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authorNils Carlson <nils.carlson@ericsson.com>2011-06-15 15:08:54 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-06-15 20:04:02 -0700
commit273ef9509b7903e50f36aaf9f1d5dc9087fca506 (patch)
tree4d1800cadbb85647b4db5cf5cb855dc43465536c /drivers/char
parent31b5f8eeece4c0d70b649bfac7759cf7e3f915dd (diff)
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>
Diffstat (limited to 'drivers/char')
-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)