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

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Date:   Wed, 23 Jan 2019 08:58:31 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Nikolaj Fogh <nikolajfogh@...il.com>, linux-usb@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.20 025/117] USB: serial: ftdi_sio: use rounding
 when calculating baud rate divisors

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@...il.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@...il.com>
>> [ johan: edit commit message slightly ]
>> Signed-off-by: Johan Hovold <johan@...nel.org>
>> Signed-off-by: Sasha Levin <sashal@...nel.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