LKM Timemark Driver

[Achim Gratz]

MLewis via devel writes:
> "Strictly speaking, the PPS pulse isn�t even necessary for this process. One could
> simply generate a pulse on the external interrupt line once per second at a known
> time. "

Yes, and if you use both measurements to eliminate the interrupt
latency, then random error propagation (assuming jitter is not
correlated between input and output) ensures that your measurement error
goes up by a factor of sqrt(2).  So you're better of just ignoring the
PPS kernel timestamp even from a theoretical standpoint.

> I'm currently using REALTIME_CLOCK. There's also the thread clock and
> the cpu clock.

Principally it should be better to use an unslewed clock source (one of
the hardware timers).  That's what the RADclock was all about, but the
API sadly never went into the mainline kernel.  But without a proper API
these are tricky to use portably and you're left with various
interference from e.g. power saving.


Regards,
Achim.
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