linux-kernel - [PATCH 3/8] tty: serial: fsl_lpuart: fix clearing of receive flag

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Date:	Thu, 9 Jun 2016 20:40:34 +0530
From:	Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>
To:	<gregkh@...uxfoundation.org>
CC:	<stefan@...er.ch>, <shawnguo@...nel.org>, <kernel@...gutronix.de>,
	<mturquette@...libre.com>, <sboyd@...eaurora.org>,
	<jslaby@...e.com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-clk@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-serial@...r.kernel.org>,
	Stefan Agner <stefan.agner@...adex.com>
Subject: [PATCH 3/8] tty: serial: fsl_lpuart: fix clearing of receive flag

From: Stefan Agner <stefan.agner@...adex.com>

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@...er.ch>
---
 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 75a2098..97c1fda 100644
--- 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);
 
-- 
2.8.3