Re: [chrony-dev] SW/HW timestamping on Linux

[Denny Page <dennypage@xxxxxx>]

On Jan 18, 2017, at 23:28, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Wed, Jan 18, 2017 at 08:41:03PM -0800, Denny Page wrote:

Tx -> Rx timestamp:
 Uncompensated                        3175ns  (stddev 110ns)
 Compensated                          3175ns

Connection types:       Expected        Error
 Regular switch          6721ns      -3546ns
 Cut-through switch      2081ns       1094ns
 loopback cable             1ns       3174ns

Intel’s predicted result is 3177ns.

Interesting. So 3174ns is what txcomp+rxcomp should be for the i211?

Yes, although I’ve seen numbers from 3172 to 3182. Intel's 3177 number is right in the middle of the range.

In the data sheet for the i210 & i211, Intel says the 100Mb min/max range for tx is 984/1024, and 2148/2228 for rx. However they go on to say that when measured against an external link partner, there is “a shift of approximately 40ns”. They go on to give the following average values as seen externally:

Parameter	Average	Comment
Tx timestamp to start of SFD on MDI	1044 ns	The range (max minus min) values measured for the Tx and Rx latency parameters are similar to the measured parameters in a stand-alone setup.
Start of SDF on MDI to Rx timestamp	2133 ns
Tx + Rx latency	3177 ns

I’m trusting these values as they correspond quite closely to my test results.

The kernel driver for the i210 uses 1024/2213. Based on both Intel’s data and my own tests, these appear to be wrong.

Denny