linux-kernel - tsc reliability for Intel Core 2 Duo "Conroe"

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Date:	Wed, 26 May 2010 10:41:43 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: tsc reliability for Intel Core 2 Duo "Conroe"

Looking through code following up on the separate TSC-related
thread, I noticed that my Intel Core 2 Duo "Conroe" box
is determined to have an unstable TSC, so falls back
to clocksource==hpet.

While a Conroe has X86_FEATURE_CONSTANT_TSC and not
X86_FEATURE_NONSTOP_TSC, a Conroe is only able to enter
C0 and C1 (unlike its sister Intel Core 2 Duo processor
"Merom" which can enter C0-C3).

I was under the impression (possibly from an earlier kernel
version?) that tsc_constant PLUS inability to enter deep-C
states would result in an acceptably stable TSC to use
as a clocksource (assuming it passes a TSC warp test).

So is this a bug?  Or is my impression incorrect?

See tsc_check_state() in drivers/acpi/processor_idle.c.
I can submit a patch, but wanted to check first.
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