linux-kernel - Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation corner case decision

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Message-ID: <87a5tn1kp6.ffs@tglx>
Date:   Fri, 15 Sep 2023 18:10:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Hilber <peter.hilber@...nsynergy.com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Hilber <peter.hilber@...nsynergy.com>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp
 interpolation corner case decision

On Fri, Aug 18 2023 at 03:20, Peter Hilber wrote:
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1247,7 +1247,8 @@ int get_device_system_crosststamp(int (*get_time_fn)
>  		 */
>  		now = tk_clock_read(&tk->tkr_mono);
>  		interval_start = tk->tkr_mono.cycle_last;
> -		if (!cycle_between(interval_start, cycles, now)) {
> +		if (!cycle_between(interval_start, cycles, now) &&
> +		    cycles != interval_start) {
>  			clock_was_set_seq = tk->clock_was_set_seq;
>  			cs_was_changed_seq = tk->cs_was_changed_seq;
>  			cycles = interval_start;

So the explanation in the changelog makes some sense, but this code
without any further explanation just makes my brain explode.

This whole thing screams for a change to cycle_between() so it becomes:

     timestamp_in_interval(start, end, ts)

and make start inclusive and not exclusive, no?

That's actually correct for both usage sites because for interpolation
the logic is the same. history_begin->cycles is a valid timestamp, no?

Thanks,

        tglx