USB: serial: cp210x: Improve baudrate support for CP2102N

[Johan Hovold <johan@kernel.org>]

On Wed, Jun 20, 2018 at 09:51:41AM +0000, Karoly Pados wrote:
> > By the way, have you tried setting other baudrates except the ones you
> > explicitly allow for here? According to the data sheet more rates should
> > be available, so perhaps just handling cp2102n as cp2108 (e.g. by not
> > trying to report back the exact rate used) or by actually calculating
> > the resulting rate could be another option?
> > 
> > Can be done later of course, just curious if you tried it.
> 
> Yeah I know, I was thinking about this too while developing the patch. 
> Officially the cp2102 and the cp2102n are fully software compatible (aside
> from baudrate aliasing), but if the cp2102n chooses different baudrates for 
> the same inputs than the older devices would then they couldn't/wouldn't be
> compatible. So I concluded it must also be doing the quantisation.

Yeah, that's probably right, but the older devices do not support
rates > 1 Mbaud so that logic does not necessarily apply there.

> Maybe I am too naive and trust the datasheet to much. I'll do some
> measurements with my scope and let you know the results.

Cool. We can keep the old behaviour for < 1Mbaud, but it would be nice
to know if you can generate rates other than the 4-5 +1Mbauds rates that
were explicitly mentioned in the data sheet.

Thanks,
Johan
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