linux-kernel - [PATCH v2 2/2] serial: core: enable FIFO after resume

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Message-Id: <20230405111559.110220-3-lma@semihalf.com>
Date:   Wed,  5 Apr 2023 13:15:59 +0200
From:   Lukasz Majczak <lma@...ihalf.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        upstream@...ihalf.com, Lukasz Majczak <lma@...ihalf.com>,
        stable@...r.kernel.org
Subject: [PATCH v2 2/2] serial: core: enable FIFO after resume

The "serial/8250: Use fifo in 8250 console driver" commit
has revealed an issue of not re-enabling FIFO after resume.
The problematic path is inside uart_resume_port() function.
First, when the console device is re-enabled,
a call to uport->ops->set_termios() internally initializes FIFO
(in serial8250_do_set_termios()), although further code
disables it by issuing ops->startup() (pointer to serial8250_do_startup,
internally calling serial8250_clear_fifos()).
There is even a comment saying "Clear the FIFO buffers and disable them.
(they will be reenabled in set_termios())", but in this scenario,
set_termios() has been called already and FIFO remains disabled.

This patch address the issue by reversing the order - first checks
if tty port is suspended and performs actions accordingly
(e.g. call to ops->startup()), then tries to re-enable
the console device after suspend (and call to uport->ops->set_termios()).

Signed-off-by: Lukasz Majczak <lma@...ihalf.com>
Cc: <stable@...r.kernel.org> # 6.1+
---
 drivers/tty/serial/serial_core.c | 54 ++++++++++++++++----------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 394a05c09d87..57a153adba3a 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2406,33 +2406,6 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
 	put_device(tty_dev);
 	uport->suspended = 0;
 
-	/*
-	 * Re-enable the console device after suspending.
-	 */
-	if (uart_console(uport)) {
-		/*
-		 * First try to use the console cflag setting.
-		 */
-		memset(&termios, 0, sizeof(struct ktermios));
-		termios.c_cflag = uport->cons->cflag;
-		termios.c_ispeed = uport->cons->ispeed;
-		termios.c_ospeed = uport->cons->ospeed;
-
-		/*
-		 * If that's unset, use the tty termios setting.
-		 */
-		if (port->tty && termios.c_cflag == 0)
-			termios = port->tty->termios;
-
-		if (console_suspend_enabled)
-			uart_change_pm(state, UART_PM_STATE_ON);
-		uport->ops->set_termios(uport, &termios, NULL);
-		if (!console_suspend_enabled && uport->ops->start_rx)
-			uport->ops->start_rx(uport);
-		if (console_suspend_enabled)
-			console_start(uport->cons);
-	}
-
 	if (tty_port_suspended(port)) {
 		const struct uart_ops *ops = uport->ops;
 		int ret;
@@ -2471,6 +2444,33 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
 		tty_port_set_suspended(port, false);
 	}
 
+	/*
+	 * Re-enable the console device after suspending.
+	 */
+	if (uart_console(uport)) {
+		/*
+		 * First try to use the console cflag setting.
+		 */
+		memset(&termios, 0, sizeof(struct ktermios));
+		termios.c_cflag = uport->cons->cflag;
+		termios.c_ispeed = uport->cons->ispeed;
+		termios.c_ospeed = uport->cons->ospeed;
+
+		/*
+		 * If that's unset, use the tty termios setting.
+		 */
+		if (port->tty && termios.c_cflag == 0)
+			termios = port->tty->termios;
+
+		if (console_suspend_enabled)
+			uart_change_pm(state, UART_PM_STATE_ON);
+		uport->ops->set_termios(uport, &termios, NULL);
+		if (!console_suspend_enabled && uport->ops->start_rx)
+			uport->ops->start_rx(uport);
+		if (console_suspend_enabled)
+			console_start(uport->cons);
+	}
+
 	mutex_unlock(&port->mutex);
 
 	return 0;
-- 
2.40.0.577.gac1e443424-goog