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Subject[34-longterm 029/209] hpet: fix unwanted interrupt due to stale irq status bit
Date
From: Clemens Ladisch <clemens@ladisch.de>

=====================================================================
| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|
=====================================================================

commit 96e9694df446d1154ec2f4fdba8908588b9cba38 upstream.

Jaswinder Singh Rajput wrote:
> By executing Documentation/timers/hpet_example.c
>
> for polling, I requested for 3 iterations but it seems iteration work
> for only 2 as first expired time is always very small.
>
> # ./hpet_example poll /dev/hpet 10 3
> -hpet: executing poll
> hpet_poll: info.hi_flags 0x0
> hpet_poll: expired time = 0x13
> hpet_poll: revents = 0x1
> hpet_poll: data 0x1
> hpet_poll: expired time = 0x1868c
> hpet_poll: revents = 0x1
> hpet_poll: data 0x1
> hpet_poll: expired time = 0x18645
> hpet_poll: revents = 0x1
> hpet_poll: data 0x1

Clearing the HPET interrupt enable bit disables interrupt generation
but does not disable the timer, so the interrupt status bit will still
be set when the timer elapses. If another interrupt arrives before
the timer has been correctly programmed (due to some other device on
the same interrupt line, or CONFIG_DEBUG_SHIRQ), this results in an
extra unwanted interrupt event because the status bit is likely to be
set from comparator matches that happened before the device was opened.

Therefore, we have to ensure that the interrupt status bit is and
stays cleared until we actually program the timer.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Reported-by: Jaswinder Singh Rajput <jaswinderlinux@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Bob Picco <bpicco@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
drivers/char/hpet.c | 15 +++++++++++++++
1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 9ded667..7447ffa 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -475,6 +475,21 @@ static int hpet_ioctl_ieon(struct hpet_dev *devp)
if (irq) {
unsigned long irq_flags;

+ if (devp->hd_flags & HPET_SHARED_IRQ) {
+ /*
+ * To prevent the interrupt handler from seeing an
+ * unwanted interrupt status bit, program the timer
+ * so that it will not fire in the near future ...
+ */
+ writel(readl(&timer->hpet_config) & ~Tn_TYPE_CNF_MASK,
+ &timer->hpet_config);
+ write_counter(read_counter(&hpet->hpet_mc),
+ &timer->hpet_compare);
+ /* ... and clear any left-over status. */
+ isr = 1 << (devp - devp->hd_hpets->hp_dev);
+ writel(isr, &hpet->hpet_isr);
+ }
+
sprintf(devp->hd_name, "hpet%d", (int)(devp - hpetp->hp_dev));
irq_flags = devp->hd_flags & HPET_SHARED_IRQ
? IRQF_SHARED : IRQF_DISABLED;
--
1.7.4.4


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