linux-kernel - Re: [PATCH v2] serial: atmel: fix incorrect baudrate setup

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Message-ID: <97b5621c-f75c-a19d-0759-b19a8a7eb83f@gmail.com>
Date:   Thu, 12 Jan 2023 17:15:10 +0100
From:   Richard Genoud <richard.genoud@...il.com>
To:     Tobias Schramm <t.schramm@...jaro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] serial: atmel: fix incorrect baudrate setup

Le 09/01/2023 à 08:29, Tobias Schramm a écrit :
> Commit ba47f97a18f2 ("serial: core: remove baud_rates when serial console
> setup") changed uart_set_options to select the correct baudrate
> configuration based on the absolute error between requested baudrate and
> available standard baudrate settings.
> Prior to that commit the baudrate was selected based on which predefined
> standard baudrate did not exceed the requested baudrate.
> This change of selection logic was never reflected in the atmel serial
> driver. Thus the comment left in the atmel serial driver is no longer
> accurate.
> Additionally the manual rounding up described in that comment and applied
> via (quot - 1) requests an incorrect baudrate. Since uart_set_options uses
> tty_termios_encode_baud_rate to determine the appropriate baudrate flags
> this can cause baudrate selection to fail entirely because
> tty_termios_encode_baud_rate will only select a baudrate if relative error
> between requested and selected baudrate does not exceed +/-2%.
> Fix that by requesting actual, exact baudrate used by the serial.
> 
> Fixes: ba47f97a18f2 ("serial: core: remove baud_rates when serial console setup")
> Signed-off-by: Tobias Schramm <t.schramm@...jaro.org>
Acked-by: Richard Genoud <richard.genoud@...il.com>

Tested-on sam9g35
> ---
>  drivers/tty/serial/atmel_serial.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index f1c06e12efa0..9cd7479b03c0 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -2657,13 +2657,7 @@ static void __init atmel_console_get_options(struct uart_port *port, int *baud,
>  	else if (mr == ATMEL_US_PAR_ODD)
>  		*parity = 'o';
>  
> -	/*
> -	 * The serial core only rounds down when matching this to a
> -	 * supported baud rate. Make sure we don't end up slightly
> -	 * lower than one of those, as it would make us fall through
> -	 * to a much lower baud rate than we really want.
> -	 */
> -	*baud = port->uartclk / (16 * (quot - 1));
> +	*baud = port->uartclk / (16 * quot);
>  }
>  
>  static int __init atmel_console_setup(struct console *co, char *options)
Thanks !


Regards,
Richard