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 Tue, Nov 13, 2018 at 08:19:44PM +0100, Nikolaj Fogh wrote:
> On 11/12/18 10:54 AM, Johan Hovold wrote:
> > On Wed, Oct 31, 2018 at 09:16:48PM +0100, Nikolaj Fogh wrote:
> >> I have experienced that the ftdi_sio driver gives less-than-optimal
> >> baud rates as the driver truncates instead of rounds to nearest
> >> during baud rate divisor calculation.

> >> This patch improves on the baud rate generation. The generated baud
> >> rate corresponds to the optimal baud rate achievable with the chip.
> >> This is what the windows driver gives as well.
> >
> > How did you verify this? Did you trace and compare the divisors
> > actually requested by the Windows driver, or did you measure the
> > resulting rates using a scope?

> I verified it by scope. Granted, I only verified it for one baud rate
> (961200). Whether it gives the same as the Windows driver in general,
> I'm not sure. However, I would think that rounding instead of flooring
> would always yield the most accurate result.

I'm not so sure in this case. The driver uses "sub-integer" divisors and
looks like it depends on truncation rather than rounding. Some
background here:

	https://www.ftdichip.com/Support/Knowledgebase/index.html?whatbaudratesareachieveabl.htm

If you want to change these calculations you need to make a stronger
case for it and verify that we don't mess up some other rate
inadvertently.

Thanks,
Johan