linux-kernel - Re: [RFC 0/3] mmc: Add dynamic frequency scaling

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Date:	Thu, 15 Jan 2015 11:04:04 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Cc:	Chris Ball <chris@...ntf.net>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	Kukjin Kim <kgene@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [RFC 0/3] mmc: Add dynamic frequency scaling

On 15 January 2015 at 10:20, Krzysztof Kozlowski
<k.kozlowski@...sung.com> wrote:
> On czw, 2015-01-15 at 09:20 +0100, Ulf Hansson wrote:
>> + Mike, Stephen (Clock maintainers)
>>
>> On 12 January 2015 at 10:23, Krzysztof Kozlowski
>> <k.kozlowski@...sung.com> wrote:
>> > Hi,
>> >
>> >
>> > I would like to hear some comments about idea of scaling MMC clock
>> > frequency. The basic idea is to lower the clock when device is
>> > completely idle or not busy enough.
>>
>> We already have host drivers that implements runtime PM support.
>> Typically that would mean the clock will be gated once the device
>> becomes runtime PM suspended.
>>
>> Why should we decrease the frequency of an already gated clock?
>
> In case of idle state you're right that clkgate would be better. But
> what about finding a compromise between high performance (high
> frequency) and energy saving for different loads on MMC?

I guess a compromise could be beneficial for some SOC and use cases.
At least I remember, ST-Ericsson's UX500 SOC had such an out of tree
hack to track MMC load.

>
> The frequency scaling could help in that case. Anyway I should prepare
> some more benchmarks for such conditions.

Seems reasonable and please do!

Kind regards
Uffe
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