This patch is borrowed from x86 hpet driver and explaind below: Due to the overly intelligent design of HPETs, we need to workaround the problem that the compare value which we write is already behind the actual counter value at the point where the value hits the real compare register. This happens for two reasons: 1) We read out the counter, add the delta and write the result to the compare register. When a NMI hits between the read out and the write then the counter can be ahead of the event already. 2) The write to the compare register is delayed by up to two HPET cycles in AMD chipsets. We can work around this by reading back the compare register to make sure that the written value has hit the hardware. But that is bad performance wise for the normal case where the event is far enough in the future. As we already know that the write can be delayed by up to two cycles we can avoid the read back of the compare register completely if we make the decision whether the delta has elapsed already or not based on the following calculation: cmp = event - actual_count; If cmp is less than 64 HPET clock cycles, then we decide that the event has happened already and return -ETIME. That covers the above #1 and #2 problems which would cause a wait for HPET wraparound (~306 seconds). Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx> --- arch/mips/loongson64/loongson-3/hpet.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/arch/mips/loongson64/loongson-3/hpet.c b/arch/mips/loongson64/loongson-3/hpet.c index bf9f1a7..a2631a5 100644 --- a/arch/mips/loongson64/loongson-3/hpet.c +++ b/arch/mips/loongson64/loongson-3/hpet.c @@ -13,6 +13,9 @@ #define SMBUS_PCI_REG64 0x64 #define SMBUS_PCI_REGB4 0xb4 +#define HPET_MIN_CYCLES 64 +#define HPET_MIN_PROG_DELTA (HPET_MIN_CYCLES + (HPET_MIN_CYCLES >> 1)) + static DEFINE_SPINLOCK(hpet_lock); DEFINE_PER_CPU(struct clock_event_device, hpet_clockevent_device); @@ -161,8 +164,9 @@ static int hpet_next_event(unsigned long delta, cnt += delta; hpet_write(HPET_T0_CMP, cnt); - res = ((int)(hpet_read(HPET_COUNTER) - cnt) > 0) ? -ETIME : 0; - return res; + res = (int)(cnt - hpet_read(HPET_COUNTER)); + + return res < HPET_MIN_CYCLES ? -ETIME : 0; } static irqreturn_t hpet_irq_handler(int irq, void *data) @@ -237,7 +241,7 @@ void __init setup_hpet_timer(void) cd->cpumask = cpumask_of(cpu); clockevent_set_clock(cd, HPET_FREQ); cd->max_delta_ns = clockevent_delta2ns(0x7fffffff, cd); - cd->min_delta_ns = 5000; + cd->min_delta_ns = clockevent_delta2ns(HPET_MIN_PROG_DELTA, cd); clockevents_register_device(cd); setup_irq(HPET_T0_IRQ, &hpet_irq); -- 2.4.6
