Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 23 Jan 2019 08:58:31 -0500
From:   Sasha Levin <sashal@kernel.org>
To:     Johan Hovold <johan@kernel.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Nikolaj Fogh <nikolajfogh@gmail.com>, 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: <20190123135831.GI202535@sasha-vm>
References: <20190108192628.121270-1-sashal@kernel.org>
 <20190108192628.121270-25-sashal@kernel.org>
 <20190109082437.GA3389@localhost>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20190109082437.GA3389@localhost>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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