On Thursday 19 August 2010, Richard Cochran wrote:
> On Wed, Aug 18, 2010 at 05:12:56PM -0700, john stultz wrote:
> > On Wed, 2010-08-18 at 09:19 +0200, Richard Cochran wrote:
> > > The timer/alarm stuff is "ancillary" and is not at all necessary. It
> > > is just a "nice to have." I will happily remove it, if it is too
> > > troubling for people.
> >
> > If there's a compelling argument for it, I'm interested to hear. But
> > again, it seems like just
> > yet-another-way-to-get-alarm/timer-functionality, so before we add an
> > extra API (or widen an existing API) I'd like to understand the need.
>
> We don't really need it, IMHO.
>
> But if we offer clockid_t CLOCK_PTP, then we get timer_settime()
> without any extra effort.
Makes sense.
> > So in thinking about this, try to focus on what the new clock_id
> > provides that the other existing clockids do not? Are they at comparable
> > levels of abstraction? 15 years from now, are folks likely to still be
> > using it? Will it be maintainable? etc...
>
> Arnd convinced me that clockid_t=CLOCK_PTP is a good fit.
My point was that a syscall is better than an ioctl based interface here,
which I definitely still think. Given that John knows much more about
clocks than I do, we still need to get agreement on the question that
he raised, which is whether we actually need to expose this clock to the
user or not.
If we can find a way to sync system time accurate enough with PTP and
PPS, user applications may not need to see two separate clocks at all.
> My plan would be to introduce just one additional syscall:
>
> SYSCALL_DEFINE3(clock_adjtime, const clockid_t, clkid,
> int, ppb, struct timespec __user *, ts)
>
> ppb - desired frequency adjustment in parts per billion
> ts - desired time step (or jump) in <sec,nsec> to correct
> a measured offset
>
> Arguably, this syscall might be useful for other clocks, too.
This is a mix of adjtime and adjtimex with the addition of
the clkid parameter, right?
Have you considered passing a struct timex instead of ppb and ts?
Is using ppb instead of the timex ppm required to get the accuracy
you want?
Arnd
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