serial: 8250: Flush DMA Rx on RLSI

commit 1980860 upstream.

Returning true from handle_rx_dma() without flushing DMA first creates
a data ordering hazard. If DMA Rx has handled any character at the
point when RLSI occurs, the non-DMA path handles any pending characters
jumping them ahead of those characters that are pending under DMA.

Fixes: 75df022 ("serial: 8250_dma: Fix RX handling")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221108121952.5497-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -1896,10 +1896,9 @@ static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
 		if (!up->dma->rx_running)
 			break;
 		fallthrough;
+	case UART_IIR_RLSI:
 	case UART_IIR_RX_TIMEOUT:
 		serial8250_rx_dma_flush(up);
-		fallthrough;
-	case UART_IIR_RLSI:
 		return true;
 	}
 	return up->dma->rx_dma(up);