On Tue, Jul 12, 2022 at 01:53:04PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Tested-by: Marek Behún <kabel@xxxxxxxxxx>
> Signed-off-by: Marek Behún <kabel@xxxxxxxxxx>
> ---
> 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
