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Date:   Wed, 9 Jan 2019 09:24:37 +0100
From:   Johan Hovold <johan@kernel.org>
To:     Sasha Levin <sashal@kernel.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Nikolaj Fogh <nikolajfogh@gmail.com>,
        Johan Hovold <johan@kernel.org>, linux-usb@vger.kernel.org
Subject: Re: [PATCH AUTOSEL 4.20 025/117] USB: serial: ftdi_sio: use rounding
 when calculating baud rate divisors
Message-ID: <20190109082437.GA3389@localhost>
References: <20190108192628.121270-1-sashal@kernel.org>
 <20190108192628.121270-25-sashal@kernel.org>
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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.

Johan