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

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

[Peter Collingbourne]

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@google.com>
Link: https://linux-review.googlesource.com/id/I2e3761d239cbf29ed41412e5338f30bffe1efc70
Fixes: 4e26b134bd17 ("serial: 8250_dw: clock rate handling for all ACPI platforms")
Cc: stable@vger.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

Re: [PATCH] serial: 8250_dw: Do not reclock if already at correct rate

[Andy Shevchenko]

On Thu, Feb 22, 2024 at 11:26:34AM -0800, Peter Collingbourne wrote:
> 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.

Makes sense to me from code perspective.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH] serial: 8250_dw: Do not reclock if already at correct rate

[Greg Kroah-Hartman]

On Thu, Feb 22, 2024 at 11:26:34AM -0800, Peter Collingbourne wrote:
> 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@google.com>
> Link: https://linux-review.googlesource.com/id/I2e3761d239cbf29ed41412e5338f30bffe1efc70

Again, why the link?  What does that help with?  Our tools add a Link:
tag to this public email thread, not a google gerrit instance.

thanks,

greg k-h