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References: <20230630171052.985577-1-peter.hilber@opensynergy.com> <20230630171052.985577-2-peter.hilber@opensynergy.com>
In-Reply-To: <20230630171052.985577-2-peter.hilber@opensynergy.com>
From:   John Stultz <jstultz@google.com>
Date:   Fri, 7 Jul 2023 15:51:32 -0700
Message-ID: <CANDhNCpaoRRKGz0XxV3zjA-ux66gU-dT_bGQ_OqDh4ty=VdXLQ@mail.gmail.com>
Subject: Re: [RFC PATCH 1/7] timekeeping: Fix cross-timestamp interpolation on
 counter wrap
To:     Peter Hilber <peter.hilber@opensynergy.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        "Christopher S. Hall" <christopher.s.hall@intel.com>
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On Fri, Jun 30, 2023 at 10:12=E2=80=AFAM Peter Hilber
<peter.hilber@opensynergy.com> wrote:
>
> cycle_between() decides whether get_device_system_crosststamp() will
> interpolate for older counter readings.
>
> cycle_between() yields wrong results for a counter wrap-around where afte=
r
> < before < test, and for the case after < test < before.
>
> Fix the comparison logic.
>
> Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supp=
orting slower devices")
> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
> ---
>  kernel/time/timekeeping.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 266d02809dbb..8f35455b6250 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1186,7 +1186,7 @@ static bool cycle_between(u64 before, u64 test, u64=
 after)
>  {
>         if (test > before && test < after)
>                 return true;
> -       if (test < before && before > after)
> +       if (before > after && (test > before || test < after))
>                 return true;
>         return false;
>  }

Thanks for catching this and sending it in.
Looks good to me. Curious: Did you actually hit such a wrap around with u64=
s?

Acked-by: John Stultz <jstultz@google.com>

thanks
-john