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@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/tsc_32.c

@@ -392,13 +392,15 @@ 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,
@@ -431,9 +433,4 @@ void __init tsc_init(void)
 		tsc_enabled = 1;
 
 	clocksource_register(&clocksource_tsc);
-
-	return;
-
-out_no_tsc:
-	setup_clear_cpu_cap(X86_FEATURE_TSC);
 }