Messages in this thread | | | Date | Wed, 23 Jan 2019 08:58:31 -0500 | From | Sasha Levin <> | Subject | Re: [PATCH AUTOSEL 4.20 025/117] USB: serial: ftdi_sio: use rounding when calculating baud rate divisors |
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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@gmail.com> >> >> [ 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@gmail.com> >> [ johan: edit commit message slightly ] >> Signed-off-by: Johan Hovold <johan@kernel.org> >> Signed-off-by: Sasha Levin <sashal@kernel.org> > >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
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