linux-kernel - RE: [PATCH] PM: fix calculation mistake in roll-over cases

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Date:	Thu, 27 Oct 2011 14:30:09 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Venu Byravarasu <vbyravarasu@...dia.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, "pavel@....cz" <pavel@....cz>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>
Subject: RE: [PATCH] PM: fix calculation mistake in roll-over cases

On Thu, 27 Oct 2011, Venu Byravarasu wrote:

> > From: venu byravarasu <vbyravarasu@...dia.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@...dia.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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