* Kevin Hilman <khilman@xxxxxxxxxxxxxxxxxxx> [120924 17:44]:
> R Sricharan <r.sricharan@xxxxxx> writes:
>
> > From: Colin Cross <ccross@xxxxxxxxxxx>
> >
> > read_persistent_clock uses a global variable, use a spinlock to
> > ensure non-atomic updates to the variable don't overlap and cause
> > time to move backwards.
> >
> > Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
> > Signed-off-by: R Sricharan <r.sricharan@xxxxxx>
>
> Acked-by: Kevin Hilman <khilman@xxxxxx>
>
> Tony, this should probably be queued up for v3.7-rc and flagged for stable.
Yes I can see that happening. But then in addition..
> > ---
> > [V2] Added signed-off-by and looped in "linux-arm-kernel" list
> >
> > arch/arm/plat-omap/counter_32k.c | 21 ++++++++++++++-------
> > 1 file changed, 14 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm/plat-omap/counter_32k.c b/arch/arm/plat-omap/counter_32k.c
> > index dbf1e03..2bc51fb 100644
> > --- a/arch/arm/plat-omap/counter_32k.c
> > +++ b/arch/arm/plat-omap/counter_32k.c
> > @@ -55,22 +55,29 @@ static u32 notrace omap_32k_read_sched_clock(void)
> > * nsecs and adds to a monotonically increasing timespec.
> > */
> > static struct timespec persistent_ts;
> > -static cycles_t cycles, last_cycles;
> > +static cycles_t cycles;
> > static unsigned int persistent_mult, persistent_shift;
> > +static DEFINE_SPINLOCK(read_persistent_clock_lock);
> > +
> > static void omap_read_persistent_clock(struct timespec *ts)
> > {
> > unsigned long long nsecs;
> > - cycles_t delta;
> > - struct timespec *tsp = &persistent_ts;
> > + cycles_t last_cycles;
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&read_persistent_clock_lock, flags);
> >
> > last_cycles = cycles;
> > cycles = sync32k_cnt_reg ? __raw_readl(sync32k_cnt_reg) : 0;
> > - delta = cycles - last_cycles;
> >
> > - nsecs = clocksource_cyc2ns(delta, persistent_mult, persistent_shift);
> > + nsecs = clocksource_cyc2ns(cycles - last_cycles,
> > + persistent_mult, persistent_shift);
..I think there's another bug here where cycles - last_cycles
returns wrong value when the timer wraps around as cycles_t is
64 bits and the counter is 32 bits. It seems it's been there
since when the read_persistent_clock was added with commit
d92cfcbe (OMAP: timekeeping: time should not stop during suspend)?
It seems that after this patch cycles should not be cycles_t
but u32, and the result of cycles - last_cycles should also
be u32.
> > + timespec_add_ns(&persistent_ts, nsecs);
> > +
> > + *ts = persistent_ts;
> >
> > - timespec_add_ns(tsp, nsecs);
> > - *ts = *tsp;
> > + spin_unlock_irqrestore(&read_persistent_clock_lock, flags);
> > }
Regards,
Tony
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