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Subject[PATCH 3.2 16/27] s390,time: revert direct ktime path for s390 clockevent device
3.2.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

commit 8adbf78ec4839c1dc4ff20c9a1f332a7bc99e6e6 upstream.

Git commit 4f37a68cdaf6dea833cfdded2a3e0c47c0f006da
"s390: Use direct ktime path for s390 clockevent device" makes use
of the CLOCK_EVT_FEAT_KTIME clockevent option to avoid the delta
calculation with ktime_get() in clockevents_program_event and the
get_tod_clock() in s390_next_event. This is based on the assumption
that the difference between the internal ktime and the hardware
clock is reflected in the wall_to_monotonic delta. But this is not
true, the ntp corrections are applied via changes to the tk->mult
multiplier and this is not reflected in wall_to_monotonic.

In theory this could be solved by using the raw monotonic clock
but it is simpler to switch back to the standard clock delta
calculation.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.2: s/get_tod_clock()/get_clock()/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/s390/kernel/time.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -110,20 +110,10 @@ static void fixup_clock_comparator(unsig
set_clock_comparator(S390_lowcore.clock_comparator);
}

-static int s390_next_ktime(ktime_t expires,
+static int s390_next_event(unsigned long delta,
struct clock_event_device *evt)
{
- struct timespec ts;
- u64 nsecs;
-
- ts.tv_sec = ts.tv_nsec = 0;
- monotonic_to_bootbased(&ts);
- nsecs = ktime_to_ns(ktime_add(timespec_to_ktime(ts), expires));
- do_div(nsecs, 125);
- S390_lowcore.clock_comparator = sched_clock_base_cc + (nsecs << 9);
- /* Program the maximum value if we have an overflow (== year 2042) */
- if (unlikely(S390_lowcore.clock_comparator < sched_clock_base_cc))
- S390_lowcore.clock_comparator = -1ULL;
+ S390_lowcore.clock_comparator = get_clock() + delta;
set_clock_comparator(S390_lowcore.clock_comparator);
return 0;
}
@@ -148,15 +138,14 @@ void init_cpu_timer(void)
cpu = smp_processor_id();
cd = &per_cpu(comparators, cpu);
cd->name = "comparator";
- cd->features = CLOCK_EVT_FEAT_ONESHOT |
- CLOCK_EVT_FEAT_KTIME;
+ cd->features = CLOCK_EVT_FEAT_ONESHOT;
cd->mult = 16777;
cd->shift = 12;
cd->min_delta_ns = 1;
cd->max_delta_ns = LONG_MAX;
cd->rating = 400;
cd->cpumask = cpumask_of(cpu);
- cd->set_next_ktime = s390_next_ktime;
+ cd->set_next_event = s390_next_event;
cd->set_mode = s390_set_mode;

clockevents_register_device(cd);


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