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

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Date:   Tue, 24 Jan 2017 09:15:56 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

On Tue, Jan 24, 2017 at 06:32:43AM +0100, Richard Cochran wrote:
> On Mon, Jan 23, 2017 at 09:06:20PM -0200, Marcelo Tosatti wrote:
> > Can you please describe what problem exists with this scheme?
> 
> This new kernel code exists just because chrony doesn't implement the
> PRECISE ioctl.  Instead of adding new "fake" modes, just teach chrony
> about the better method.

The latest development code of chrony now supports the PRECISE ioctl.

I did some tests with an e1000e NIC (i219) and it seemed the stability
was slightly worse than with the non-PRECISE ioctl, but there was a
400-500ns offset between the two, so it should be much more accurate
(we finally have something that avoids the asymmetry on the PCI-e
bus?). Configuring the refclock with a shorter dpoll should compensate
for the decrease in stability. In any case, there is a "nocrossts"
option to not use the PRECISE ioctl even if it's supported.

-- 
Miroslav Lichvar