This combination can lead to data loss."
+#  endif
 
-#if defined(CONFIG_USART3_RXDMA) && defined(CONFIG_USART3_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for USART3. \
-            This combination can lead to data loss."
-#endif
+#  if defined(CONFIG_USART3_RXDMA) && defined(CONFIG_USART3_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for USART3. \
+              This combination can lead to data loss."
+#  endif
 
-#if defined(CONFIG_USART6_RXDMA) && defined(CONFIG_USART6_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for USART6. \
-            This combination can lead to data loss."
-#endif
+#  if defined(CONFIG_USART6_RXDMA) && defined(CONFIG_USART6_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for USART6. \
+              This combination can lead to data loss."
+#  endif
 
-#if defined(CONFIG_UART7_RXDMA) && defined(CONFIG_UART7_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for UART7. \
-            This combination can lead to data loss."
-#endif
-
-#if defined(CONFIG_UART8_RXDMA) && defined(CONFIG_UART8_IFLOWCONTROL)
-#  warning "RXDMA and IFLOWCONTROL both enabled for UART8. \
-            This combination can lead to data loss."
-#endif
+#  if defined(CONFIG_UART7_RXDMA) && defined(CONFIG_UART7_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for UART7. \
+              This combination can lead to data loss."
+#  endif
 
+#  if defined(CONFIG_UART8_RXDMA) && defined(CONFIG_UART8_IFLOWCONTROL)
+#    warning "RXDMA and IFLOWCONTROL both enabled for UART8. \
+              This combination can lead to data loss."
+#  endif
+#endif /* CONFIG_STM32F7_FLOWCONTROL_BROKEN */
 
 /****************************************************************************
  * Private Types