Re: [PATCH AUTOSEL 4.20 025/117] USB: serial: ftdi_sio: use rounding when calculating baud rate divisors

[Sasha Levin]

On Wed, Jan 09, 2019 at 09:24:37AM +0100, Johan Hovold wrote:
> On Tue, Jan 08, 2019 at 02:24:53PM -0500, Sasha Levin wrote:
> > From: Nikolaj Fogh <nikolajfogh@xxxxxxxxx>
> >
> > [ Upstream commit 6abd837104a3a8e1cda64fc4d7675f6c3ece9d8b ]
> >
> > 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 is significantly improved
> > for some rates (e.g. 961200), 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>
> > [ johan: edit commit message slightly ]
> > Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>
> The stable tag was left out on purpose for this one to avoid breaking
> working setups which may have compensated and adapted to this driver
> behaviour which has been there since the driver was merged ages ago.

I'll drop it, thank you.

--
Thanks,
Sasha