[time-nuts] (fwd) Re: ``direct'' RS-232 vs. RS-232 via USB vs. PPSdecoding cards

[qb4 at comcast.net]

>
>I think that's enough to synchronize the counter on the Ethernet card with 
>the CPU counter.  You need either something like a PPS connected to both the 
>Ethernet and some pin on the CPU, or you need some pin the CPU can control 
>connected over to the Ethernet.

It seems like it would be pretty easy synchronize the frequencies on
the card and processor if the processor has an Always Running APIC
timer (this is apparently a different thing than the Skylake Always
Running Timer...) because In the case of the i250/i350 ethernet
controller and post-core2 intel chipsets they both run off a 25MHz XO.
For time transfer you'd still be relying on latency of running code
rather than hardware-synchronized register capture.

>Do you have any info on the Linux support?  What magic term to I search for?  
>(I'm guessing it's a magic ioctl.)

You're right, the ioctl is named PTP_SYS_OFFSET_PRECISE  -- there is a
capability bit that can be checked, but it looks like up to kernel 4.9
it's supported only on Skylake onboard ethernet.