linux-kernel - [PATCH] serial: 8250_dw: Do not reclock if already at correct rate

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Message-Id: <20240222192635.1050502-1-pcc@google.com>
Date: Thu, 22 Feb 2024 11:26:34 -0800
From: Peter Collingbourne <pcc@...gle.com>
To: "Ilpo Järvinen" <ilpo.jarvinen@...ux.intel.com>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc: Peter Collingbourne <pcc@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH] serial: 8250_dw: Do not reclock if already at correct rate

When userspace opens the console, we call set_termios() passing a
termios with the console's configured baud rate. Currently this causes
dw8250_set_termios() to disable and then re-enable the UART clock at
the same frequency as it was originally. This can cause corruption
of any concurrent console output. Fix it by skipping the reclocking
if we are already at the correct rate.

Signed-off-by: Peter Collingbourne <pcc@...gle.com>
Link: https://linux-review.googlesource.com/id/I2e3761d239cbf29ed41412e5338f30bffe1efc70
Fixes: 4e26b134bd17 ("serial: 8250_dw: clock rate handling for all ACPI platforms")
Cc: stable@...r.kernel.org
---
 drivers/tty/serial/8250/8250_dw.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 2d1f350a4bea..c1d43f040c43 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -357,9 +357,9 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	long rate;
 	int ret;
 
-	clk_disable_unprepare(d->clk);
 	rate = clk_round_rate(d->clk, newrate);
-	if (rate > 0) {
+	if (rate > 0 && p->uartclk != rate) {
+		clk_disable_unprepare(d->clk);
 		/*
 		 * Note that any clock-notifer worker will block in
 		 * serial8250_update_uartclk() until we are done.
@@ -367,8 +367,8 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 		ret = clk_set_rate(d->clk, newrate);
 		if (!ret)
 			p->uartclk = rate;
+		clk_prepare_enable(d->clk);
 	}
-	clk_prepare_enable(d->clk);
 
 	dw8250_do_set_termios(p, termios, old);
 }
-- 
2.44.0.rc1.240.g4c46232300-goog