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

Peter Hilber
2023-07-27T10:21:51Z

On 08.07.23 00:51, John Stultz wrote:
> On Fri, Jun 30, 2023 at 10:12 AM 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 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@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 u64s?

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

Thanks,

Peter