[PATCH] x86: quick TSC calibration, improve

[Ingo Molnar]

- make sure the final TSC timestamp is reliable too

- make sure nobody increases QUICK_PIT_MS so that
  QUICK_PIT_ITERATIONS can get larger than 0xff, breaking the iteration.
  (It would take about 60 msecs to reach that limit.)

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 arch/x86/kernel/tsc.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 839070b..4832a40 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -304,6 +304,11 @@ static unsigned long quick_pit_calibrate(void)
 	outb(0xff, 0x42);
 	outb(0xff, 0x42);
 
+	/*
+	 * The iteration assumes that expect never goes below zero:
+	 */
+	BUILD_BUG_ON(QUICK_PIT_ITERATIONS >= 0xff);
+
 	if (pit_expect_msb(0xff)) {
 		int i;
 		u64 t1, t2, delta;
@@ -317,6 +322,12 @@ static unsigned long quick_pit_calibrate(void)
 		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
--
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