netdev - RE: [PATCH net V4 1/2] igb: add PTP Hardware Clock code

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Date:	Tue, 24 Jan 2012 21:23:45 +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>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH net V4 1/2] igb: add PTP Hardware Clock code



> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@...il.com]
> Sent: Tuesday, January 24, 2012 9:44 AM
> To: Keller, Jacob E
> Cc: netdev@...r.kernel.org; e1000-devel@...ts.sourceforge.net; Kirsher,
> Jeffrey T; Ronciak, John; John Stultz; Thomas Gleixner
> Subject: Re: [PATCH net V4 1/2] igb: add PTP Hardware Clock code
> 
> On Mon, Jan 23, 2012 at 06:39:44PM +0000, Keller, Jacob E wrote:
> 
> > > +static int ptp_82576_adjfreq(struct ptp_clock_info *ptp, s32 ppb)
> > > +{
> > > +	u64 rate;
> > > +	u32 incvalue;
> > > +	int neg_adj = 0;
> > > +	struct igb_adapter *igb = container_of(ptp, struct igb_adapter,
> caps);
> > > +	struct e1000_hw *hw = &igb->hw;
> > > +
> > > +	if (ppb < 0) {
> > > +		neg_adj = 1;
> > > +		ppb = -ppb;
> > > +	}
> > > +	rate = ppb;
> > > +	rate <<= 14;
> > > +	rate = div_u64(rate, 1953125);
> > > +
> >
> > So is the rate ppb accumulating? I was under the impression that it
> > calculated from the current clock frequency so it would need to be
> > applied to the current incvalue, not the base... Is this not the
> > case? What was the intention of the ppb?
> 
> The ppb is simply the desired rate offset in parts per billion. It is
> not a delta from the current offset, but rather fixed from the clock's
> nominal frequency. This comes from the NTP timex.freq field (but the
> unit here is ppb, timex.freq is ppm with a 16 bit fraction.)
> 
> If the clock servo is a typical PI controller for example, then the
> servo output already represents the accumulated offset.
> 
> > > +	incvalue = 16 << IGB_82576_TSYNC_SHIFT;
> > > +
> > > +	if (neg_adj)
> > > +		incvalue -= rate;
> > > +	else
> > > +		incvalue += rate;
> > > +
> > > +	wr32(E1000_TIMINCA, INCPERIOD_82576 | (incvalue &
> INCVALUE_82576_MASK));
> > > +
> > > +	return 0;
> > > +}
> 
> > > +		/* Dial the nominal frequency. */
> > > +		wr32(E1000_TIMINCA, INCPERIOD_82576 | INCVALUE_82576);
> > > +		break;
> > > +
> >
> > It would be good to check whether the link speed has an impact on
> > the SYSTIME register rate. I know on the 10 Gb parts the systime
> > registers are driven by the DMA clock which is partitioned
> > differently at slower speeds so the values have to be updated
> > whenever the link speed changes..
> 
> If this is true for 82576, then it certainly is not documented. I
> don't have that card, so maybe someone from Intel can answer this?
> 

I'll look into this.

> I think the documentation for the 82580 is clear about the basic rate
> always being the same.

Yeah, I believe the 82580 stays the same regardless of the device link speed.

> 
> Thanks,
> Richard
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