On Wed, May 31, 2017 at 5:18 PM, A.S. Dong [off-list ref] wrote:

quoted

On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng [off-list ref] 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.

quoted

quoted

+       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.

quoted

+        * Loop to find the best OSR value possible, one that generates

minimum

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+        * baud_diff iterate through the rest of the supported values of

OSR.

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+        *
+        * Calculation Formula:
+        *  Baud Rate = baud clock / ((OSR+1) ? SBR)
+        */
+       baud_diff = baudrate;
+       osr = 0;
+       sbr = 0;
+

quoted

+       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?

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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