On 16/03/2021 16:59, Miroslav Lichvar wrote:
> On Tue, Mar 16, 2021 at 03:52:36PM +0100, Frantisek Rysanek wrote:
>> On 16 Mar 2021 at 11:25, Miroslav Lichvar wrote:
>>>
>>> In the Intel's igc driver I saw few SYNCE registers defined,
>>> but no code using them.
>>>
>> Whoa. igc ? oh there's an i225... didn't know about this one, thanks
>> for the pointer, this definitely looks promising :-)
>> Looks like a successor to the i210 generation.
>
> I'm sorry for misleading you. I meant the ice driver (speeds up to
> 100Gb). I didn't see an out-of-tree version of the igc driver.
>
> The I225 might be an interesting NIC for experiments for a different
> reason. It seems it supports the PCIe Precision Time Measurement (PTM)
> feature. It is a hardware implementation of an NTP-like protocol over
> PCIe, which should allow a highly accurate synchronization of the
> system clock, avoiding the asymmetry on PCIe. It is not supported in
> the igc driver yet, but there were some patches submitted for it.
>
> I measured the PCIe asymmetry with the I210 on few different
> boards+CPUs and it changed a lot (between about -100ns and 100ns), so
> I think PTM could make a significant improvement.
>
The "Precision Time Measurement" sound a cool feature.
The PCI is probably asymmetric.
As long as the traffic is not overflow (less then 99%),
The transfer time of the PCI have a limit of ~120 ms (on my PC).
Using hardware timestamps, the timestamps are always taken on the NIC
hardware, before the PCI. So, the PCI should not change the measurement.
The i210 does have inter gap issue, but it should not be relevant for
PTP measure.
I did some testing with TSN streaming with i210.
By adding guards on the i210 drivers to over come the inter gap issue, I
manage to get a precision of less then 20 ns on the streams.
My team did notice that the oscillator of the i210 was not precise
enough. We use the i210 as client (slave mode), with a good master.
The results should be higher.
Erez
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