linux-kernel - Re: [RFC PATCH 1/7] timekeeping: Fix cross-timestamp interpolation on counter wrap

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Date:   Thu, 27 Jul 2023 12:21:38 +0200
From:   Peter Hilber <peter.hilber@...nsynergy.com>
To:     John Stultz <jstultz@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH 1/7] timekeeping: Fix cross-timestamp interpolation on
 counter wrap

On 08.07.23 00:51, John Stultz wrote:
> On Fri, Jun 30, 2023 at 10:12 AM Peter Hilber
> <peter.hilber@...nsynergy.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 after
>> < before < test, and for the case after < test < before.
>>
>> Fix the comparison logic.
>>
>> Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
>> Signed-off-by: Peter Hilber <peter.hilber@...nsynergy.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 u64s?

No, I just saw this when fixing the bug in the next patch.

Thanks,

Peter