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

John Stultz
2023-07-07T22:51:52Z

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?

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

thanks
-john