[PATCH] serial: 8250_dma: Clear stale RX state on shutdown

[Cunhao Lu]

serial8250_release_dma() terminates RX DMA and releases the channel, but
leaves rx_running set.  If the port is closed while an RX transfer is
active, the stale state remains while rxchan is NULL until the channel is
requested again on the next open.

The DesignWare BUSY workaround added by commit a7b9ce39fbe4
("serial: 8250_dw: Ensure BUSY is deasserted") calls
serial8250_rx_dma_flush() from the LCR write path during startup.  This
happens before serial8250_request_dma() obtains a new RX channel.  On
reopen, the stale rx_running state therefore makes the flush path pass a
NULL channel to dmaengine_pause(), causing a kernel Oops.

Clear rx_running after terminating RX DMA, matching the TX cleanup.  Also
make the flush helper return if the DMA object or RX channel is not
available so startup and teardown paths cannot pass a NULL channel to the
DMAengine API.

Fixes: 0fcb7901f9d6 ("tty: serial: 8250_dma: keep own book keeping about RX transfers")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Cunhao Lu <1579567540@xxxxxx>
---
 drivers/tty/serial/8250/8250_dma.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 3b6452e759d5..5a83e5269b41 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -211,11 +211,12 @@ void serial8250_rx_dma_flush(struct uart_8250_port *p)
 {
 	struct uart_8250_dma *dma = p->dma;
 
-	if (dma->rx_running) {
-		dmaengine_pause(dma->rxchan);
-		__dma_rx_complete(p);
-		dmaengine_terminate_async(dma->rxchan);
-	}
+	if (!dma || !dma->rxchan || !dma->rx_running)
+		return;
+
+	dmaengine_pause(dma->rxchan);
+	__dma_rx_complete(p);
+	dmaengine_terminate_async(dma->rxchan);
 }
 EXPORT_SYMBOL_GPL(serial8250_rx_dma_flush);
 
@@ -324,6 +325,7 @@ void serial8250_release_dma(struct uart_8250_port *p)
 
 	/* Release RX resources */
 	dmaengine_terminate_sync(dma->rxchan);
+	dma->rx_running = 0;
 	dma_free_coherent(dma->rxchan->device->dev, dma->rx_size, dma->rx_buf,
 			  dma->rx_addr);
 	dma_release_channel(dma->rxchan);

---
base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
change-id: 20260727-master-992af64f2a97

Best regards,
--  
Cunhao Lu <1579567540@xxxxxx>