On Thursday 19 August 2010, Richard Cochran wrote:

On Wed, Aug 18, 2010 at 05:12:56PM -0700, john stultz wrote:

quoted

On Wed, 2010-08-18 at 09:19 +0200, Richard Cochran wrote:

quoted

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.

quoted

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