linux-kernel - Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method

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Message-ID: <CAHp75VcSToSfbUZ2TP0OsLRjV4TNpq7D_=4tnB_+rDvS2jp22Q@mail.gmail.com>
Date:   Fri, 9 Jun 2017 12:26:20 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "A.S. Dong" <aisheng.dong@....com>
Cc:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Andy Duan <fugang.duan@....com>,
        Stefan Agner <stefan@...er.ch>,
        Mingkai Hu <mingkai.hu@....com>, "Y.B. Lu" <yangbo.lu@....com>,
        Dong Aisheng <dongas86@...il.com>
Subject: Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate
 calculation method

On Fri, Jun 9, 2017 at 11:01 AM, A.S. Dong <aisheng.dong@....com> wrote:

>> >> > +       u32 sbr, osr, baud_diff, tmp_osr, tmp_sbr, tmp_diff, tmp;
>> >> > +       u32 clk = sport->port.uartclk;
>> >> > +
>> >> > +       /*
>> >> > +        * The idea is to use the best OSR (over-sampling rate)
>> possible.
>> >> > +        * Note, OSR is typically hard-set to 16 in other LPUART
>> >> instantiations.
>> >> > +        * Loop to find the best OSR value possible, one that
>> >> > + generates
>> >> minimum
>> >> > +        * baud_diff iterate through the rest of the supported
>> >> > + values of
>> >> OSR.
>> >> > +        *
>> >> > +        * Calculation Formula:
>> >> > +        *  Baud Rate = baud clock / ((OSR+1) × SBR)
>> >> > +        */
>> >> > +       baud_diff = baudrate;
>> >> > +       osr = 0;
>> >> > +       sbr = 0;
>> >> > +
>> >>
>> >> > +       for (tmp_osr = 4; tmp_osr <= 32; tmp_osr++) {
>>
>> I missed one thing, what happened by default to OSR? What is the value in
>> use?
>>
>
> No valid default value. (osc/sbr are 0 by default)
> If no proper osc and sbr calculated, a WARNING will show.

Okay, so, it means the maximum supported speed is UART clock / 4. Correct?

>> So, the algo is the following:
>>
>> Assume the ranges like this:
>> OSR = [4 ... 32]
>> SBR = [2 ... 8192]
>>
>
> Baud Rate = baud clock / ((OSR+1) × SBR)
>
> In HW:
> OSR range : 3 – 31
> SBR range: 1 – 8191

I've read that, but think outside the box.

>> Then:
>>
>> 1. Get ratio factor as
>>       ratio = CLK / desired baud rate
>> 2. If ratio < 8192 * 9 / 2, just use (ratio / 4, 4) as (OSR, SBR) setting.
>> (Needs clarification on OSR < 4)
>
> Sorry that I'm a bit mess here.
> What is 8192 * 9 /2 meaning?

I forgot the details...

> And for (ratio / 4, 4) as (OSR,SBR), take 115200 as an example:
> Assuming baud clock 24Mhz.
>
> Ratio = 24000000 / 115200 = 208
> OSR = Ratio / 4 = 52
> Then OSR is out of range which seems wrong.

...yes...

>> 3. if ratio >= 8192 * 31, just use those
>> two numbers (8192, 31). You can't do anything better there.
>
> This actually may not happen.
> Even take a 9600 as example, the clk becomes:
> 8191 * 31 * 9600 = 2.4GHz
> Which is theoretically not exist.
>
>> 4. Otherwise, get a minimum required factor of OSR
>>       osr_min = ratio / 8192
>> 5. Start your loop from osr_min + 1 to 31.
>>
>> 6 (optional). Of course you may not consider baud_diff > osr_min, it's I
>> suppose obvious
>>
>> P.S. Note, all divisions by 2^n are just simple right shifts. Diffs are
>> calculated as multiplication of OSR and SBR in comparison to ratio. One
>> division so far.

> I'm not quite understand the approach.

...lemme prepare a python script demonstrating it.

> How about you send a separate baud algorithm improvement patch later?

Why not to do it right a way?

Just describe it in a comment if you afraid of reader can't understand
from the code.

-- 
With Best Regards,
Andy Shevchenko