[PATCH] x86-64: - Ignore long SMI interrupts in clock calibration

Ensure that no SMI interrupts occur between the read of the HPET & TSC
in the clock calibration loop.

I noticed that a 2.66GHz system incorrectly detected the processor
clock speed about 1/7 of the time:

	time.c: Detected 2660.005 MHz processor.	(most of the time)
	time.c: Detected 2988.203 MHz processor.	(sometime)

The problem is caused by an SMI interrupt occuring in hpet_calibrate_tsc()
between the read of the HPET & TSC. Prior to switching the BIOS into
ACPI mode, it appears that every 27msec an SMI interrupt occurs. The
SMI interrupt takes 4.8 msec to process.

Note: On my test system, TICK_MIN had to be >380. I picked 5000
to minimize risk of having a value that is too small for other
platforms.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andi Kleen <ak@suse.de>

 arch/x86_64/kernel/time.c |   25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

arch/x86_64/kernel/time.c

@@ -656,6 +656,25 @@ core_initcall(cpufreq_tsc);
  */
 
 #define TICK_COUNT 100000000
+#define TICK_MIN   5000
+
+/*
+ * Some platforms take periodic SMI interrupts with 5ms duration. Make sure none
+ * occurs between the reads of the hpet & TSC.
+ */
+static void __init read_hpet_tsc(int *hpet, int *tsc)
+{
+	int tsc1, tsc2, hpet1;
+
+	do {
+		tsc1 = get_cycles_sync();
+		hpet1 = hpet_readl(HPET_COUNTER);
+		tsc2 = get_cycles_sync();
+	} while (tsc2 - tsc1 > TICK_MIN);
+	*hpet = hpet1;
+	*tsc = tsc2;
+}
+
 
 static unsigned int __init hpet_calibrate_tsc(void)
 {
@@ -666,13 +685,11 @@ static unsigned int __init hpet_calibrate_tsc(void)
 	local_irq_save(flags);
 	local_irq_disable();
 
-	hpet_start = hpet_readl(HPET_COUNTER);
-	rdtscl(tsc_start);
+	read_hpet_tsc(&hpet_start, &tsc_start);
 
 	do {
 		local_irq_disable();
-		hpet_now = hpet_readl(HPET_COUNTER);
-		tsc_now = get_cycles_sync();
+		read_hpet_tsc(&hpet_now, &tsc_now);
 		local_irq_restore(flags);
 	} while ((tsc_now - tsc_start) < TICK_COUNT &&
 		 (hpet_now - hpet_start) < TICK_COUNT);