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SubjectRe: [patch V2 40/64] hwmon: ibmaem: Use ktime_get_ns()
On Mon, Jul 21, 2014 at 10:37:38AM +0200, Jean Delvare wrote:
> Hi Thomas,
>
> Le Wednesday 16 July 2014 à 21:04 +0000, Thomas Gleixner a écrit :
> > Using the wall clock time for delta time calculations is wrong to
> > begin with because wall clock time can be set from userspace and NTP.
> > Such data wants to be based on clock monotonic.
> >
> > The calcuations also are done on a nanosecond basis. Use the
>
> Typo: calculations
>
> > nanoseconds based interface right away.
> >
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Jean Delvare <jdelvare@suse.de>
> > ---
> > drivers/hwmon/ibmaem.c | 6 ++----
> > 1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > Index: tip/drivers/hwmon/ibmaem.c
> > ===================================================================
> > --- tip.orig/drivers/hwmon/ibmaem.c
> > +++ tip/drivers/hwmon/ibmaem.c
> > @@ -842,11 +842,10 @@ static ssize_t aem_show_power(struct dev
> > struct aem_data *data = dev_get_drvdata(dev);
> > u64 before, after, delta, time;
> > signed long leftover;
> > - struct timespec b, a;
> >
> > mutex_lock(&data->lock);
> > update_aem_energy_one(data, attr->index);
> > - getnstimeofday(&b);
> > + time = ktime_get_ns();
> > before = data->energy[attr->index];
> >
> > leftover = schedule_timeout_interruptible(
> > @@ -858,11 +857,10 @@ static ssize_t aem_show_power(struct dev
> > }
> >
> > update_aem_energy_one(data, attr->index);
> > - getnstimeofday(&a);
> > + time = ktime_get_ns() - time;
> > after = data->energy[attr->index];
> > mutex_unlock(&data->lock);
> >
> > - time = timespec_to_ns(&a) - timespec_to_ns(&b);
> > delta = (after - before) * UJ_PER_MJ;
> >
> > return sprintf(buf, "%llu\n",
>
> I'm not familiar with the driver and I can't test it, and I'm not
> familiar with kernel timekeeping either. All I can say is that the
> changes look reasonable and good.
>
> Acked-by: Jean Delvare <jdelvare@suse.de>
>
> Darrick, maybe you want to comment or test?

Looks ok to me from a theoretical point of view, but since I no longer work at
IBM, I don't have hardware for a real test. :(

--D
>
> --
> Jean Delvare
> SUSE L3 Support
>
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