Re: [RFC PATCH v2 3/6] timekeeping: Fix cross-timestamp interpolation for non-x86

[John Stultz]

On Thu, Aug 17, 2023 at 6:20 PM Peter Hilber
<peter.hilber@xxxxxxxxxxxxxxx> wrote:
>
> So far, get_device_system_crosststamp() unconditionally passes
> system_counterval.cycles to timekeeping_cycles_to_ns(). But when
> interpolating system time (do_interp == true), system_counterval.cycles is
> before tkr_mono.cycle_last, contrary to the timekeeping_cycles_to_ns()
> expectations.
>
> On x86, CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE will mitigate on
> interpolating, setting delta to 0. With delta == 0, xtstamp->sys_monoraw
> and xtstamp->sys_realtime are then set to the last update time, as
> implicitly expected by adjust_historical_crosststamp(). On other
> architectures, the resulting nonsense xtstamp->sys_monoraw and
> xtstamp->sys_realtime corrupt the xtstamp (ts) adjustment in
> adjust_historical_crosststamp().
>
> Fix this by deriving xtstamp->sys_monoraw and xtstamp->sys_realtime from
> the last update time when interpolating, by using the local variable
> "cycles". The local variable already has the right value when
> interpolating, unlike system_counterval.cycles.
>
> Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
> Signed-off-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>

Thanks again for iterating on this.  This looks much better!

Now, I've never had an environment that used this logic, so I'm
trusting you've tested it well?

Assuming so:
Acked-by: John Stultz <jstultz@xxxxxxxxxx>

thanks
-john