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path: root/drivers/char/hpet.c
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Diffstat (limited to 'drivers/char/hpet.c')
-rw-r--r--drivers/char/hpet.c25
1 files changed, 23 insertions, 2 deletions
diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 051474c65b78..34d6a1cab8de 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)