1 files changed, 23 insertions, 2 deletions

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 051474c65b7..34d6a1cab8d 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -163,11 +163,32 @@ static irqreturn_t hpet_interrupt(int irq, void *data)
          * This has the effect of treating non-periodic like periodic.
          */
         if ((devp->hd_flags & (HPET_IE | HPET_PERIODIC)) == HPET_IE) {
-                unsigned long m, t;
+                unsigned long m, t, mc, base, k;
+                struct hpet __iomem *hpet = devp->hd_hpet;
+                struct hpets *hpetp = devp->hd_hpets;
 
                 t = devp->hd_ireqfreq;
                 m = read_counter(&devp->hd_timer->hpet_compare);
-                write_counter(t + m, &devp->hd_timer->hpet_compare);
+                mc = read_counter(&hpet->hpet_mc);
+                /* The time for the next interrupt would logically be t + m,
+                 * however, if we are very unlucky and the interrupt is delayed
+                 * for longer than t then we will completely miss the next
+                 * interrupt if we set t + m and an application will hang.
+                 * Therefore we need to make a more complex computation assuming
+                 * that there exists a k for which the following is true:
+                 * k * t + base < mc + delta
+                 * (k + 1) * t + base > mc + delta
+                 * where t is the interval in hpet ticks for the given freq,
+                 * base is the theoretical start value 0 < base < t,
+                 * mc is the main counter value at the time of the interrupt,
+                 * delta is the time it takes to write the a value to the
+                 * comparator.
+                 * k may then be computed as (mc - base + delta) / t .
+                 */
+                base = mc % t;
+                k = (mc - base + hpetp->hp_delta) / t;
+                write_counter(t * (k + 1) + base,
+                              &devp->hd_timer->hpet_compare);
         }
 
         if (devp->hd_flags & HPET_SHARED_IRQ)