Re: [PATCH v2 5/7] USB: serial: ftdi_sio: Fix baud rate rounding for ASYNC_SPD_CUST

[Johan Hovold <johan@kernel.org>]

On Tue, Jul 12, 2022 at 01:53:04PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@kernel.org>
> 
> To compute more accurate baud rate when user uses ASYNC_SPD_CUST API,
> use DIV_ROUND_CLOSEST() instead of just rounding down.
> 
> Rationale:
>   Application uses old API, so it computes divisor D for baud rate B.
> 
>   The driver then tries to compute back the requested baud rate B, but
>   rounds down in the division.
> 
>   Using rounding to closest value instead should increate accuracy here.
> 
> Signed-off-by: Pali Rohár <pali@kernel.org>
> Tested-by: Marek Behún <kabel@kernel.org>
> Signed-off-by: Marek Behún <kabel@kernel.org>
> ---
>  drivers/usb/serial/ftdi_sio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
> index cdbba1a9edd9..5db1293bb7a2 100644
> --- a/drivers/usb/serial/ftdi_sio.c
> +++ b/drivers/usb/serial/ftdi_sio.c
> @@ -1317,7 +1317,7 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
>  	if (baud == 38400 &&
>  	    ((priv->flags & ASYNC_SPD_MASK) == ASYNC_SPD_CUST) &&
>  	     (priv->custom_divisor)) {
> -		baud = priv->baud_base / priv->custom_divisor;
> +		baud = DIV_ROUND_CLOSEST(priv->baud_base, priv->custom_divisor);

Sure, why not, but this should only be used for the following debug
statement. (See next patch).

>  		dev_dbg(dev, "%s - custom divisor %d sets baud rate to %d\n",
>  			__func__, priv->custom_divisor, baud);
>  	}

Johan