On Mon, Mar 13, 2017 at 08:04:14PM +0100, Thomas Gleixner wrote:
> On Mon, 13 Mar 2017, Peter Zijlstra wrote:
>
> >
> > Subhransu reported that convert_art_to_tsc() isn't working for him.
> >
> > It turns out that because of commit:
> >
> > 57779dc2b3b7 ("x86, tsc: Skip refined tsc calibration on systems with
> > reliable TSC")
>
> That's the wrong culprit. The problem was introduced with the ART support
> patches. I fixed the changelog up.
Right; thanks for untangling that history.
> > Systems with TSC_KNOWN_FREQ (due to having CPUID.15h) will not run the
> > refined calibration thing, and then forget to set the ART clocksource
> > relation.
> >
> > We cannot set the ART relation prior to running the refined calibration;
> > therefore duplicate the lines.
> >
> > XXX: should we clear this in mark_tsc_unstable() ?
>
> It's already handled. get_device_system_timestamp() which convert ART to
> clock monotonic checks whether the timekeeper clocksource is the same as
> the correlated ART clocksource, i.e. TSC. So when timekeeping switches away
> from TSC get_device_system_timestamp() returns -ENODEV.
Ah right. Couldn't find that in a hurry.
