[3.16.y-ckt stable] Patch "tty: serial: fsl_lpuart: clear receive flag on FIFO flush" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    tty: serial: fsl_lpuart: clear receive flag on FIFO flush

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 96fbe89f47b363e7881c1124a7b441bdc11b352b Mon Sep 17 00:00:00 2001
From: Stefan Agner <stefan at agner.ch>
Date: Fri, 13 Mar 2015 14:51:51 +0100
Subject: tty: serial: fsl_lpuart: clear receive flag on FIFO flush

commit 8e4934c6d6c659e22b1b746af4196683e77ce6ca upstream.

When the receiver was enabled during startup, a character could
have been in the FIFO when the UART get initially used. The
driver configures the (receive) watermark level, and flushes the
FIFO. However, the receive flag (RDRF) could still be set at that
stage (as mentioned in the register description of UARTx_RWFIFO).
This leads to an interrupt which won't be handled properly in
interrupt mode: The receive interrupt function lpuart_rxint checks
the FIFO count, which is 0 at that point (due to the flush
during initialization). The problem does not manifest when using
DMA to receive characters.

Fix this situation by explicitly read the status register, which
leads to clearing of the RDRF flag. Due to the flush just after
the status flag read, a explicit data read is not to required.

Signed-off-by: Stefan Agner <stefan at agner.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/tty/serial/fsl_lpuart.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 05a033e7446c..2b49f2abd8b5 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -608,6 +608,9 @@ 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);