Commit ed5316d4 authored Jan 11, 2007 by Jack Steiner Committed by Andi Kleen Jan 11, 2007

[PATCH] x86-64: - Ignore long SMI interrupts in clock calibration



Ensure that no SMI interrupts occur between the read of the HPET & TSC
in the clock calibration loop.

I noticed that a 2.66GHz system incorrectly detected the processor
clock speed about 1/7 of the time:

	time.c: Detected 2660.005 MHz processor.	(most of the time)
	time.c: Detected 2988.203 MHz processor.	(sometime)

The problem is caused by an SMI interrupt occuring in hpet_calibrate_tsc()
between the read of the HPET & TSC. Prior to switching the BIOS into
ACPI mode, it appears that every 27msec an SMI interrupt occurs. The
SMI interrupt takes 4.8 msec to process.

Note: On my test system, TICK_MIN had to be >380. I picked 5000
to minimize risk of having a value that is too small for other
platforms.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andi Kleen <ak@suse.de>

 arch/x86_64/kernel/time.c |   25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

parent 9d24a81e

arch/x86_64/kernel/time.c

+21 −4

Original line number	Diff line number	Diff line
		@@ -656,6 +656,25 @@ core_initcall(cpufreq_tsc);
		*/

		#define TICK_COUNT 100000000
		#define TICK_MIN 5000

		/*
		* Some platforms take periodic SMI interrupts with 5ms duration. Make sure none
		* occurs between the reads of the hpet & TSC.
		*/
		static void __init read_hpet_tsc(int hpet, int tsc)
		{
		int tsc1, tsc2, hpet1;

		do {
		tsc1 = get_cycles_sync();
		hpet1 = hpet_readl(HPET_COUNTER);
		tsc2 = get_cycles_sync();
		} while (tsc2 - tsc1 > TICK_MIN);
		*hpet = hpet1;
		*tsc = tsc2;
		}


		static unsigned int __init hpet_calibrate_tsc(void)
		{
		@@ -666,13 +685,11 @@ static unsigned int __init hpet_calibrate_tsc(void)
		local_irq_save(flags);
		local_irq_disable();

		hpet_start = hpet_readl(HPET_COUNTER);
		rdtscl(tsc_start);
		read_hpet_tsc(&hpet_start, &tsc_start);

		do {
		local_irq_disable();
		hpet_now = hpet_readl(HPET_COUNTER);
		tsc_now = get_cycles_sync();
		read_hpet_tsc(&hpet_now, &tsc_now);
		local_irq_restore(flags);
		} while ((tsc_now - tsc_start) < TICK_COUNT &&
		(hpet_now - hpet_start) < TICK_COUNT);