[PATCH] Fix periodic-emulation in HPET for delayed interrupts

From: Nils Carlson
Date: Wed Jun 01 2011 - 07:48:44 EST

When interrupts are delayed due to interrupt masking or due
to other interrupts being serviced the HPET periodic-emuation
would fail. This happened because given an interval t and
a time for the current interrupt m we would compute the next
time as t + m. This works until we are delayed for > t, in
which case we would be writing a new value which is in fact
in the past.

This can be solved by computing the next time instead as
(k * t) + m where k is large enough to be in the future.
The exact computation of k is described in a comment to
the code.

Signed-off-by: Nils Carlson <nils.carlson@xxxxxxxxxxxx>
---
drivers/char/hpet.c | 25 +++++++++++++++++++++++--
1 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 051474c..34d6a1c 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;
- write_counter(t + m, &devp->hd_timer->hpet_compare);
+ /* 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)
--
1.7.2.5

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