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

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Message-ID: <ce764b91-11b0-44a5-9cf9-96977652bbd5@default>
Date:	Mon, 24 May 2010 11:13:09 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	john stultz <johnstul@...ibm.com>,
	Brian Bloniarz <bmb@...enacr.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

> From: john stultz [mailto:johnstul@...ibm.com]
> 
> Yea, the relative instability of the tsc calibration at boot is an
> issue for folks who want very very precise timekeeping immediately
> after a reboot.
> 
> I proposed a solution to this awhile back via a boot option users
> could use to specify the tsc_khz freq, so it would be consistent from
> boot to boot. See: https://patchwork.kernel.org/patch/22492/
> 
> It didn't really go anywhere due to a lack of public interest.
> However, if you're interested in playing with it, I can try to revive
> the patch.

Another possibility: Optionally trust the stamped rate for the part?

I understand that on Nehalem this value is available in
MSR_PLATFORM_INFO[15:8] (google for MSR_PLATFORM_INFO 15 8),
but I don't know if this MSR is available on older (or AMD)
processors.

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?
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