linux-kernel - Re: [PATCH] cpuidle: Change ktime_get() with local

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Message-ID: <20160420125837.GZ3430@twins.programming.kicks-ass.net>
Date:	Wed, 20 Apr 2016 14:58:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	rjw@...ysocki.net, mingo@...nel.org,
	"open list:CPUIDLE DRIVERS" <linux-pm@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuidle: Change ktime_get() with local_clock()

On Wed, Apr 20, 2016 at 02:30:11PM +0200, Daniel Lezcano wrote:
> On Wed, Apr 20, 2016 at 02:13:15PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 14, 2016 at 09:23:54PM +0200, Daniel Lezcano wrote:
> > > @@ -217,7 +217,11 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
> > >  	if (!cpuidle_state_is_coupled(drv, entered_state))
> > >  		local_irq_enable();
> > >  
> > > -	diff = ktime_to_us(ktime_sub(time_end, time_start));
> > > +	/*
> > > +	 * local_clock() returns the time in nanosecond, let's shift
> > > +	 * by 10 (divide by 1024) to have microsecond based time.
> > > +	 */
> > > +	diff = (time_end - time_start) >> 10;
> > 
> > Changelog fails to explain the ramifications of this change...
> 
> Sorry, I don't get the point of your comment. Do you mean I should elaborate 
> the comment above in the changelog?

Yeah, why is /1024 good enough?