[v2] USB: serial: ftdi_sio: Improve the accuracy of the baud rate generator by using round-to-closest instead of truncating when calculating baud rate divisors.

[Johan Hovold <johan@kernel.org>]

On Fri, Nov 16, 2018 at 10:44:21PM +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>

Thanks for updated commit message. I was just about to apply it, but
checkpatch reported that the Signed-off address doesn't match the author
address (From). Would you mind fixing that up?

Also, please use a less verbose subject (patch summary), try to make it
fit within 72 cols or so.

Thanks,
Johan