serial: stm32: fix rx data length when parity enabled

- Fixes a rx data error when data length < 8 bits and parity is enabled.
RDR register MSB is used for parity bit reception.
- Adds a mask to ignore MSB when data is get from RDR.

Fixes: 34891872 ("serial: stm32: adding dma support")
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

@@ -194,8 +194,8 @@ static int stm32_pending_rx(struct uart_port *port, u32 *sr, int *last_res,
 	return 0;
 }
 
-static unsigned long
-stm32_get_char(struct uart_port *port, u32 *sr, int *last_res)
+static unsigned long stm32_get_char(struct uart_port *port, u32 *sr,
+				    int *last_res)
 {
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
@@ -205,10 +205,13 @@ stm32_get_char(struct uart_port *port, u32 *sr, int *last_res)
 		c = stm32_port->rx_buf[RX_BUF_L - (*last_res)--];
 		if ((*last_res) == 0)
 			*last_res = RX_BUF_L;
-		return c;
 	} else {
-		return readl_relaxed(port->membase + ofs->rdr);
+		c = readl_relaxed(port->membase + ofs->rdr);
+		/* apply RDR data mask */
+		c &= stm32_port->rdr_mask;
 	}
+
+	return c;
 }
 
 static void stm32_receive_chars(struct uart_port *port, bool threaded)
@@ -679,6 +682,7 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
 		cr2 |= USART_CR2_STOP_2B;
 
 	bits = stm32_get_databits(termios);
+	stm32_port->rdr_mask = (BIT(bits) - 1);
 
 	if (cflag & PARENB) {
 		bits++;

drivers/tty/serial/stm32-usart.h

@@ -254,6 +254,7 @@ struct stm32_port {
 	bool hw_flow_control;
 	bool fifoen;
 	int wakeirq;
+	int rdr_mask;		/* receive data register mask */
 };
 
 static struct stm32_port stm32_ports[STM32_MAX_PORTS];