netdev - Re: [PATCH net-next 2/3] ptp: igb: Use the high resolution frequency method.

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Message-ID: <20161109131146.GA6491@localhost.localdomain>
Date:   Wed, 9 Nov 2016 14:11:46 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     "Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Manfred.Rudigier@...cron.at" <Manfred.Rudigier@...cron.at>,
        "ulrik.debie-os@...ig.org" <ulrik.debie-os@...ig.org>,
        "stefan.sorensen@...ctralink.com" <stefan.sorensen@...ctralink.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "john.stultz@...aro.org" <john.stultz@...aro.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.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.

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.

HTH,
Richard