linux-kernel - Re: [PATCH v2 2/2] mmc: dw_mmc: Add the ability to set the ciu clock frequency

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Message-ID: <CAD=FV=Wy5X9C=NOUVyAzZK5+BKD1w4FTmoTHzKipq2j_Y9rdtg@mail.gmail.com>
Date:	Tue, 18 Jun 2013 08:15:37 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Jaehoon Chung <jh80.chung@...sung.com>
Cc:	Chris Ball <cjb@...top.org>, Olof Johansson <olof@...om.net>,
	Andrew Bresticker <abrestic@...omium.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Abhilash Kesavan <a.kesavan@...sung.com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Seungwon Jeon <tgih.jun@...sung.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	Will Newton <will.newton@...il.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>, linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>
Subject: Re: [PATCH v2 2/2] mmc: dw_mmc: Add the ability to set the ciu clock frequency

Jaehoon,

On Mon, Jun 17, 2013 at 9:51 PM, Jaehoon Chung <jh80.chung@...sung.com> wrote:
> Hi Doug,
>
> I have one question for using <clock-frequency>.
> I found the fixed-rate-clocks feature.
> If we want to set <clock-frequency>, then can we use the fixed-rate-clocks?
> i'm not sure how use the fixed-rate-clocks. but it seems to set fixed-rate value for clock frequency.
>
> clk_set_rate() didn't ensure to set the <clock-frequency> value.

I'm not sure I understand the question.  I don't think that the
fixed-rate-clocks have a close relation to the clock-frequency or the
ciu clock.  The fixed-rate-clock entries for a board usually specify
the root clock source for a board.  For instance in exynos5250-snow
you can see:

fixed-rate-clocks {
  xxti {
    compatible = "samsung,clock-xxti";
    clock-frequency = <24000000>;
  };
};

Other clocks in the board are derived from this clock through PLLs,
muxes, dividers, gates, etc.  On 5250 we have:

 fin_pll (xxti) -> fout_mpll -> fout_mplldiv2 -> mout_mpll_fout ->
 sclk_mpll -> sclk_mpll_user -> mout_mmc1 -> div_mmc1
 div_mmc_pre1 -> sclk_mmc1

In 5250 the ciu clock for mmc1 is sclk_mmc1, which is a simple gate.
When you "enable" this clock it, ungates it.  The sclk_mmc1 has the
flag CLK_SET_RATE_PARENT on it.  That means when you try to set the
rate it will involve the parent clock (div_mmc_pre1).  The parent
clock also has CLK_SET_RATE_PARENT, so it can also involve div_mmc1.
I haven't dug through to see how the clock framework splits up divides
between div_mmc1 and div_mmc_pre1, but it's supposed to handle that.

We don't allow clk_set_rate to percolate any higher (no
CLK_SET_RATE_PARENT at mout_mmc1).

-Doug
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