Re: [RFC v3 net-next 13/18] net/sched: Introduce the TBS Qdisc

[Richard Cochran <richardcochran@gmail.com>]

On Wed, Apr 11, 2018 at 04:38:44PM -0700, Jesus Sanchez-Palencia wrote:
> Just breaking this down a bit, yes, TAI is the network time base, and the NICs
> PTP clock use that because PTP is (commonly) based on TAI. After the PHCs have
> been synchronized over the network (e.g. with ptp4l), my understanding is that
> if applications want to use the clockid_t CLOCK_TAI as a network clock reference
> it's required that something (i.e. phc2sys) is synchronizing the PHCs and the
> system clock, and also that something calls adjtime to apply the TAI vs UTC
> offset to CLOCK_TAI.

Yes.  I haven't seen any distro that sets the TAI-UTC offset after
boot, nor are there any user space tools for this.  The kernel is
ready, though.

> I was thinking about the full offload use-cases, thus when no scheduling is
> happening inside the qdiscs. Applications could just read the time from the PHC
> clocks directly without having to rely on any of the above. On this case,
> userspace would use DYNAMIC_CLOCK just to flag that this is the case, but I must
> admit it's not clear to me how common of a use-case that is, or even if it makes
> sense.

1588 allows only two timescales, TAI and ARB-itrary.  Although it
doesn't make too much sense to use ARB, still people will do strange
things.  Probably some people use UTC.  I am not advocating supporting
alternate timescales, just pointing out the possibility.

Thanks,
Richard