linux-kernel - Re: [PATCH 3/3] serial: 8250_dw: drop bogus uartclk optimisation

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Date:   Fri, 15 Oct 2021 17:25:31 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] serial: 8250_dw: drop bogus uartclk optimisation

On Fri, Oct 15, 2021 at 01:14:22PM +0200, Johan Hovold wrote:
> 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.

Acked-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Signed-off-by: Johan Hovold <johan@...nel.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
> 

-- 
With Best Regards,
Andy Shevchenko