linux-kernel - Re: [PATCH] mmc: dw_mmc: try pick the exact same voltage as vmmc for vqmmc

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Message-ID: <CAD=FV=UZT91rp2rgPCh5mY5s+g8J3iOrDsHKjuXAgEqoye5goQ@mail.gmail.com>
Date:	Mon, 24 Nov 2014 21:29:28 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	addy ke <addy.ke@...k-chips.com>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"tgih.jun@...sung.com" <tgih.jun@...sung.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Chris Ball <chris@...ntf.net>,
	Dinh Nguyen <dinguyen@...era.com>,
	Heiko Stübner <heiko@...ech.de>,
	Olof Johansson <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Alexandru Stan <amstan@...omium.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"zhenfu.fang" <zhenfu.fang@...k-chips.com>,
	Eddie Cai <cf@...k-chips.com>, lintao <lintao@...k-chips.com>,
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	Kever Yang <kever.yang@...k-chips.com>,
	zhangqing <zhangqing@...k-chips.com>,
	Lin Huang <hl@...k-chips.com>
Subject: Re: [PATCH] mmc: dw_mmc: try pick the exact same voltage as vmmc for vqmmc

Ulf,

On Mon, Nov 24, 2014 at 5:29 AM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
>> 2. Several people I've talked to have expressed concerns that our
>> minimum value is 2.7V.  Apparently that's really on the edge and makes
>> EEs a little nervous.  The quick sample of cards sitting on my desk
>> shows that they seem to claim 0x00ff8000, which doesn't include 2.7V.
>
> 0x00ff8000 states what values of VDD levels the device supports. Not VIO.

Yup, I was just pointing out that possibly others were trying to get a
little bit of margin (not going all the way down to 2.7V) too.

>> Both of the above make me feel like dw_mmc should try its best to pick
>> a value for vqmmc that is closest to the value of vmmc (and >= 2.7V).
>> That also happens to make us work exactly like hosts where vmmc and
>> vqmmc are supplied by the same supply.
>
> I do see your point. And I agree that it would be nice to achieve
> something like this.
>
> The question is how to do this. For sure, we need to involve the mmc
> core to handle this correctly.

You could add a function to the core that we could call from
dw_mci_switch_voltage() and it would do all the work except trying to
set the UHS register.  That would certainly make it easy.  That could
try to set the highest voltage that is <= vmmc when we're at
MMC_SIGNAL_VOLTAGE_330.

-Doug
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