linux-kernel - Re: [PATCH] mmc: dw_mmc: add fixed divider for ciu

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Date:	Fri, 10 Apr 2015 13:08:41 -0500
From:	Dinh Nguyen <dinh.linux@...il.com>
To:	Doug Anderson <dianders@...omium.org>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>
CC:	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	setka@...s.cz, Seungwon Jeon <tgih.jun@...sung.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Chris Ball <chris@...ntf.net>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Alexandru Stan <amstan@...omium.org>,
	Heiko Stübner <heiko@...ech.de>
Subject: Re: [PATCH] mmc: dw_mmc: add fixed divider for ciu_clk on SoCFPGA



On 4/10/15 10:15 AM, Doug Anderson wrote:
> Dinh,
> 
> On Fri, Apr 10, 2015 at 6:56 AM,  <dinguyen@...nsource.altera.com> wrote:
>> From: Dinh Nguyen <dinguyen@...nsource.altera.com>
>>
>> The ciu_clk(Card Interface Unit Clock) on the SoCFPGA platform has a fixed
>> divider of 4. Add the fixed clock divide code in the platform's clock
>> setup code.
> 
> It might actually be better to do this a different way for SoCFPGA.  I
> sorta wish we had done it differently for Rockchip as well, but at
> this point you end up with the complexity of changing device tree
> bindings in conjunction with code and it gets ugly.

Yes, I started going down this path and realized that.

> 
> Specifically, you've probably got the following clocks:
> 
> SD_prediv = 400MHz
> -> SD postdiv = 100MHz
> -> SD sample = 100MHz, shifted
> -> SD drive = 100MHz, shifted
> 
> Right now you're specifying "SD_prediv" as the SD card clock.  If you
> instead expose "SD postdiv" as a new clock (from your clock driver)
> that is "SD prediv" divided by 4 then you'll magically get all the
> behavior that you want with no modifications to dw_mmc.  Just make
> sure that "SD postdiv" passes on rate changes to its parent (that's
> just a flag in the common clock framework).
> 

That's a great idea, thanks for pointing that out.

> 
> At some point in time you'll also want to expose the sample and drive
> clocks once you get UHS modes working.  Alexandru posted some patches
> for this a while ago to support tuning in dw_mmc using just drive and
> sample clocks, but the patch still needed some more work.  Either he
> or I will probably pick it up again soon.

I think I have already done this by representing the sdmmc_clk with a
"clk-phase" property for this clock.

Dinh
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