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

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Date:	Mon, 4 Jul 2011 17:58:27 +0900
From:	MyungJoo Ham <myungjoo.ham@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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.

>
> Thanks,
> Rafael
>

Thank you.

- MyungJoo


-- 
MyungJoo Ham, Ph.D.
Mobile Software Platform Lab,
Digital Media and Communications (DMC) Business
Samsung Electronics
cell: 82-10-6714-2858
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