[PATCH 3/3] serial: 8250_dw: drop bogus uartclk optimisation

From: Johan Hovold <johan@kernel.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Serge Semin <Sergey.Semin@baikalelectronics.ru>,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	Johan Hovold <johan@kernel.org>
Subject: [PATCH 3/3] serial: 8250_dw: drop bogus uartclk optimisation
Date: Fri, 15 Oct 2021 13:14:22 +0200	[thread overview]
Message-ID: <20211015111422.1027-4-johan@kernel.org> (raw)
In-Reply-To: <20211015111422.1027-1-johan@kernel.org>

The driver was updating the port uartclk before setting the new rate in
an attempt to avoid having the clock notifier redundantly update the
divisors.

The set_termios() callback is however called under the termios semaphore
and tty-port mutex so the worker scheduled by the clock notifier will
block in serial8250_update_uartclk() until the uartclk and divisors have
been updated anyway.

Drop the unnecessary swaps and incorrect comment and simply update the
uartclk field if the clock-rate change was successful.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/tty/serial/8250/8250_dw.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index a3a0154da567..52e03dd0eb3b 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -338,15 +338,12 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	rate = clk_round_rate(d->clk, newrate);
 	if (rate > 0) {
 		/*
-		 * Premilinary set the uartclk to the new clock rate so the
-		 * clock update event handler caused by the clk_set_rate()
-		 * calling wouldn't actually update the UART divisor since
-		 * we about to do this anyway.
+		 * Note that any clock-notifer worker will block in
+		 * serial8250_update_uartclk() until we are done.
 		 */
-		swap(p->uartclk, rate);
 		ret = clk_set_rate(d->clk, newrate);
-		if (ret)
-			swap(p->uartclk, rate);
+		if (!ret)
+			p->uartclk = rate;
 	}
 	clk_prepare_enable(d->clk);
 
-- 
2.32.0

