x86: TSC make the calibration loop smarter

The last changes made the calibration loop 250ms long which is far
too much. Try to do that more clever.

Experiments have shown that using a 10ms delay for the PIT based calibration
gives us a good enough value. If we have a reference (HPET/PMTIMER) and the
result of the PIT and the reference is close enough, then we can break out of
the calibration loop on a match right away and use the reference value.

Otherwise we just loop 3 times and decide then, which value to take.

One caveat is that for virtualized environments the PIT calibration often does
not work at all and I found out that 10us is a bit too short as well for the
reference to give a sane result. The solution here is to make the last loop
longer when the first two PIT calibrations failed.

Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

Author: KAGA-KOKO

arch/x86/kernel/tsc.c

@@ -159,9 +159,14 @@ static unsigned long calc_pmtimer_ref(u64 deltatsc, u64 pm1, u64 pm2)
 	return (unsigned long) deltatsc;
 }
 
-#define CAL_MS		50
+#define CAL_MS		10
 #define CAL_LATCH	(CLOCK_TICK_RATE / (1000 / CAL_MS))
-#define CAL_PIT_LOOPS	5000
+#define CAL_PIT_LOOPS	1000
+
+#define CAL2_MS		50
+#define CAL2_LATCH	(CLOCK_TICK_RATE / (1000 / CAL2_MS))
+#define CAL2_PIT_LOOPS	5000
+
 
 /*
  * Try to calibrate the TSC against the Programmable
@@ -170,7 +175,7 @@ static unsigned long calc_pmtimer_ref(u64 deltatsc, u64 pm1, u64 pm2)
  *
  * Return ULONG_MAX on failure to calibrate.
  */
-static unsigned long pit_calibrate_tsc(void)
+static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
 {
 	u64 tsc, t1, t2, delta;
 	unsigned long tscmin, tscmax;
@@ -185,8 +190,8 @@ static unsigned long pit_calibrate_tsc(void)
 	 * (LSB then MSB) to begin countdown.
 	 */
 	outb(0xb0, 0x43);
-	outb(CAL_LATCH & 0xff, 0x42);
-	outb(CAL_LATCH >> 8, 0x42);
+	outb(latch & 0xff, 0x42);
+	outb(latch >> 8, 0x42);
 
 	tsc = t1 = t2 = get_cycles();
 
@@ -207,18 +212,18 @@ static unsigned long pit_calibrate_tsc(void)
 	/*
 	 * Sanity checks:
 	 *
-	 * If we were not able to read the PIT more than PIT_MIN_LOOPS
+	 * If we were not able to read the PIT more than loopmin
 	 * times, then we have been hit by a massive SMI
 	 *
 	 * If the maximum is 10 times larger than the minimum,
 	 * then we got hit by an SMI as well.
 	 */
-	if (pitcnt < CAL_PIT_LOOPS || tscmax > 10 * tscmin)
+	if (pitcnt < loopmin || tscmax > 10 * tscmin)
 		return ULONG_MAX;
 
 	/* Calculate the PIT value */
 	delta = t2 - t1;
-	do_div(delta, CAL_MS);
+	do_div(delta, ms);
 	return delta;
 }
 
@@ -230,8 +235,8 @@ unsigned long native_calibrate_tsc(void)
 {
 	u64 tsc1, tsc2, delta, ref1, ref2;
 	unsigned long tsc_pit_min = ULONG_MAX, tsc_ref_min = ULONG_MAX;
-	unsigned long flags;
-	int hpet = is_hpet_enabled(), i;
+	unsigned long flags, latch, ms;
+	int hpet = is_hpet_enabled(), i, loopmin;
 
 	/*
 	 * Run 5 calibration loops to get the lowest frequency value
@@ -257,7 +262,13 @@ unsigned long native_calibrate_tsc(void)
 	 * calibration delay loop as we have to wait for a certain
 	 * amount of time anyway.
 	 */
-	for (i = 0; i < 5; i++) {
+
+	/* Preset PIT loop values */
+	latch = CAL_LATCH;
+	ms = CAL_MS;
+	loopmin = CAL_PIT_LOOPS;
+
+	for (i = 0; i < 3; i++) {
 		unsigned long tsc_pit_khz;
 
 		/*
@@ -268,7 +279,7 @@ unsigned long native_calibrate_tsc(void)
 		 */
 		local_irq_save(flags);
 		tsc1 = tsc_read_refs(&ref1, hpet);
-		tsc_pit_khz = pit_calibrate_tsc();
+		tsc_pit_khz = pit_calibrate_tsc(latch, ms, loopmin);
 		tsc2 = tsc_read_refs(&ref2, hpet);
 		local_irq_restore(flags);
 
@@ -290,6 +301,35 @@ unsigned long native_calibrate_tsc(void)
 			tsc2 = calc_pmtimer_ref(tsc2, ref1, ref2);
 
 		tsc_ref_min = min(tsc_ref_min, (unsigned long) tsc2);
+
+		/* Check the reference deviation */
+		delta = ((u64) tsc_pit_min) * 100;
+		do_div(delta, tsc_ref_min);
+
+		/*
+		 * If both calibration results are inside a 10% window
+		 * then we can be sure, that the calibration
+		 * succeeded. We break out of the loop right away. We
+		 * use the reference value, as it is more precise.
+		 */
+		if (delta >= 90 && delta <= 110) {
+			printk(KERN_INFO
+			       "TSC: PIT calibration matches %s. %d loops\n",
+			       hpet ? "HPET" : "PMTIMER", i + 1);
+			return tsc_ref_min;
+		}
+
+		/*
+		 * Check whether PIT failed more than once. This
+		 * happens in virtualized environments. We need to
+		 * give the virtual PC a slightly longer timeframe for
+		 * the HPET/PMTIMER to make the result precise.
+		 */
+		if (i == 1 && tsc_pit_min == ULONG_MAX) {
+			latch = CAL2_LATCH;
+			ms = CAL2_MS;
+			loopmin = CAL2_PIT_LOOPS;
+		}
 	}
 
 	/*
@@ -309,7 +349,7 @@ unsigned long native_calibrate_tsc(void)
 		/* The alternative source failed as well, disable TSC */
 		if (tsc_ref_min == ULONG_MAX) {
 			printk(KERN_WARNING "TSC: HPET/PMTIMER calibration "
-			       "failed due to SMI disturbance.\n");
+			       "failed.\n");
 			return 0;
 		}
 
@@ -328,37 +368,18 @@ unsigned long native_calibrate_tsc(void)
 
 	/* The alternative source failed, use the PIT calibration value */
 	if (tsc_ref_min == ULONG_MAX) {
-		printk(KERN_WARNING "TSC: HPET/PMTIMER calibration failed due "
-		       "to SMI disturbance. Using PIT calibration\n");
+		printk(KERN_WARNING "TSC: HPET/PMTIMER calibration failed. "
+		       "Using PIT calibration\n");
 		return tsc_pit_min;
 	}
 
-	/* Check the reference deviation */
-	delta = ((u64) tsc_pit_min) * 100;
-	do_div(delta, tsc_ref_min);
-
-	/*
-	 * If both calibration results are inside a 5% window, the we
-	 * use the lower frequency of those as it is probably the
-	 * closest estimate.
-	 */
-	if (delta >= 95 && delta <= 105) {
-		printk(KERN_INFO "TSC: PIT calibration confirmed by %s.\n",
-		       hpet ? "HPET" : "PMTIMER");
-		printk(KERN_INFO "TSC: using %s calibration value\n",
-		       tsc_pit_min <= tsc_ref_min ? "PIT" :
-		       hpet ? "HPET" : "PMTIMER");
-		return tsc_pit_min <= tsc_ref_min ? tsc_pit_min : tsc_ref_min;
-	}
-
-	printk(KERN_WARNING "TSC: PIT calibration deviates from %s: %lu %lu.\n",
-	       hpet ? "HPET" : "PMTIMER", tsc_pit_min, tsc_ref_min);
-
 	/*
 	 * The calibration values differ too much. In doubt, we use
 	 * the PIT value as we know that there are PMTIMERs around
-	 * running at double speed.
+	 * running at double speed. At least we let the user know:
 	 */
+	printk(KERN_WARNING "TSC: PIT calibration deviates from %s: %lu %lu.\n",
+	       hpet ? "HPET" : "PMTIMER", tsc_pit_min, tsc_ref_min);
 	printk(KERN_INFO "TSC: Using PIT calibration value\n");
 	return tsc_pit_min;
 }