STM32L4 serial: Allow configuring Rx DMA buffer size

arch/arm/src/stm32l4/Kconfig

@@ -3414,6 +3414,18 @@ config UART5_RXDMA
 	---help---
 		In high data rate usage, Rx DMA may eliminate Rx overrun errors
 
+config STM32L4_SERIAL_RXDMA_BUFFER_SIZE
+	int "Rx DMA buffer size"
+	default 32
+	depends on USART1_RXDMA || USART2_RXDMA || USART3_RXDMA || UART4_RXDMA || UART5_RXDMA
+	---help---
+		The DMA buffer size when using RX DMA to emulate a FIFO.
+
+		When streaming data, the generic serial layer will be called
+		every time the FIFO receives half this number of bytes.
+
+		Value given here will be rounded up to next multiple of 32 bytes.
+
 config SERIAL_DISABLE_REORDERING
 	bool "Disable reordering of ttySx devices."
 	depends on STM32L4_USART1 || STM32L4_USART2 || STM32L4_USART3 || STM32L4_UART4 || STM32L4_UART5

arch/arm/src/stm32l4/stm32l4_serial.c

@@ -133,9 +133,18 @@
  *
  * When streaming data, the generic serial layer will be called
  * every time the FIFO receives half this number of bytes.
+ *
+ * If there ever is a STM32L4 with D-cache, the buffer size
+ * should be an even multiple of ARMV7M_DCACHE_LINESIZE, so that it
+ * can be individually invalidated.
  */
 
-#  define RXDMA_BUFFER_SIZE   32
+#  if !defined(CONFIG_STM32L4_SERIAL_RXDMA_BUFFER_SIZE) || \
+      CONFIG_STM32L4_SERIAL_RXDMA_BUFFER_SIZE == 0
+#    define RXDMA_BUFFER_SIZE 32
+#  else
+#    define RXDMA_BUFFER_SIZE ((CONFIG_STM32L4_SERIAL_RXDMA_BUFFER_SIZE + 31) & ~31)
+#  endif
 
 /* DMA priority */