> -----Original Message----- > From: Richard Cochran [mailto:richardcoch...@gmail.com] > Sent: Wednesday, November 09, 2016 5:12 AM > To: Keller, Jacob E <jacob.e.kel...@intel.com> > Cc: netdev@vger.kernel.org; t...@linutronix.de; manfred.rudig...@omicron.at; > ulrik.debie...@e2big.org; stefan.soren...@spectralink.com; > da...@davemloft.net; Kirsher, Jeffrey T <jeffrey.t.kirs...@intel.com>; > john.stu...@linaro.org; intel-wired-...@lists.osuosl.org > Subject: Re: [PATCH net-next 2/3] ptp: igb: Use the high resolution frequency > method. > > On Tue, Nov 08, 2016 at 10:02:22PM +0000, Keller, Jacob E wrote: > > On Tue, 2016-11-08 at 22:49 +0100, Richard Cochran wrote: > > > - rate = ppb; > > > - rate <<= 26; > > > - rate = div_u64(rate, 1953125); > > > + rate = scaled_ppm; > > > + rate <<= 13; > > > + rate = div_u64(rate, 15625); > > > > I'm curious how you generate the new math here, since this can be > > tricky, and I could use more examples in order to port to some of the > > other drivers implementations. I'm not quit sure how to handle the > > value when the lower 16 bits are fractional. > > TL;DR version: > > In ptp_clock.c we convert scaled_ppm to ppb like this. > > ppb = scaled_ppm * 10^3 * 2^-16 > > If you already have a working driver that does > > regval = ppb * SOMEMATH; > > then just substitute > > regval = (scaled_ppm * 10^3 * 2^-16) * SOMEMATH; > = (scaled_ppm * 5^3 * 2^-13) * SOMEMATH; > > and simplify by combining the 5^3 and 2^-13 constants into SOMEMATH. >
Thanks, this makes more sense :) > Longer explanation: > > You have to consider how the frequency adjustment HW works, case by > case. Both the i210 and the phyter have an adjustment register that > holds units of 2^-32 nanoseconds per 8 nanosecond clock period, and so > the rate from adjustment value 1 is (2^-32 / 8). > > Then with the old interface, the conversion from "adjustment unit" to > ppb was (2^-32 / 8 * 10^9) or (2^-26 * 5^9). The conversion the other > way needs the inverse, and so the code did (ppb << 26) / 5^9. > > With the new interface, the conversion from "adjustment unit" to > scaled_ppm is (2^-32 / 8 * 10^6 * 2^16) or (2^-13 * 5^6). The code > converts the other direction using the inverse, (s_ppm << 13) / 5^6. > Right. This helps. Thanks, Jake > HTH, > Richard
