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    SubjectRe: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support
    > There is no other scalable time source available for fast timer access
    > than the time stamp counter in the cpu. Other time source require
    > memory accesses which is inherently slower.

    On what hardware ?

    > An accurate other time source is used to adjust this clock. NTP does that
    > via the clock interfaces from user space which has its problems with
    > accuracy. PTP can provide the network synced time access
    > that would a more accurate calibration of the time.

    Accuracy does not require speed of access. Accuracy requires predictible
    latency of access.

    > Userland code would introduce latencies that would make sub microsecond
    > time sync very difficult.

    You can take a multiple micro-second I/O stall or SMI trap on a PC so you
    already lost the battle on the platform you seem to be discussing.

    > You dont need the user APIs if you directly use the PTP time source to
    > steer the system clock. In fact I think you have to do it in kernel space
    > since user space latencies will degrade accuracy otherwise.

    PTP is not a 'time source' it is one or more source of time. The
    distinction is rather important.

    > It implies clock tuning in userspace for a potential sub microsecond
    > accurate clock. The clock accuracy will be limited by user space
    > latencies and noise. You wont be able to discipline the system clock
    > accurately.

    Noise matters, latency doesn't. And the kernel is getting more and more
    real time support all the time.

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