linux-kernel - Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance

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Date:	8 May 2009 11:27:06 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	johnstul@...ibm.com, linux@...izon.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	ulrich.windl@...uni-regensburg.de, williams@...hat.com,
	zippel@...ux-m68k.org
Subject: Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance

> I'd suggest the following:

I should add the documentation patch that goes with it:
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 90b3924..6747881 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2391,10 +2391,20 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	tsc=		Disable clocksource-must-verify flag for TSC.
 			Format: <string>
-			[x86] reliable: mark tsc clocksource as reliable, this
-			disables clocksource verification at runtime.
-			Used to enable high-resolution timer mode on older
-			hardware, and in virtualized environment.
+			[x86] if value is "reliable", mark tsc clocksource
+			as reliable.  This disables clocksource verification
+			at runtime.  Used to enable high-resolution timer mode
+			on older hardware, and in virtualized environment.
+
+	tsc_khz=	[x86] Set the TSC freq value.
+			Format: <khz>
+			Used to avoid TSC calibration error.  The TSC frequency
+			is re-measured each boot, and changes by a few parts
+			per hundred thousand each time.  This is noticeable to
+			time synchronization applications like NTP.
+			This option makes it consistent across boots.
+			If the specified value differs from the measured one
+			by more than 0.1%, it is ignored.
 
 	turbografx.map[2|3]=	[HW,JOY]
 			TurboGraFX parallel port interface
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