diff mbox series

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

Message ID 20220712115306.26471-6-kabel@kernel.org
State New
Headers show
Series ftdi_sio driver improvements | expand

Commit Message

Marek Behún July 12, 2022, 11:53 a.m. UTC 
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(-)

Comments

Johan Hovold July 24, 2022, 12:26 p.m. UTC | #1 
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
diff mbox series

Patch

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