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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <02874ECE860811409154E81DA85FBB5857C68AFE@ORSMSX115.amr.corp.intel.com>
Date:   Wed, 9 Nov 2016 21:40:04 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Richard Cochran <richardcochran@...il.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.

> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@...il.com]
> Sent: Wednesday, November 09, 2016 5:12 AM
> To: Keller, Jacob E <jacob.e.keller@...el.com>
> Cc: netdev@...r.kernel.org; tglx@...utronix.de; Manfred.Rudigier@...cron.at;
> ulrik.debie-os@...ig.org; stefan.sorensen@...ctralink.com;
> davem@...emloft.net; Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>;
> john.stultz@...aro.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.
> 

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