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Date:   Mon, 23 Jan 2017 11:19:17 -0200
From:   Marcelo Tosatti <>
To:     Richard Cochran <>
        Paolo Bonzini <>,
        Radim Krcmar <>,
        Miroslav Lichvar <>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

On Fri, Jan 20, 2017 at 09:25:02PM +0100, Richard Cochran wrote:
> On Fri, Jan 20, 2017 at 10:20:29AM -0200, Marcelo Tosatti wrote:
> > Emulate PTP_SYS_OFFSET by using an arithmetic mean of the
> > realtime samples from ->getcrosststamp callback.
> This change log is not very informative.
> Yes, I can see that the new code calculates a mean.
> But WHY is this needed?

This is needed to generate the PTP_SYS_OFFSET data: a table with read
from realtime clock, read from device clock, read from realtime clock,
... :

time ->
device clock	|	|sample2|	|sample4|	|sample6| ...
realtime clock  |sample1|       |sample3|	|sample5|

Where sampleN is the read of the respective clock.

>From the following PTP_SYS_OFFSET_PRECISE data:

device clock	|sample1P,deviceclock|       |sample2P,deviceclock|
realtime clock  |sample1P,realtimeclock|     |sample2P,realtimeclock|

The mean of {sample2P,realtimeclock} and {sample1P,realtimeclock} is
used to generate sample1 for the realtime clock.

> And why is this kvm case so special, that it requires its own new flag
> within the generic PHC subsystem?

We get PTP_SYS_OFFSET_PRECISE support for users and use that to 
implement PTP_SYS_OFFSET as well. Other drivers could do the same, 
as long as their ->getcrosststamp callbacks return PTP synchronized

Since the clock is moving forward, approximating the clock read at
"sample3" with the mean of "sample1" and "sample5" (in the first table
above) is a very good approximation.

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