Re: x86/tsc: Fix ART for TSC_KNOWN_FREQ

[Peter Zijlstra]

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.