Hi Andy, > -----Original Message----- > From: Andy Shevchenko [mailto:andy.shevchenko@xxxxxxxxx] > Sent: Saturday, June 03, 2017 1:11 AM > To: A.S. Dong > Cc: linux-serial@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-arm > Mailing List; Greg Kroah-Hartman; Jiri Slaby; Andy Duan; Stefan Agner; > Mingkai Hu; Y.B. Lu > Subject: Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud > rate calculation method > > On Wed, May 31, 2017 at 5:18 PM, A.S. Dong <aisheng.dong@xxxxxxx> wrote: > >> On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng <aisheng.dong@xxxxxxx> > 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? > No valid default value. (osc/sbr are 0 by default) If no proper osc and sbr calculated, a WARNING will show. > >> 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] > Baud Rate = baud clock / ((OSR+1) × SBR) In HW: OSR range : 3 – 31 SBR range: 1 – 8191 > 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? 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. > 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. How about you send a separate baud algorithm improvement patch later? Then it first can provide us a good patch history and also better to understand for review. Last, very appreciate for your kind and professional review. Regards Dong Aisheng ��.n��������+%������w��{.n�����{��ǫ����{ay� ʇڙ���f���h������_�(�階�ݢj"��������G����?���&��
