From ae21cf9248584d9b3776bfe2ebec47256bf098f8 Mon Sep 17 00:00:00 2001 From: Nils Carlson Date: Wed, 23 Sep 2009 15:57:13 -0700 Subject: hpet: hpet driver periodic timer setup bug fixes The periodic interrupt from drivers/char/hpet.c does not work correctly, both when using the periodic capability of the hardware and while emulating the periodic interrupt (when hardware does not support periodic mode). With timers capable of periodic interrupts, the comparator field is first set with the period value followed by set of hidden accumulator, which has the side effect of overwriting the comparator value. This results in wrong periodicity for the interrupts. For, periodic interrupts to work, following steps are necessary, in that order. * Set config with Tn_VAL_SET_CNF bit * Write to hidden accumulator, the value written is the time when the first interrupt should be generated * Write compartor with period interval for subsequent interrupts (http://www.intel.com/hardwaredesign/hpetspec_1.pdf ) When emulating periodic timer with timers not capable of periodic interrupt, driver is adding the period to counter value instead of comparator value, which causes slow drift when using this emulation. Also, driver seems to add hpetp->hp_delta both while setting up periodic interrupt and while emulating periodic interrupts with timers not capable of doing periodic interrupts. This hp_delta will result in slower than expected interrupt rate and should not be used while setting the interval. Signed-off-by: Venkatesh Pallipadi Signed-off-by: Nils Carlson Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/char/hpet.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) (limited to 'drivers') diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c index 4a9f3492b92..70a770ac013 100644 --- a/drivers/char/hpet.c +++ b/drivers/char/hpet.c @@ -166,9 +166,8 @@ static irqreturn_t hpet_interrupt(int irq, void *data) unsigned long m, t; t = devp->hd_ireqfreq; - m = read_counter(&devp->hd_hpet->hpet_mc); - write_counter(t + m + devp->hd_hpets->hp_delta, - &devp->hd_timer->hpet_compare); + m = read_counter(&devp->hd_timer->hpet_compare); + write_counter(t + m, &devp->hd_timer->hpet_compare); } if (devp->hd_flags & HPET_SHARED_IRQ) @@ -504,21 +503,25 @@ static int hpet_ioctl_ieon(struct hpet_dev *devp) g = v | Tn_32MODE_CNF_MASK | Tn_INT_ENB_CNF_MASK; if (devp->hd_flags & HPET_PERIODIC) { - write_counter(t, &timer->hpet_compare); g |= Tn_TYPE_CNF_MASK; - v |= Tn_TYPE_CNF_MASK; - writeq(v, &timer->hpet_config); - v |= Tn_VAL_SET_CNF_MASK; + v |= Tn_TYPE_CNF_MASK | Tn_VAL_SET_CNF_MASK; writeq(v, &timer->hpet_config); local_irq_save(flags); - /* NOTE: what we modify here is a hidden accumulator + /* + * NOTE: First we modify the hidden accumulator * register supported by periodic-capable comparators. * We never want to modify the (single) counter; that - * would affect all the comparators. + * would affect all the comparators. The value written + * is the counter value when the first interrupt is due. */ m = read_counter(&hpet->hpet_mc); write_counter(t + m + hpetp->hp_delta, &timer->hpet_compare); + /* + * Then we modify the comparator, indicating the period + * for subsequent interrupt. + */ + write_counter(t, &timer->hpet_compare); } else { local_irq_save(flags); m = read_counter(&hpet->hpet_mc); -- cgit v1.2.3