1 files changed, 12 insertions, 9 deletions

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 4a9f3492b921..70a770ac0138 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);