stm32: serial Allow configuring Rx DMA buffer size

arch/arm/src/stm32/Kconfig

@@ -6165,6 +6165,19 @@ if STM32_SERIALDRIVER
 
 comment "Serial Driver Configuration"
 
+config STM32_SERIAL_RXDMA_BUFFER_SIZE
+	int "Rx DMA buffer size"
+	default 32
+	range 32 4096
+	depends on USART1_RXDMA || USART2_RXDMA || USART3_RXDMA || UART4_RXDMA || UART5_RXDMA || USART6_RXDMA || UART7_RXDMA || UART8_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 or this number of bytes.
+
+		Value given here will be rounded up to next multiple of 4 bytes.
+
 config SERIAL_DISABLE_REORDERING
 	bool "Disable reordering of ttySx devices."
 	depends on STM32_USART1 || STM32_USART2 || STM32_USART3 || STM32_UART4 || STM32_UART5 || STM32_USART6 || STM32_UART7 || STM32_UART8

arch/arm/src/stm32/stm32_serial.c

@@ -182,8 +182,12 @@
  * When streaming data, the generic serial layer will be called
  * every time the FIFO receives half this number of bytes.
  */
-
-#  define RXDMA_BUFFER_SIZE   32
+#  if !defined(CONFIG_STM32_SERIAL_RXDMA_BUFFER_SIZE)
+#    define CONFIG_STM32_SERIAL_RXDMA_BUFFER_SIZE 32
+#  endif
+#  define RXDMA_MUTIPLE  4
+#  define RXDMA_BUFFER_SIZE   ((CONFIG_STM32_SERIAL_RXDMA_BUFFER_SIZE \
+                                + RXDMA_MUTIPLE) & ~RXDMA_MUTIPLE)
 
 /* DMA priority */