linux-kernel - Re: [patch V2 40/64] hwmon: ibmaem: Use ktime_get

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Message-ID: <20140721213825.GA2862@birch.djwong.org>
Date:	Mon, 21 Jul 2014 14:38:25 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Jean Delvare <jdelvare@...e.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [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@...utronix.de>
> > Cc: Jean Delvare <jdelvare@...e.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@...e.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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