serial: stm32: add RX and TX FIFO flush

Adds a flush of RX and TX FIFOs, and fixes some errors:
- adds RX FIFO flush in startup fonction
- removes the useless transmitter enabling in startup fonction
  (e.g. receiver only, see Documentation/serial/driver)
- configures FIFO threshold before enabling it, rather than after
- flushes both TX and RX in set_termios function
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -599,11 +599,11 @@ static int stm32_startup(struct uart_port *port)
 	if (ret)
 		return ret;
 
-	val = stm32_port->cr1_irq | USART_CR1_TE | USART_CR1_RE;
-	if (stm32_port->fifoen)
-		val |= USART_CR1_FIFOEN;
-	stm32_set_bits(port, ofs->cr1, val);
+	/* RX FIFO Flush */
+	if (ofs->rqr != UNDEF_REG)
+		stm32_set_bits(port, ofs->rqr, USART_RQR_RXFRQ);
 
+	/* Tx and RX FIFO configuration */
 	if (stm32_port->fifoen) {
 		val = readl_relaxed(port->membase + ofs->cr3);
 		val &= ~(USART_CR3_TXFTCFG_MASK | USART_CR3_RXFTCFG_MASK);
@@ -612,6 +612,12 @@ static int stm32_startup(struct uart_port *port)
 		writel_relaxed(val, port->membase + ofs->cr3);
 	}
 
+	/* RX FIFO enabling */
+	val = stm32_port->cr1_irq | USART_CR1_RE;
+	if (stm32_port->fifoen)
+		val |= USART_CR1_FIFOEN;
+	stm32_set_bits(port, ofs->cr1, val);
+
 	return 0;
 }
 
@@ -694,8 +700,12 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
 	/* Stop serial port and reset value */
 	writel_relaxed(0, port->membase + ofs->cr1);
 
-	cr1 = USART_CR1_TE | USART_CR1_RE;
+	/* flush RX & TX FIFO */
+	if (ofs->rqr != UNDEF_REG)
+		stm32_set_bits(port, ofs->rqr,
+			       USART_RQR_TXFRQ | USART_RQR_RXFRQ);
 
+	cr1 = USART_CR1_TE | USART_CR1_RE;
 	if (stm32_port->fifoen)
 		cr1 |= USART_CR1_FIFOEN;
 	cr2 = 0;