On 3/5/2020 1:11 AM, Frantisek Rysanek wrote:
> And another sideways question is: in the i210 hardware, there's a 
> register called SYSTIMR, supposedly holding the fraction of a 
> nanosecond (= sub-nanosecond resolution). And this register is 
> ignored by the igb driver in Linux - first and foremost because the 
> internal timestamping infrastructure only supports nanosecond 
> resolution. I know that a "ns fraction" field is present in the PTP 
> frames, but everybody except the White Rabbit just leave that field 
> empty (all zeroes). I'm wondering if this SYSTIMR register in the 
> i210 hardware has some practical use, or is always zero, or what.
> Well for my practical purposes, the SYSTIMR does not get reflected in 
> the two AUXSTMP registers - so I can probably just ignore SYSTIMR 
> too.

So, the SYSTIMR field is not "used" directly, but it holds and maintains
fractional nanoseconds. When you adjust the increment value slightly,
these get added to the SYSTIMR field of the system time. As that slowly
increments and eventually overflows, it will then increment the SYSTIML

Essentially we always round down by cutting off SYSTIMR.

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