serial: sh-sci: Fix race condition causing garbage during shutdown

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>

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 71707e8e6e3ffe768ad76e52b118b74e32ad35e0..21b06cf1f15bc642dd73857e16dc9667301a62ea 100644 (file)
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1558,7 +1558,16 @@ 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 = 0;
+}
+#else /* !CONFIG_SERIAL_SH_SCI_DMA */
 static inline void sci_request_dma(struct uart_port *port)
 {
 }
@@ -1566,7 +1575,9 @@ 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 */
 
 static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
 {
@@ -2581,6 +2592,7 @@ static const struct uart_ops sci_uart_ops = {
        .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,