[v2] USB: serial: ftdi_sio: Use rounding instead of truncating when calculating baud rate divisors.

[Johan Hovold <johan@kernel.org>]

On Thu, Nov 22, 2018 at 09:27:46PM +0100, Nikolaj Fogh wrote:
> Improve baud-rate generation by using rounding-to-closest instead of
> truncation in divisor calculation.
> 
> Results have been verified by logic analyzer on an FT232RT (232BM) chip.
> The following table shows the wanted baud rate, the baud rate obtained
> with the old method (truncation), with the new method (rounding) and the
> baud rate generated by the windows 10 driver. The numbers in parentheses
> is the error.
> 
> +- Wanted --+------ Old -------+------ New -------+------ Win -------+
> |   9600    |   9600  (0.00%)  |   9604  (0.05%)  |   9605 (0.05%)  |
> |   19200   |   19200 (0.00%)  |   19199 (0.01%)  |   19198 (0.01%)  |
> |   38400   |   38395 (0.01%)  |   38431 (0.08%)  |   38394 (0.02%)  |
> |   57600   |   57725 (0.22%)  |   57540 (0.10%)  |   57673 (0.13%)  |
> |  115200   |  115307 (0.09%)  |  115330 (0.11%)  |  115320 (0.10%)  |
> |  921600   |  919963 (0.18%)  |  920386 (0.13%)  |  920810 (0.09%)  |
> |  961200   |  996512 (3.67%)  |  956480 (0.49%)  |  956937 (0.44%)  |
> +-----------+------------------+------------------+------------------+
> 
> The error due to noise in the measurements is in the order of a few
> tenths of a %. As can be seen, the baud rate for 961200 is significantly
> improved for some rates, and corresponds to the output given by the
> windows driver.
> 
> The theoretical baud rate has been calculated for all baud rates from 1
> to 3M, and as expected, the error is centered around 0, with a triangle
> shape instead of a sawtooth, so the maximum error is decreased to half.
> 
> Signed-off-by: Nikolaj Fogh <nikolajfogh@gmail.com>
> 
> ---
>   drivers/usb/serial/ftdi_sio.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
> index 609198d9594c..0edbd3427548 100644
> --- a/drivers/usb/serial/ftdi_sio.c
> +++ b/drivers/usb/serial/ftdi_sio.c
> @@ -1130,7 +1130,7 @@ static unsigned short int 
> ftdi_232am_baud_base_to_divisor(int baud, int base)
>   {
>       unsigned short int divisor;
>       /* divisor shifted 3 bits to the left */
> -    int divisor3 = base / 2 / baud;
> +    int divisor3 = DIV_ROUND_CLOSEST(base, 2 * baud);
>       if ((divisor3 & 0x7) == 7)
>           divisor3++; /* round x.7/8 up to x+1 */
>       divisor = divisor3 >> 3;

This version turned out to be white-space damaged by gmail (?) with tabs
replaced by spaces.

I fixed it up manually, but next time try sending the patch to yourself
and make sure can apply it (and/or run checkpatch on the result).

I also shortened the summary further and edited the commit message very
slightly. The result can be seen here (soon):

	https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/commit/?h=usb-next&id=6abd837104a3a8e1cda64fc4d7675f6c3ece9d8b

Again, thanks for fixing this.

Johan