[PATCH AUTOSEL 3.18 64/98] tty: serial: fsl_lpuart: fix clearing of receive flag

Sasha Levin
2018-10-25T14:18:19Z

From: Stefan Agner <stefan@agner.ch>

[ Upstream commit d68827c62a105eec547945daedf4d1d3e283717d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/serial/fsl_lpuart.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index af3047d8322a..92b7a5bf7c4d 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -910,13 +910,16 @@ 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);

--
2.17.1