linux-kernel - Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

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Message-ID: <20170120202502.GA10368@localhost.localdomain>
Date:   Fri, 20 Jan 2017 21:25:02 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

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?

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

Thanks,
Richard