FAILED: patch "[PATCH] serial: pch: don't overwrite xmit->buf[0] by x_char" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d9f3af4fbb1d955bbaf872d9e76502f6e3e803cb Mon Sep 17 00:00:00 2001
From: Jiri Slaby <jirislaby@kernel.org>
Date: Tue, 3 May 2022 10:08:03 +0200
Subject: [PATCH] serial: pch: don't overwrite xmit->buf[0] by x_char

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@vger.kernel.org>
Fixes: 3c6a483275f4 (Serial: EG20T: add PCH_UART driver)
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220503080808.28332-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index f872613a5e83..6cb631487383 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -624,22 +624,6 @@ static int push_rx(struct eg20t_port *priv, const unsigned char *buf,
 	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;
@@ -889,9 +873,10 @@ static unsigned int handle_tx(struct eg20t_port *priv)
 
 	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--;
 	}
@@ -948,9 +933,11 @@ static unsigned int dma_handle_tx(struct eg20t_port *priv)
 	}
 
 	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--;
 	}