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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Geert Uytterhoeven <geert+renesas@glider.be>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.4 049/167] serial: sh-sci: Fix race condition
 causing garbage during shutdown
Thread-Topic: [PATCH AUTOSEL for 4.4 049/167] serial: sh-sci: Fix race
 condition causing garbage during shutdown
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Date:   Mon, 19 Mar 2018 16:06:22 +0000
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From: Geert Uytterhoeven <geert+renesas@glider.be>

[ Upstream commit 1cf4a7efdc71cab84c42cfea7200608711ea954f ]

If DMA is enabled and used, a burst of old data may be seen on the
serial console during "poweroff" or "reboot".  uart_flush_buffer()
clears the circular buffer, but sci_port.tx_dma_len is not reset.
This leads to a circular buffer overflow, dumping (UART_XMIT_SIZE -
sci_port.tx_dma_len) bytes.

To fix this, add a .flush_buffer() callback that resets
sci_port.tx_dma_len.

Inspired by commit 31ca2c63fdc0aee7 ("tty/serial: atmel: fix race
condition (TX+DMA)").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 drivers/tty/serial/sh-sci.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 80d0ffe7abc1..b2a7c3d9dcaf 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1455,7 +1455,16 @@ static void sci_free_dma(struct uart_port *port)
 	if (s->chan_rx)
 		sci_rx_dma_release(s, false);
 }
-#else
+
+static void sci_flush_buffer(struct uart_port *port)
+{
+	/*
+	 * In uart_flush_buffer(), the xmit circular buffer has just been
+	 * cleared, so we have to reset tx_dma_len accordingly.
+	 */
+	to_sci_port(port)->tx_dma_len =3D 0;
+}
+#else /* !CONFIG_SERIAL_SH_SCI_DMA */
 static inline void sci_request_dma(struct uart_port *port)
 {
 }
@@ -1463,7 +1472,9 @@ static inline void sci_request_dma(struct uart_port *=
port)
 static inline void sci_free_dma(struct uart_port *port)
 {
 }
-#endif
+
+#define sci_flush_buffer	NULL
+#endif /* !CONFIG_SERIAL_SH_SCI_DMA */
=20
 static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
 {
@@ -2203,6 +2214,7 @@ static struct uart_ops sci_uart_ops =3D {
 	.break_ctl	=3D sci_break_ctl,
 	.startup	=3D sci_startup,
 	.shutdown	=3D sci_shutdown,
+	.flush_buffer	=3D sci_flush_buffer,
 	.set_termios	=3D sci_set_termios,
 	.pm		=3D sci_pm,
 	.type		=3D sci_type,
--=20
2.14.1