Messages in this thread | | | Date | Mon, 11 Oct 2021 05:58:15 -0700 | From | Richard Cochran <> | Subject | Re: [PATCH net-next] ptp: add vclock timestamp conversion IOCTL |
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On Fri, Oct 08, 2021 at 09:13:58AM +0200, Sebastien Laveze wrote: > Of course, so what tests and measurements can we bring on the table to > convince you that it doesn't lead to chaos ?
So, to restate what I've said before, you cannot adjust both the physical clock and the virtual clock at the same time.
Here is a simple example that has no solution, AFAICT.
- Imagine one physical and one virtual clock based on it.
- A user space program using the virtual clock synchronizes to within 100 nanoseconds of its upstream PTP Domain. So far, so good.
- Now a second program using the physical clock starts up, and proceeds to measure, then correct the gross phase offset to its upstream PTP Domain.
- The driver must now add, as your proposal entails, the reverse correction into the virtual clock's timecounter/cyclecounter.
- However, this particular physical clock uses a RMW pattern to program the offset correction.
- Boom. Now the duration of the RMW becomes an offset error in the virtual clock. The magnitude may be microseconds or even milliseconds for devices behind slow MDIO buses, for example.
End of story.
Thanks, Richard
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