[PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"

[Ferry Toth <ftoth@exalondelft.nl>]

This reverts commit 7885af04df6e4efd72910200c1bfc079d61202e4.

Commit 7885af04df6e ("serial: 8250_port: Drop duplicate NULL check")
added `up->dma = NULL;` to serial8250_release_dma(), with the stated
intent of making sure DMA would not be used again after release.

However, up->dma is assigned once at probe time.  serial8250_do_startup()
only calls serial8250_request_dma() when up->dma is non-NULL, so after
the first close the pointer stays NULL and DMA is never re-acquired.

On Intel Tangier (Edison) with HSU DMA this causes a regression: the
first port open uses TX DMA, the second and later opens transmit in PIO
mode, producing 64-byte FIFO bursts with ~50us gaps between them.

Fixes: 7885af04df6e ("serial: 8250_port: Drop duplicate NULL check")
Cc: stable@vger.kernel.org
Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
---
 drivers/tty/serial/8250/8250_port.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 38fa45e74a37..19d0663ee756 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2428,8 +2428,8 @@ void serial8250_do_shutdown(struct uart_port *port)
 
 	synchronize_irq(port->irq);
 
-	serial8250_release_dma(up);
-	up->dma = NULL;
+	if (up->dma)
+		serial8250_release_dma(up);
 
 	scoped_guard(uart_port_lock_irqsave, port) {
 		if (port->flags & UPF_FOURPORT) {
-- 
2.53.0