tty: serial: fsl_lpuart: fix DMA operation when using IOMMU

The DMA channel might not be available at probe time. This is esp. the
case if the DMA controller has an IOMMU mapping.

There is also another caveat. If there is no DMA controller at all,
dma_request_chan() will also return -EPROBE_DEFER. Thus we cannot test
for -EPROBE_DEFER in probe(). Otherwise the lpuart driver will fail to
probe if, for example, the DMA driver is not enabled in the kernel
configuration.

To workaround this, we request the DMA channel in _startup(). Other
serial drivers do it the same way.
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20200306214433.23215-2-michael@walle.ccSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -1493,8 +1493,21 @@ static void rx_dma_timer_init(struct lpuart_port *sport)
 static void lpuart_tx_dma_startup(struct lpuart_port *sport)
 {
 	u32 uartbaud;
+	int ret;
+
+	sport->dma_tx_chan = dma_request_chan(sport->port.dev, "tx");
+	if (IS_ERR(sport->dma_tx_chan)) {
+		dev_info_once(sport->port.dev,
+			      "DMA tx channel request failed, operating without tx DMA (%ld)\n",
+			      PTR_ERR(sport->dma_tx_chan));
+		sport->dma_tx_chan = NULL;
+		goto err;
+	}
+
+	ret = lpuart_dma_tx_request(&sport->port);
+	if (!ret)
+		goto err;
 
-	if (sport->dma_tx_chan && !lpuart_dma_tx_request(&sport->port)) {
 	init_waitqueue_head(&sport->dma_wait);
 	sport->lpuart_dma_tx_use = true;
 	if (lpuart_is_32(sport)) {
@@ -1505,14 +1518,30 @@ static void lpuart_tx_dma_startup(struct lpuart_port *sport)
 		writeb(readb(sport->port.membase + UARTCR5) |
 		       UARTCR5_TDMAS, sport->port.membase + UARTCR5);
 	}
-	} else {
+
+	return;
+
+err:
 	sport->lpuart_dma_tx_use = false;
-	}
 }
 
 static void lpuart_rx_dma_startup(struct lpuart_port *sport)
 {
-	if (sport->dma_rx_chan && !lpuart_start_rx_dma(sport)) {
+	int ret;
+
+	sport->dma_rx_chan = dma_request_chan(sport->port.dev, "rx");
+	if (IS_ERR(sport->dma_rx_chan)) {
+		dev_info_once(sport->port.dev,
+			      "DMA rx channel request failed, operating without rx DMA (%ld)\n",
+			      PTR_ERR(sport->dma_rx_chan));
+		sport->dma_rx_chan = NULL;
+		goto err;
+	}
+
+	ret = lpuart_start_rx_dma(sport);
+	if (ret)
+		goto err;
+
 	/* set Rx DMA timeout */
 	sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
 	if (!sport->dma_rx_timeout)
@@ -1520,9 +1549,11 @@ static void lpuart_rx_dma_startup(struct lpuart_port *sport)
 
 	sport->lpuart_dma_rx_use = true;
 	rx_dma_timer_init(sport);
-	} else {
+
+	return;
+
+err:
 	sport->lpuart_dma_rx_use = false;
-	}
 }
 
 static int lpuart_startup(struct uart_port *port)
@@ -1615,6 +1646,11 @@ static void lpuart_dma_shutdown(struct lpuart_port *sport)
 			dmaengine_terminate_all(sport->dma_tx_chan);
 		}
 	}
+
+	if (sport->dma_tx_chan)
+		dma_release_channel(sport->dma_tx_chan);
+	if (sport->dma_rx_chan)
+		dma_release_channel(sport->dma_rx_chan);
 }
 
 static void lpuart_shutdown(struct uart_port *port)
@@ -2520,16 +2556,6 @@ static int lpuart_probe(struct platform_device *pdev)
 
 	sport->port.rs485_config(&sport->port, &sport->port.rs485);
 
-	sport->dma_tx_chan = dma_request_slave_channel(sport->port.dev, "tx");
-	if (!sport->dma_tx_chan)
-		dev_info(sport->port.dev, "DMA tx channel request failed, "
-				"operating without tx DMA\n");
-
-	sport->dma_rx_chan = dma_request_slave_channel(sport->port.dev, "rx");
-	if (!sport->dma_rx_chan)
-		dev_info(sport->port.dev, "DMA rx channel request failed, "
-				"operating without rx DMA\n");
-
 	return 0;
 
 failed_attach_port: