linux-kernel - Re: clock_gettime

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Date:	Wed, 04 Sep 2013 15:59:28 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Arun Sharma <asharma@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Kumar Sundararajan <kumar@...com>
Subject: Re: clock_gettime_ns

On 09/04/2013 03:29 PM, H. Peter Anvin wrote:
> On 09/04/2013 01:54 PM, John Stultz wrote:
>>> 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.
> Not so (see below).
>
>>  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.
>>
> But not fractional-second information,right?  I believe it would be
> desirable if we can create a small structure (<= 16 bytes) for this.

Well, depending on if STA_NANO is set, adjtimex returns either nsec or
usec precision via the timex.time field.

> UTC - TAI is always an integral number of seconds, possibly negative
> (unlikely, but...)
>
> Something like:
>
> 	struct time_ns {
> 		u64 tai_s;
> 		u32 tai_ns;
> 		s16 utcdelta;	/* TAI - UTC */
> 		u8 leap;	/* Positive leap second in progress */
> 		u8 pad;		/* Something useful here maybe? */
> 	};
>
> Why the leap second flag?  It is necessary to represent the 61st second
> in a minute during a positive leap second.  Consider the below
> (artificial) cases:
>
> (leap second)
> TAI	31536000	31536001	31536002	31536003
> Delta	2		2		?		3
> UTC	23:59:58	23:59:59	23:59:60	00:00:00
>
> (no leap second)
> TAI	31536000	31536001	31536002	31536003
> Delta	2		2		2		2
> UTC	23:59:58	23:59:59	00:00:00	00:00:01
>
> (no leap second)
> TAI	31536000	31536001	31536002	31536003
> Delta	3		3		3		3
> UTC	23:59:57	23:59:58	23:59:59	00:00:00
>
> There simply is no sufficiently meaningful value that can be put on the
> delta during a positive leap second.  Both 2 and 3 would be wrong in the
> above example, giving UTC of either 00:00:00 or 23:59:59.
>
> There is a way to do without the leap second flag by making UTC the main
> time; this does have the advantage of higher compatibility with time_t,
> struct timespec, etc:
>
> 	struct timespecx {
> 		time_t tx_sec;		/* POSIX UTC seconds */
> 		u32 tx_ns;		/* Nanoseconds */
> 		s32 tx_taidelta;	/* TAI - UTC */
> 	};


And again, most of the detail above is already there w/ adjtimex (though
admittedly not in a very tight format).

My concern with adding these details to the timespec-like structure this
is with most clockids I'm not sure taidelta would make sense.

Also, there's been talk of a slewed-leap-second clockid, basically UTC
but around the leapsecond it slows down to absorb the extra second. This
means that clockid would have a subsecond offset from TAI.

thanks
-john
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