linux-kernel - Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit

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Message-ID: <20150602194350.GN1187@tassilo.jf.intel.com>
Date:	Tue, 2 Jun 2015 12:43:50 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Adrian Hunter <adrian.hunter@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in
 quick_pit_calibrate()

On Tue, Jun 02, 2015 at 12:41:27PM -0700, Andy Lutomirski wrote:
> On Tue, Jun 2, 2015 at 12:33 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Thu, 21 May 2015, Adrian Hunter wrote:
> >
> >> If it takes longer than 12us to read the PIT counter lsb/msb,
> >> then the error margin will never fall below 500ppm within 50ms,
> >> and Fast TSC calibration will always fail.
> >
> > So finally the legacy PIT emulation takes longer than the 30 years old
> > hardware implementation. Progress!
> >
> >> This patch detects when that will happen and switches to using
> >> a slightly different algorithm that takes advantage of the PIT's
> >> latch comand.
> >
> > Is there really no smarter way to figure out the TSC frequency on
> > modern systems?
> 
> I just asked Len this question yesterday.  intel_pstate can do it,
> although the algorithm is a bit gross.

intel_pstate needs to know the model number. If you know the 
model number sure you can do a lot better (e.g. using the 
ref-clock fixed counter or some other methods)

But if you don't you need something else. And at some point
the only thing left over is the PIT.

-Andi
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