linux-kernel - Re: clock_gettime

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Date:	Wed, 4 Sep 2013 13:33:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Arun Sharma <asharma@...com>, LKML <linux-kernel@...r.kernel.org>,
	Kumar Sundararajan <kumar@...com>
Subject: Re: clock_gettime_ns

On Wed, Sep 4, 2013 at 12:20 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Wed, Sep 4, 2013 at 11:51 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> I think that most of the hangup was a lack of agreement on how the API
>> should work wrt leap seconds.
>
> I don't recall this objection. The interface uses existing clockids,
> so it probably should keep the existing leap-second behavior of those
> clockids.
>
>> I've always thought that the Right Way to represent a UTC time is
>> nanoseconds since some epoch, where every potential leap second
>> counts.
>
> Check out the CLOCK_TAI clockid merged in 3.10.
>

I never really liked that -- CLOCK_TAI doesn't tell what time it is in
any format that normal people understand.

I'd advocate for going whole hog and returning, atomically:

 - TAI (nanoseconds from epoch)
 - UTC - TAI (seconds or nanoseconds) *
 - TAI - CLOCK_MONOTONIC (nanoseconds)
 - a leap second flag.

* There are various ways to define this.  My fancy UTC - TAI wouldn't
actually need the leap-second flag, since the UTC time would indicate
leap seconds directly.  With the conventional approach, someone would
have to decide whether the leap second count increments at the
beginning or the end of the leap second.

--Andy
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