linux-kernel - Re: [PATCH V2 2/2] cpufreq: Change freq before suspending governors

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Date:	Fri, 22 Nov 2013 12:39:24 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Stephen Warren <swarren@...dia.com>,
	Kukjin Kim <kgene@...nel.org>,
	Amit Daniel Kachhap <amit.daniel@...sung.com>,
	Sachin Kamat <sachin.kamat@...aro.org>
CC:	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lan Tianyu <tianyu.lan@...el.com>, Nishanth Menon <nm@...com>,
	jinchoi@...adcom.com,
	Sebastian Capella <sebastian.capella@...aro.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>
Subject: Re: [PATCH V2 2/2] cpufreq: Change freq before suspending governors

On 11/22/2013 05:52 AM, Viresh Kumar wrote:
> On 22 November 2013 18:07, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>> On Friday, November 22, 2013 04:59:49 PM Viresh Kumar wrote:
>>> Some platforms might want to change frequency before suspending governors. Like:
>>> - Some platform which want to set freq to max to speed up suspend/hibernation
>>>   process.
>>> - Some platform (like: Tegra or exynos), set this to min or bootloader's
>>>   frequency.
>>>
>>> This patch adds an option for those, so that they can specify this at call to
>>> ->init(), so that cpufreq core can take care of this before suspending system.
>>>
>>> If this variable is not updated by ->init() then its value would be zero and so
>>> core wouldn't do anything.
>>>
>>> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>>
>> I don't think this is generally necessary, because the suspend/resume routines
>> added by patch [1/2] will be executed very late during suspend or very early
>> during resume and it shouldn't really matter what performance levels the CPUs
>> are at then.
> 
> There are few things here:
> - I feel that the current place from where we have suspended stuff is not gonna
> fly. We are doing that in noirq and probably devices which might be required
> during frequency transitions might already be down.. So we *may* need to
> move that in dpm_suspend()..
> - Secondly I want to understand why Tegra/Exynos has such code which I
> mentioned above..
> 
> @Stephen, Kukjin and other samsung folks: Please provide some input here,
> before your systems break in mainline :)

I believe we set the clock to a low value because fast clocks consume
more power. Tegra architecturally supports a number of different suspend
levels. Only some of those actually power off or gate the clock source
itself.
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