[PATCH 5.10 414/452] serial: pch: dont overwrite xmit->buf[0] by x_char

[Greg Kroah-Hartman]

From: Jiri Slaby <jslaby@xxxxxxx>

commit d9f3af4fbb1d955bbaf872d9e76502f6e3e803cb upstream.

When x_char is to be sent, the TX path overwrites whatever is in the
circular buffer at offset 0 with x_char and sends it using
pch_uart_hal_write(). I don't understand how this was supposed to work
if xmit->buf[0] already contained some character. It must have been
lost.

Remove this whole pop_tx_x() concept and do the work directly in the
callers. (Without printing anything using dev_dbg().)

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 3c6a483275f4 (Serial: EG20T: add PCH_UART driver)
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Link: https://lore.kernel.org/r/20220503080808.28332-1-jslaby@xxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/serial/pch_uart.c |   27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -628,22 +628,6 @@ static int push_rx(struct eg20t_port *pr
 	return 0;
 }
 
-static int pop_tx_x(struct eg20t_port *priv, unsigned char *buf)
-{
-	int ret = 0;
-	struct uart_port *port = &priv->port;
-
-	if (port->x_char) {
-		dev_dbg(priv->port.dev, "%s:X character send %02x (%lu)\n",
-			__func__, port->x_char, jiffies);
-		buf[0] = port->x_char;
-		port->x_char = 0;
-		ret = 1;
-	}
-
-	return ret;
-}
-
 static int dma_push_rx(struct eg20t_port *priv, int size)
 {
 	int room;
@@ -893,9 +877,10 @@ static unsigned int handle_tx(struct eg2
 
 	fifo_size = max(priv->fifo_size, 1);
 	tx_empty = 1;
-	if (pop_tx_x(priv, xmit->buf)) {
-		pch_uart_hal_write(priv, xmit->buf, 1);
+	if (port->x_char) {
+		pch_uart_hal_write(priv, &port->x_char, 1);
 		port->icount.tx++;
+		port->x_char = 0;
 		tx_empty = 0;
 		fifo_size--;
 	}
@@ -950,9 +935,11 @@ static unsigned int dma_handle_tx(struct
 	}
 
 	fifo_size = max(priv->fifo_size, 1);
-	if (pop_tx_x(priv, xmit->buf)) {
-		pch_uart_hal_write(priv, xmit->buf, 1);
+
+	if (port->x_char) {
+		pch_uart_hal_write(priv, &port->x_char, 1);
 		port->icount.tx++;
+		port->x_char = 0;
 		fifo_size--;
 	}