Re: [PATCH v8 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource

Thomas Gleixner
2016-02-24T11:00:31Z

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On Mon, 22 Feb 2016, Christopher S. Hall wrote:

> On modern Intel systems TSC is derived from the new Always Running Timer
> (ART). ART can be captured simultaneous to the capture of
> audio and network device clocks, allowing a correlation between timebases
> to be constructed. Upon capture, the driver converts the captured ART
> value to the appropriate system clock using the correlated clocksource
> mechanism.
>
> On systems that support ART a new CPUID leaf (0x15) returns parameters
> “m” and “n” such that:
>
> TSC_value = (ART_value * m) / n + k [n >= 2]
>
> [k is an offset that can adjusted by a privileged agent. The
> IA32_TSC_ADJUST MSR is an example of an interface to adjust k.
> See 17.14.4 of the Intel SDM for more details]
>
> Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
> [jstultz: Tweaked to fix build issue, also reworked math for
> 64bit division on 32bit systems]
> Signed-off-by: John Stultz <john.stultz@linaro.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>