| Re: [chrony-dev] SW/HW timestamping on Linux |
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On Thu, Jan 05, 2017 at 10:15:08AM -0800, Denny Page wrote:
> Yes, comp is an abbreviation for compensation. One will always an add and the other is always a subtract. This is because the timestamps are defined as being at the php layer (last bit of the SFD), but the timestamps are being taken at the mii layer. For inbound, the phy layer precedes the mii layer which means that the timestamp has to be adjusted backward in time, hence the subtract. For outbound, the phy later succeeds the mii layer which means that the timestamp has to be adjusted forward in time, hence the add. Have a look at the igb kernel driver or linuxptp for examples.
Ok. I thought the drivers were supposed to adjust the timestamp for
the (minimum?) MAC-PHY error.
> I understand the desire to keep everything consistent in terms of seconds. I am concerned that this will lead to a lot of errors on people’s part because the values are always going to be two digit microseconds or under. For 100Mb on an i354 (more compensation than an i210/i211) the values would be:
You can specify nanoseconds with the e-9 exponent. Some options in the
man page already show that.
> rxcomp
> Specifies the difference in nanoseconds between the reported receive
> time stamp and the actual reception time at the physical layer. This value
> will be subtracted from receive time stamps obtained from the network
> card. The default is 0.
>
> Does this work for you?
Yes, thanks. I was wondering if it should try to explain the
transposition based on the TX l2 length and other problems that may
need to be compensated.
Could be the measured peer delay used to check if the combined TX/RX
compensation is correct? If I have two identical NICs connected
directly with a short cable on 1gbit, what is the expected peer delay?
With no compensation I see now delay of about 1450 ns between two i210
NICs.
Anyway, the compensation is now in git (and 3.0-pre3). Please test.
--
Miroslav Lichvar
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