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 <redacted>
---
 drivers/tty/serial/fsl_lpuart.c | 84 +++++++++++++++++++++------------
 1 file changed, 53 insertions(+), 31 deletions(-)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index c31b8f3db6bf..0b8c477b32a3 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c

@@ -1493,36 +1493,63 @@ static void rx_dma_timer_init(struct lpuart_port *sport)
 static void lpuart_tx_dma_startup(struct lpuart_port *sport)
 {
 	u32 uartbaud;
+	int ret;
 
-	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)) {
-			uartbaud = lpuart32_read(&sport->port, UARTBAUD);
-			lpuart32_write(&sport->port,
-				       uartbaud | UARTBAUD_TDMAE, UARTBAUD);
-		} else {
-			writeb(readb(sport->port.membase + UARTCR5) |
-				UARTCR5_TDMAS, sport->port.membase + UARTCR5);
-		}
+	sport->dma_tx_chan = dma_request_slave_channel(sport->port.dev, "tx");
+	if (!sport->dma_tx_chan) {
+		dev_info_once(sport->port.dev,
+			      "DMA tx channel request failed, operating without tx DMA\n");