netdev - Re: [Intel-wired-lan] [PATCH next-queue v2 3/3] igc: Add support for PTP getcrosststamp()

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Date:   Wed, 11 Nov 2020 16:38:16 -0800
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     intel-wired-lan@...ts.osuosl.org, andre.guedes@...el.com,
        linux-pci@...r.kernel.org, netdev@...r.kernel.org,
        bhelgaas@...gle.com
Subject: Re: [Intel-wired-lan] [PATCH next-queue v2 3/3] igc: Add support
 for PTP getcrosststamp()

Hi,

Miroslav Lichvar <mlichvar@...hat.com> writes:

> On Mon, Nov 09, 2020 at 10:10:19PM -0800, Vinicius Costa Gomes wrote:
>> i225 has support for PCIe PTM, which allows us to implement support
>> for the PTP_SYS_OFFSET_PRECISE ioctl(), implemented in the driver via
>> the getcrosststamp() function.
>
> Would it be possible to provide the PTM measurements with the
> PTP_SYS_OFFSET_EXTENDED ioctl instead of PTP_SYS_OFFSET_PRECISE?
>
> As I understand it, PTM is not cross timestamping. It's basically
> NTP over PCIe, which provides four timestamps with each "dialog". From
> the other constants added to the header file it looks like they could
> all be obtained and then they could be converted to the triplets
> returned by the EXTENDED ioctl.
>

There might be a problem, the PTM dialogs start from the device, the
protocol is more or less this:

 1. NIC sends "Request" message, takes T1 timestamp;
 2. Host receives "Request" message, takes T2 timestamp;
 3. Host sends "Response" message, takes T3 timestamp;
 4. NIC receives "Response" message, takes T4 timestamp;

So, T2 and T3 are "host" timestamps and T1 and T4 are NIC timestamps.

That means that the timestamps I have "as is" are a bit different than
the expectations of the EXTENDED ioctl().

Of course I could use T3 (as the "pre" timestamp), T4 as the device
timestamp, and calculate the delay[1], apply it to T3 and get something
T3' as the "post" timestamp (T3' = T3 + delay). But I feel that this
"massaging" would defeat the purpose of using the EXTENDED variant.

Does it make sense? Am I worrying too much?

[1] 
	delay = ((T4 - T1) - (T3 - T2)) / 2

Cheers,
-- 
Vinicius