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

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Message-Id: <201110251508.40854.rjw@sisk.pl>
Date:	Tue, 25 Oct 2011 15:08:40 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	vbyravarasu@...dia.com
Cc:	pavel@....cz, len.brown@...el.com, gregkh@...e.de,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] PM: fix calculation mistake in roll-over cases

On Tuesday, October 25, 2011, vbyravarasu@...dia.com 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;
> +	unsigned long max_num = ~0;
>  
>  	delta = now - dev->power.accounting_timestamp;
>  
> -	if (delta < 0)
> -		delta = 0;
> +	if (now < dev->power.accounting_timestamp)
> +		delta = max_num - dev->power.accounting_timestamp + now;
>  
>  	dev->power.accounting_timestamp = now;

I'm not sure how this is supposed to improve things.  Care to give more
details?

Rafael
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