Re: [PATCH v2 1/5] serial: core: tidy invalid baudrate handling in uart_get_baud_rate

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Sep 19, 2023 at 07:26:40PM -0700, Max Filippov wrote:
> uart_get_baud_rate has input parameters 'min' and 'max' limiting the
> range of acceptable baud rates from the caller's perspective. If neither
> current or old termios structures have acceptable baud rate setting and
> 9600 is not in the min/max range either the function returns 0 and
> issues a warning.
> However for a UART that does not support speed of 9600 baud this is
> expected behavior.
> Clarify that 0 can be (and always could be) returned from the
> uart_get_baud_rate. Don't issue a warning in that case.
> Move the warinng to the uart_get_divisor instead, which is often called
> with the uart_get_baud_rate return value.
> 
> Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
> ---
>  drivers/tty/serial/serial_core.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 7bdc21d5e13b..a8e2915832e8 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -431,7 +431,7 @@ EXPORT_SYMBOL(uart_update_timeout);
>   * baud.
>   *
>   * If the new baud rate is invalid, try the @old termios setting. If it's still
> - * invalid, we try 9600 baud.
> + * invalid, we try 9600 baud. If that is also invalid 0 is returned.
>   *
>   * The @termios structure is updated to reflect the baud rate we're actually
>   * going to be using. Don't do this for the case where B0 is requested ("hang
> @@ -515,8 +515,6 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
>  							max - 1, max - 1);
>  		}
>  	}
> -	/* Should never happen */
> -	WARN_ON(1);

I'm ok with this removal, but:

>  	return 0;
>  }
>  EXPORT_SYMBOL(uart_get_baud_rate);
> @@ -539,6 +537,7 @@ uart_get_divisor(struct uart_port *port, unsigned int baud)
>  {
>  	unsigned int quot;
>  
> +	WARN_ON(baud == 0);

Why is this needed?  If this isn't happening today, then there's no need
to check for this here.  Or if it can happen, we should return an error,
not cause a possible reboot of the system if panic-on-warn is enabled.

thanks,

greg k-h