Hi, Miroslav Lichvar <mlichvar@xxxxxxxxxx> 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
