linux-kernel - Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps for PTP_SYS_OFFSET

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Date: Wed, 8 May 2024 19:53:58 -0700
From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Yuliang Li <yuliangli@...gle.com>, 
	Don Hatchett <hatch@...gle.com>
Cc: Netdev <netdev@...r.kernel.org>, Linux <linux-kernel@...r.kernel.org>, 
	David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Richard Cochran <richardcochran@...il.com>, Arnd Bergmann <arnd@...db.de>, 
	Sagi Maimon <maimon.sagi@...il.com>, Jonathan Corbet <corbet@....net>, John Stultz <jstultz@...gle.com>, 
	Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps
 for PTP_SYS_OFFSET_EXTENDED

On Wed, May 8, 2024 at 12:35 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Thu, May 02 2024 at 14:10, Mahesh Bandewar wrote:
> > The ability to read the PHC (Physical Hardware Clock) alongside
> > multiple system clocks is currently dependent on the specific
> > hardware architecture. This limitation restricts the use of
> > PTP_SYS_OFFSET_PRECISE to certain hardware configurations.
> >
> > The generic soultion which would work across all architectures
> > is to read the PHC along with the latency to perform PHC-read as
> > offered by PTP_SYS_OFFSET_EXTENDED which provides pre and post
> > timestamps.  However, these timestamps are currently limited
> > to the CLOCK_REALTIME timebase. Since CLOCK_REALTIME is affected
> > by NTP (or similar time synchronization services), it can
> > experience significant jumps forward or backward. This hinders
> > the precise latency measurements that PTP_SYS_OFFSET_EXTENDED
> > is designed to provide.
>
> This is really a handwavy argument.
>
> Fact is that the time jumps of CLOCK_REALTIME caused by NTP (etc) are
> rare and significant enough to be easily filtered out. That's why this
> interface allows you to retrieve more than one sample.
>
> Can you please explain which problem you are actually trying to solve?
>
> It can't be PTP system time synchronization as that obviously requires
> CLOCK_REALTIME.
>
Let me add a couple of folks from the clock team. @Yuliang Li  @Don Hatchett
I'm just a nomad-kernel-net guy trying to fill-in gaps :(

> Thanks,
>
>         tglx