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

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Date:	Thu, 7 Jul 2011 23:08:38 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	MyungJoo Ham <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:
> On Mon, Jul 4, 2011 at 5:43 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > 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).
> 
> Ah.. that was for the 64bit operations.
> 
> Both busy_time and current_frequency are 32bit and current_frequency
> may be a big number.
> 
> Thus, in order to get "freq" value without losing bits (e.g., if
> current_frequency = 1GHz and busy_time = 8000, we get an overflow
> without 64bit operations), I've inserted 64bit operations with the
> conversion. For the cosmetic reasons, it appears that "u64" looks
> better though.

You wouldn't need the explicit type casting if you did

a = stat.busy_time;
a *= stat.current_frequency;

Thanks,
Rafael
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