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@xxxxxxxxx>
>
> ---
> 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
