Commit 5aac644a authored Jun 03, 2015 by Adrian Hunter Committed by Thomas Gleixner Jul 06, 2015

x86/tsc: Let high latency PIT fail fast in quick_pit_calibrate()



If it takes longer than 12us to read the PIT counter lsb/msb,
then the error margin will never fall below 500ppm within 50ms,
and Fast TSC calibration will always fail.

This patch detects when that will happen and fails fast. Note
the failure message is not printed in that case because:
1. it will always happen on that class of hardware
2. the absence of the message is more informative than its
presence

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/556EB717.9070607@intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent d770e558

arch/x86/kernel/tsc.c

+10 −1

Original line number	Diff line number	Diff line
		@@ -598,10 +598,19 @@ static unsigned long quick_pit_calibrate(void)
		if (!pit_expect_msb(0xff-i, &delta, &d2))
		break;

		delta -= tsc;

		/*
		* Extrapolate the error and fail fast if the error will
		* never be below 500 ppm.
		*/
		if (i == 1 &&
		d1 + d2 >= (delta * MAX_QUICK_PIT_ITERATIONS) >> 11)
		return 0;

		/*
		* Iterate until the error is less than 500 ppm
		*/
		delta -= tsc;
		if (d1+d2 >= delta >> 11)
		continue;

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