tty/serial: atmel: fix race condition (TX+DMA)

commit 31ca2c63 upstream.

If uart_flush_buffer() is called between atmel_tx_dma() and
atmel_complete_tx_dma(), the circular buffer has been cleared, but not
atmel_port->tx_len.
That leads to a circular buffer overflow (dumping (UART_XMIT_SIZE -
atmel_port->tx_len) bytes).
Tested-by: Nicolas Ferre <nicolas.ferre@microchip.com>
[rg] backport to 3.12
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/tty/serial/atmel_serial.c

@@ -1690,6 +1690,11 @@ static void atmel_flush_buffer(struct uart_port *port)
 		UART_PUT_TCR(port, 0);
 		atmel_port->pdc_tx.ofs = 0;
 	}
+	/*
+	 * in uart_flush_buffer(), the xmit circular buffer has just
+	 * been cleared, so we have to reset its length accordingly.
+	 */
+	sg_dma_len(&atmel_port->sg_tx) = 0;
 }
 
 /*