linux-kernel - Re: [PATCH v3 2/3] PM / DEVFREQ: add example governors

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Date:	Mon, 4 Jul 2011 10:43:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	myungjoo.ham@...il.com
Cc:	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Jiejing Zhang <kzjeef@...il.com>,
	Colin Cross <ccross@...gle.com>, Nishanth Menon <nm@...com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 2/3] PM / DEVFREQ: add example governors

On Monday, July 04, 2011, MyungJoo Ham wrote:
> Hello,
> 
> 2011/7/3 Rafael J. Wysocki <rjw@...k.pl>:
> > Hi,
> >
> > On Friday, May 27, 2011, MyungJoo Ham wrote:
> >> Three CPUFREQ-like governors are provided as examples.
> >>
> >> powersave: use the lowest frequency possible. The user (device) should
> >> set the polling_ms as 0 because polling is useless for this governor.
> >>
> >> performance: use the highest freqeuncy possible. The user (device)
> >> should set the polling_ms as 0 because polling is useless for this
> >> governor.
> >>
> >> simple_ondemand: simplified version of CPUFREQ's ONDEMAND governor.
> >>
> >> When a user updates OPP entries (enable/disable/add), OPP framework
> >> automatically notifies DEVFREQ to update operating frequency
> >> accordingly. Thus, DEVFREQ users (device drivers) do not need to update
> >> DEVFREQ manually with OPP entry updates or set polling_ms for powersave
> >> , performance, or any other "static" governors.
> >>
> >> Signed-off-by: MyungJoo Ham <myungjoo.ham@...sung.com>
> >> Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> []
> >> +
> >> +     /* Set the desired frequency based on the load */
> >> +     a = (unsigned long long) stat.busy_time * stat.current_frequency;
> >
> > What's the purpose of the conversion?
> 
> Assuming that the work speed of a device is proportional to its
> frequency, it measures the amount of work done.
> It's time * work/time. For example, during the last 10 second, if the
> busy_time was 5 sec and frequency was 10MHz,
> it's "50M", which is same as 20MHz and 2.5 sec.

I understand that, but my question was why you're doing a forced conversion
to (unsigned long long).

Thanks,
Rafael
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