linux-kernel - Re: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <4BFAC34C.5020606@zytor.com>
Date:	Mon, 24 May 2010 11:19:56 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	john stultz <johnstul@...ibm.com>,
	Brian Bloniarz <bmb@...enacr.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs

On 05/24/2010 11:13 AM, Dan Magenheimer wrote:
> Just wondering:  If one were to put an ultra-precise scope on
> a processor, how far off would the calibrated value be?  I'd
> imagine the process of calibrating one unknown crystal against
> a second crystal which has a known-but-not-highly-precise
> frequency, though good enough for most purposes, is not particularly
> accurate. In other words, maybe the stamped rate is more accurate
> than the calibrated rate anyway?

No.  Not even close.

A spread-spectrum clock is inaccurate by entire percentage points.
A non-spread clock is typically ±50 ppm with typical consumer PC
oscillators, ±1 ppm with non-crappy but still cheap oscillators (e.g.
used in cell phones.)

	-hpa
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