linux-kernel - Re: clock_gettime

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Message-ID: <52279E1D.3060907@linaro.org>
Date:	Wed, 04 Sep 2013 13:54:53 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Arun Sharma <asharma@...com>, LKML <linux-kernel@...r.kernel.org>,
	Kumar Sundararajan <kumar@...com>
Subject: Re: clock_gettime_ns

On 09/04/2013 01:33 PM, Andy Lutomirski wrote:
> 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.

Well, adjtimex() gives you UTC & tai offset & leapsecond flag in one go.

thanks
-john

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