linux-kernel - [PATCH 1/2] x86: If we cannot calibrate the TSC, we panic.

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Message-Id: <200803042307.51645.rusty@rustcorp.com.au>
Date:	Tue, 4 Mar 2008 23:07:50 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Ahmed S. Darwish" <darwish.07@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>, lguest@...abs.org,
	akpm <akpm@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...source.com>
Subject: [PATCH 1/2] x86: If we cannot calibrate the TSC, we panic.

The current tsc_init() clears the TSC feature bit if the TSC khz
cannot be calculated, causing us to panic in
arch/x86/kernel/cpu/bugs.c check_config().  We should simply mark it
unstable.

Frankly, someone should take an axe to this code.  mark_tsc_unstable()
not only marks it unstable, but sets tsc_enabled to 0, which seems
redundant but is actually important here because means it won't be
used by sched_clock() either.  Perhaps a tristate enum "UNUSABLE,
UNSTABLE, OK" would be clearer, and separate mark_tsc_unstable() and
mark_tsc_broken() functions?

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff -r f9a80502dc46 arch/x86/kernel/tsc_32.c
--- a/arch/x86/kernel/tsc_32.c	Tue Mar 04 21:06:41 2008 +1100
+++ b/arch/x86/kernel/tsc_32.c	Tue Mar 04 22:55:10 2008 +1100
@@ -394,13 +394,15 @@ void __init tsc_init(void)
 	int cpu;
 
 	if (!cpu_has_tsc)
-		goto out_no_tsc;
+		return;
 
 	cpu_khz = calculate_cpu_khz();
 	tsc_khz = cpu_khz;
 
-	if (!cpu_khz)
-		goto out_no_tsc;
+	if (!cpu_khz) {
+		mark_tsc_unstable("could not calculate TSC khz");
+		return;
+	}
 
 	printk("Detected %lu.%03lu MHz processor.\n",
 				(unsigned long)cpu_khz / 1000,
@@ -433,9 +435,4 @@ void __init tsc_init(void)
 		tsc_enabled = 1;
 
 	clocksource_register(&clocksource_tsc);
-
-	return;
-
-out_no_tsc:
-	setup_clear_cpu_cap(X86_FEATURE_TSC);
 }
--
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