serial: 8250_dw: drop bogus uartclk optimisation

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.

Tested-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211015111422.1027-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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) {
 		/*
-		 * Preliminary 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);