serial: 8250_dma: power off device after TX is done

When any 8250 based driver sets up DMA and has UART_CAP_RPM capability enabled
the device is left powered on after transfer is done. We need to schedule a
device suspend operation when DMA completes the transfer.

The patch is based on the work done by the reporter.

Reported-by: Huiquan Zhong <huiquan.zhong@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dma.c

@@ -72,10 +72,15 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 	struct dma_async_tx_descriptor	*desc;
 	int ret;
 
-	if (uart_tx_stopped(&p->port) || dma->tx_running ||
-	    uart_circ_empty(xmit))
+	if (dma->tx_running)
 		return 0;
 
+	if (uart_tx_stopped(&p->port) || uart_circ_empty(xmit)) {
+		/* We have been called from __dma_tx_complete() */
+		serial8250_rpm_put_tx(p);
+		return 0;
+	}
+
 	dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
 
 	desc = dmaengine_prep_slave_single(dma->txchan,