tty: serial: pch: don't crash if DMA enabled but not loaded

if the DMA driver isn't loaded "on time" then we crash in the irq handler:
| pch_uart 0000:02:0a.4: pch_request_dma:dma_request_channel FAILS(Tx)
| BUG: unable to handle kernel NULL pointer dereference at   (null)
| IP: [<c0676ed9>] pch_uart_interrupt+0x739/0x940

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index 0aa2b528ef3d..6bb7e90c8f22 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -1508,10 +1508,14 @@ static int pch_uart_verify_port(struct uart_port *port,
                         __func__);
                 return -EOPNOTSUPP;
 #endif
-                dev_info(priv->port.dev, "PCH UART : Use DMA Mode\n");
-                if (!priv->use_dma)
+                if (!priv->use_dma) {
                         pch_request_dma(port);
-                priv->use_dma = 1;
+                        if (priv->chan_rx)
+                                priv->use_dma = 1;
+                }
+                dev_info(priv->port.dev, "PCH UART: %s\n",
+                                priv->use_dma ?
+                                "Use DMA Mode" : "No DMA");
         }
 
         return 0;