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

[Andy Shevchenko]

On Wed, May 31, 2017 at 5:18 PM, A.S. Dong <aisheng.dong at nxp.com> wrote:
>> On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng <aisheng.dong at nxp.com> wrote:

By some reason my previous message went privately.
It didn't have anything major anyway and here I'm suggesting
optimization of finding factors of the formula in use. See below.

>> > +       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?

>> I _think_ you may simplify this and avoid for-loop if you reconsider
>> approach.

> But there is indeed a optimization way, see below.

> To optimize the looping, we probably could do:
> If (!baud_diff)
>         Break;

It's a small one, we may have more interesting approach.

So, the algo is the following:

Assume the ranges like this:
OSR = [4 ... 32]
SBR = [2 ... 8192]

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)
3. if ratio >= 8192 * 31, just use those two numbers (8192, 31). You
can't do anything better there.
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.

-- 
With Best Regards,
Andy Shevchenko