1 files changed, 65 insertions, 45 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 9c8b71531ca8..dfa6f7b73ff5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -274,30 +274,43 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
  * use the TSC value at the transitions to calculate a pretty
  * good value for the TSC frequencty.
  */
-static inline int pit_expect_msb(unsigned char val)
+static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap)
 {
-	int count = 0;
+	int count;
+	u64 tsc = 0;
 
 	for (count = 0; count < 50000; count++) {
 		/* Ignore LSB */
 		inb(0x42);
 		if (inb(0x42) != val)
 			break;
+		tsc = get_cycles();
 	}
-	return count > 50;
+	*deltap = get_cycles() - tsc;
+	*tscp = tsc;
+
+	/*
+	 * We require _some_ success, but the quality control
+	 * will be based on the error terms on the TSC values.
+	 */
+	return count > 5;
 }
 
 /*
- * How many MSB values do we want to see? We aim for a
- * 15ms calibration, which assuming a 2us counter read
- * error should give us roughly 150 ppm precision for
- * the calibration.
+ * How many MSB values do we want to see? We aim for
+ * a maximum error rate of 500ppm (in practice the
+ * real error is much smaller), but refuse to spend
+ * more than 25ms on it.
  */
-#define QUICK_PIT_MS 15
-#define QUICK_PIT_ITERATIONS (QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)
+#define MAX_QUICK_PIT_MS 25
+#define MAX_QUICK_PIT_ITERATIONS (MAX_QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)
 
 static unsigned long quick_pit_calibrate(void)
 {
+	int i;
+	u64 tsc, delta;
+	unsigned long d1, d2;
+
 	/* Set the Gate high, disable speaker */
 	outb((inb(0x61) & ~0x02) | 0x01, 0x61);
 
@@ -316,45 +329,52 @@ static unsigned long quick_pit_calibrate(void)
 	outb(0xff, 0x42);
 	outb(0xff, 0x42);
 
-	if (pit_expect_msb(0xff)) {
-		int i;
-		u64 t1, t2, delta;
-		unsigned char expect = 0xfe;
-
-		t1 = get_cycles();
-		for (i = 0; i < QUICK_PIT_ITERATIONS; i++, expect--) {
-			if (!pit_expect_msb(expect))
-				goto failed;
+	/*
+	 * The PIT starts counting at the next edge, so we
+	 * need to delay for a microsecond. The easiest way
+	 * to do that is to just read back the 16-bit counter
+	 * once from the PIT.
+	 */
+	inb(0x42);
+	inb(0x42);
+
+	if (pit_expect_msb(0xff, &tsc, &d1)) {
+		for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
+			if (!pit_expect_msb(0xff-i, &delta, &d2))
+				break;
+
+			/*
+			 * Iterate until the error is less than 500 ppm
+			 */
+			delta -= tsc;
+			if (d1+d2 < delta >> 11)
+				goto success;
 		}
-		t2 = get_cycles();
-
-		/*
-		 * Make sure we can rely on the second TSC timestamp:
-		 */
-		if (!pit_expect_msb(expect))
-			goto failed;
-
-		/*
-		 * Ok, if we get here, then we've seen the
-		 * MSB of the PIT decrement QUICK_PIT_ITERATIONS
-		 * times, and each MSB had many hits, so we never
-		 * had any sudden jumps.
-		 *
-		 * As a result, we can depend on there not being
-		 * any odd delays anywhere, and the TSC reads are
-		 * reliable.
-		 *
-		 * kHz = ticks / time-in-seconds / 1000;
-		 * kHz = (t2 - t1) / (QPI * 256 / PIT_TICK_RATE) / 1000
-		 * kHz = ((t2 - t1) * PIT_TICK_RATE) / (QPI * 256 * 1000)
-		 */
-		delta = (t2 - t1)*PIT_TICK_RATE;
-		do_div(delta, QUICK_PIT_ITERATIONS*256*1000);
-		printk("Fast TSC calibration using PIT\n");
-		return delta;
 	}
-failed:
+	printk("Fast TSC calibration failed\n");
 	return 0;
+
+success:
+	/*
+	 * Ok, if we get here, then we've seen the
+	 * MSB of the PIT decrement 'i' times, and the
+	 * error has shrunk to less than 500 ppm.
+	 *
+	 * As a result, we can depend on there not being
+	 * any odd delays anywhere, and the TSC reads are
+	 * reliable (within the error). We also adjust the
+	 * delta to the middle of the error bars, just
+	 * because it looks nicer.
+	 *
+	 * kHz = ticks / time-in-seconds / 1000;
+	 * kHz = (t2 - t1) / (I * 256 / PIT_TICK_RATE) / 1000
+	 * kHz = ((t2 - t1) * PIT_TICK_RATE) / (I * 256 * 1000)
+	 */
+	delta += (long)(d2 - d1)/2;
+	delta *= PIT_TICK_RATE;
+	do_div(delta, i*256*1000);
+	printk("Fast TSC calibration using PIT\n");
+	return delta;
 }
 
 /**