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    SubjectRe: [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@nxp.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

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