Explain why HPET T0_CMP register is written twice From: Vojtech Pavlik After writing the CFG register, the first value written to the T0_CMP register is the value at which next interrupt should be triggered, every value after that sets the period of the interrupt. For that reason, the code needs to write the value twice - to set both the phase and period. Signed-off-by: Vojtech Pavlik Signed-off-by: Andi Kleen --- arch/x86_64/kernel/time.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) Index: linux/arch/x86_64/kernel/time.c =================================================================== --- linux.orig/arch/x86_64/kernel/time.c +++ linux/arch/x86_64/kernel/time.c @@ -795,8 +795,8 @@ static int hpet_timer_stop_set_go(unsign if (hpet_use_timer) { hpet_writel(HPET_TN_ENABLE | HPET_TN_PERIODIC | HPET_TN_SETVAL | HPET_TN_32BIT, HPET_T0_CFG); - hpet_writel(hpet_tick, HPET_T0_CMP); - hpet_writel(hpet_tick, HPET_T0_CMP); /* AK: why twice? */ + hpet_writel(hpet_tick, HPET_T0_CMP); /* next interrupt */ + hpet_writel(hpet_tick, HPET_T0_CMP); /* period */ cfg |= HPET_CFG_LEGACY; } /*