From a6a80e1d8cf82b46a69f88e659da02749231eb36 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 17 Mar 2009 07:58:26 -0700 Subject: [PATCH] Fix potential fast PIT TSC calibration startup glitch During bootup, when we reprogram the PIT (programmable interval timer) to start counting down from 0xffff in order to use it for the fast TSC calibration, we should also make sure to delay a bit afterwards to allow the PIT hardware to actually start counting with the new value. That will happens at the next CLK pulse (1.193182 MHz), so the easiest way to do that is to just wait at least one microsecond after programming the new PIT counter value. We do that by just reading the counter value back once - which will take about 2us on PC hardware. Reported-and-tested-by: john stultz Signed-off-by: Linus Torvalds --- arch/x86/kernel/tsc.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 599e581..9e80207 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -315,6 +315,15 @@ static unsigned long quick_pit_calibrate(void) outb(0xff, 0x42); outb(0xff, 0x42); + /* + * 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)) { int i; u64 t1, t2, delta; -- 2.7.4