Messages in this thread | | | From | Andy Shevchenko <> | Date | Fri, 2 Jun 2017 20:11:24 +0300 | Subject | Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method |
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On Wed, May 31, 2017 at 5:18 PM, A.S. Dong <aisheng.dong@nxp.com> wrote: >> On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng <aisheng.dong@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
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