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

From: Miroslav Lichvar
Date: Tue Jan 24 2017 - 03:16:03 EST

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