drivers/char/hpet.c: fix periodic-emulation for delayed interrupts

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.

More detail:

Assuming an interval of 5 between each expected interrupt we have a normal
case of

t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t5: interrupt, read t5 from comparator, set next interrupt t5 + 5
t10: interrupt, read t10 from comparator, set next interrupt t10 + 5
...

So, what happens when the interrupt is serviced too late?

t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t11: delayed interrupt serviced, read t5 from comparator, set next
interrupt t5 + 5, which is in the past!
... counter loops ...
t10: Much much later, get the next interrupt.

This can happen either because we have interrupts masked for too long
(some stupid driver goes on a printk rampage) or just because we are
pushing the limits of the interval (too small a period), or both most
probably.

My solution is to read the main counter as well and set the next interrupt
to occur at the right interval, for example:

t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t11: delayed interrupt serviced, read t5 from comparator, set next
interrupt t15 as t10 has been missed.
t15: back on track.

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 051474c65b783c5e9f3315de2fb3732516810027..34d6a1cab8def622d95c46b138b02a7d8a0eeeec 100644 (file)
--- 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)