linux-kernel - Re: [PATCH v6 2/3] mmc: dw_mmc: Honor requests to set the clock to 0 (turn off clock)

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Message-ID: <CAD=FV=XL-J700CBgwgWfy6JNKL9rNcbPNsqXH=--kGsNJJPS2Q@mail.gmail.com>
Date:	Sun, 25 Aug 2013 20:38:41 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Jaehoon Chung <jh80.chung@...sung.com>
Cc:	Chris Ball <cjb@...top.org>, Seungwon Jeon <tgih.jun@...sung.com>,
	James Hogan <james.hogan@...tec.com>,
	Grant Grundler <grundler@...omium.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Abhilash Kesavan <a.kesavan@...sung.com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Olof Johansson <olof@...om.net>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] mmc: dw_mmc: Honor requests to set the clock to 0
 (turn off clock)

Jaehoon,

On Sun, Aug 25, 2013 at 6:31 PM, Jaehoon Chung <jh80.chung@...sung.com> wrote:
> Hi Doug,
>
> On 08/24/2013 05:40 AM, Doug Anderson wrote:
>> Jaehoon,
>>
>> On Fri, Aug 23, 2013 at 6:21 AM, Jaehoon Chung <jh80.chung@...sung.com> wrote:
>>> Hi Doug,
>>>
>>> If the clock-gating is enabled, then maybe it's continuously printed the kernel message for Bus_speed.
>>
>> Can you explain?  I don't think dw_mmc has support for clock gating
>> right now.  ...or are there some patches that I'm not aware of?  I
>> could believe that if you've got some non-upstream clock gating
>> patches that these would need to be modified to handle it...  ...but
>> unless those are slated to land upstream it seems like I can't take
>> them into account, can I?
> If i enabled the CONFIG_MMC_CLK_GATE, the i have found the below message whenever some operation is run.
> I will test more with your patch.

Ah, sorry!  I wasn't aware of that config option.  I was thinking of
automatic clock gating based on something like the common clock
framework.  When there are no more users of a gate clock it will get
turned off.  To have that work dw_mmc would need to release its biu /
ciu clocks at some point.

If I had to guess, I'd speculate that perhaps we should just change
the printout to a dev_debug(), though I do find that printout
incredibly useful.  If I had to guess I'd say that the mmc core is
switching between a clock of 0 and a full speed clock constantly.  If
that's true then it means that dw_mmc used to treat that like a no-op.
 Now it actually gates the clock.  If you comment out the printout, do
things still work?  Does your power consumption go down?

Let me know if you find anything.  Otherwise I can try to reproduce this week.

-Doug
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