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

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Date:	Wed, 24 Feb 2016 12:00:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Christopher S. Hall" <christopher.s.hall@...el.com>
cc:	richardcochran@...il.com, mingo@...hat.com, john.stultz@...aro.org,
	hpa@...or.com, jeffrey.t.kirsher@...el.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
	netdev@...r.kernel.org, kevin.b.stanton@...el.com,
	kevin.j.clarke@...el.com
Subject: Re: [PATCH v8 6/8] x86: tsc: Always Running Timer (ART) correlated
 clocksource

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@...el.com>
> [jstultz: Tweaked to fix build issue, also reworked math for
> 64bit division on 32bit systems]
> Signed-off-by: John Stultz <john.stultz@...aro.org>

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