Messages in this thread | | | From | Mahesh Bandewar (महेश बंडेवार) <> | Date | Wed, 8 May 2024 19:53:58 -0700 | Subject | Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps for PTP_SYS_OFFSET_EXTENDED |
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On Wed, May 8, 2024 at 12:35 AM Thomas Gleixner <tglx@linutronix.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
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