tty: serial: fsl_lpuart: fix clearing of receive flag

Commit 8e4934c6d6c6 ("tty: serial: fsl_lpuart: clear receive flag on FIFO
flush") implemented clearing of the receive flag by reading the status register
only. It turned out that even though we flush the FIFO afterwards, a explicit
read of the data register is still required.

This leads to a FIFO underrun. To avoid this, follow the advice in the overrun
"Operation section": Unconditionally clear RXUF after using RXFLUSH.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index e46cffd..7164632 100644 (file)
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -935,13 +935,16 @@ static void lpuart_setup_watermark(struct lpuart_port *sport)
        writeb(val | UARTPFIFO_TXFE | UARTPFIFO_RXFE,
                        sport->port.membase + UARTPFIFO);
 
-       /* explicitly clear RDRF */
-       readb(sport->port.membase + UARTSR1);
-
        /* flush Tx and Rx FIFO */
        writeb(UARTCFIFO_TXFLUSH | UARTCFIFO_RXFLUSH,
                        sport->port.membase + UARTCFIFO);
 
+       /* explicitly clear RDRF */
+       if (readb(sport->port.membase + UARTSR1) & UARTSR1_RDRF) {
+               readb(sport->port.membase + UARTDR);
+               writeb(UARTSFIFO_RXUF, sport->port.membase + UARTSFIFO);
+       }
+
        writeb(0, sport->port.membase + UARTTWFIFO);
        writeb(1, sport->port.membase + UARTRWFIFO);