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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.9 071/310] serial: sh-sci: Fix race condition causing garbage during shutdown
Date:   Wed, 11 Apr 2018 20:33:30 +0200
Message-Id: <20180411183625.365271302@linuxfoundation.org>
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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/sh-sci.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1545,7 +1545,16 @@ static void sci_free_dma(struct uart_por
 	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 = 0;
+}
+#else /* !CONFIG_SERIAL_SH_SCI_DMA */
 static inline void sci_request_dma(struct uart_port *port)
 {
 }
@@ -1553,7 +1562,9 @@ static inline void sci_request_dma(struc
 static inline void sci_free_dma(struct uart_port *port)
 {
 }
-#endif
+
+#define sci_flush_buffer	NULL
+#endif /* !CONFIG_SERIAL_SH_SCI_DMA */
 
 static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
 {
@@ -2551,6 +2562,7 @@ static const struct uart_ops sci_uart_op
 	.break_ctl	= sci_break_ctl,
 	.startup	= sci_startup,
 	.shutdown	= sci_shutdown,
+	.flush_buffer	= sci_flush_buffer,
 	.set_termios	= sci_set_termios,
 	.pm		= sci_pm,
 	.type		= sci_type,