Merge branch 'x86-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'x86-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  x86: Fix serialization in pit_expect_msb()

1 files changed, 22 insertions, 7 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6e1a368d21d..71f4368b357 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -275,15 +275,20 @@ 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_verify_msb(unsigned char val)
+{
+	/* Ignore LSB */
+	inb(0x42);
+	return inb(0x42) == val;
+}
+
 static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap)
 {
 	int count;
 	u64 tsc = 0;
 
 	for (count = 0; count < 50000; count++) {
-		/* Ignore LSB */
-		inb(0x42);
-		if (inb(0x42) != val)
+		if (!pit_verify_msb(val))
 			break;
 		tsc = get_cycles();
 	}
@@ -336,8 +341,7 @@ static unsigned long quick_pit_calibrate(void)
 	 * to do that is to just read back the 16-bit counter
 	 * once from the PIT.
 	 */
-	inb(0x42);
-	inb(0x42);
+	pit_verify_msb(0);
 
 	if (pit_expect_msb(0xff, &tsc, &d1)) {
 		for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
@@ -348,8 +352,19 @@ static unsigned long quick_pit_calibrate(void)
 			 * Iterate until the error is less than 500 ppm
 			 */
 			delta -= tsc;
-			if (d1+d2 < delta >> 11)
-				goto success;
+			if (d1+d2 >= delta >> 11)
+				continue;
+
+			/*
+			 * Check the PIT one more time to verify that
+			 * all TSC reads were stable wrt the PIT.
+			 *
+			 * This also guarantees serialization of the
+			 * last cycle read ('d2') in pit_expect_msb.
+			 */
+			if (!pit_verify_msb(0xfe - i))
+				break;
+			goto success;
 		}
 	}
 	printk("Fast TSC calibration failed\n");