From 273ef9509b7903e50f36aaf9f1d5dc9087fca506 Mon Sep 17 00:00:00 2001 From: Nils Carlson Date: Wed, 15 Jun 2011 15:08:54 -0700 Subject: 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 Cc: John Stultz Cc: Thomas Gleixner Cc: Clemens Ladisch Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/char/hpet.c | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) (limited to 'drivers/char') 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) -- cgit v1.2.3-70-g09d2