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SubjectRE: [PATCH] PM: fix calculation mistake in roll-over cases
On Thu, 27 Oct 2011, Venu Byravarasu wrote:

> > From: venu byravarasu <vbyravarasu@nvidia.com>
> >
> > In case of jiffies roll over, delta is made zero.
> > Hence fixing it, after taking roll over into consideration.
> >
> > Signed-off-by: venu byravarasu <vbyravarasu@nvidia.com>
> > ---
> > When jiffies roll over, calculation of time spent in the
> > current state (stored in variable 'delta') is incorrect.
> > Hence fixing it, after taking roll over into consideration.
> >
> > drivers/base/power/runtime.c | 7 ++++---
> > 1 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index 1079e03..bd93fb0 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -30,12 +30,13 @@ static int rpm_suspend(struct device *dev, int rpmflags);
> > void update_pm_runtime_accounting(struct device *dev)
> > {
> > unsigned long now = jiffies;
> > - int delta;
> > + unsigned long delta;

That change is correct.

> > + unsigned long max_num = ~0;

max_num is not needed.

> >
> > delta = now - dev->power.accounting_timestamp;
> >
> > - if (delta < 0)
> > - delta = 0;

That change is correct.

> > + if (now < dev->power.accounting_timestamp)
> > + delta = max_num - dev->power.accounting_timestamp + now;

These two lines are not needed.

> >
> > dev->power.accounting_timestamp = now;
>
> > I'm not sure how this is supposed to improve things. Care to give more
> > details?
> Below two items were taken care with this change:
> 1. Value of now is jiffies which is unsigned long.
> As it is being stored in delta of 'int' type, for all values of now > 0x80000000,
> Delta will be made 0 with the original if condition.
> By changing delta to unsigned long, this is taken care.

Yes, that's the right thing to do.

> 2. Even if delta is made unsigned, in cases of jiffies roll over, delta will be zero.
> That is also being taken care with the code added as part of if condition.

Since delta is now unsigned, the "if (delta < 0)" test can never
succeed. Therefore it can be removed.

The new lines you added with max_num don't seem to serve any useful
purpose. All they do is recalculate the same value that delta had
before, but with an off-by-one error.

Alan Stern



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