[PATCH] x86_64: Fix drift with HPET timer enabled

If the HPET timer is enabled, the clock can drift by ~3 seconds a day.
This is due to the HPET timer not being initialized with the correct
setting (still using PIT count).

If HZ changes, this drift can become even more pronounced.

HPET patch initializes tick_nsec with correct tick_nsec settings for
HPET timer.

Vojtech comments:

  "It's not entirely correct (it assumes the HPET ticks totally
   exactly), but it's significantly better than assuming the PIT error
   there."

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c
index 2eeaa95edff0..7392570f975d 100644
--- a/arch/x86_64/kernel/time.c
+++ b/arch/x86_64/kernel/time.c
@@ -917,6 +917,8 @@ void __init time_init(void)
                 vxtime.hpet_address = 0;
 
         if (hpet_use_timer) {
+                /* set tick_nsec to use the proper rate for HPET */
+                tick_nsec = TICK_NSEC_HPET;
                 cpu_khz = hpet_calibrate_tsc();
                 timename = "HPET";
 #ifdef CONFIG_X86_PM_TIMER