linux-kernel - Re: [PATCH 06/11] Documentation: mmc: sdhci-of-arasan: Add ability to export card clock

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Date:	Wed, 8 Jun 2016 13:52:20 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Rob Herring <robh@...nel.org>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Kishon Vijay Abraham I <kishon@...com>,
	Heiko Stuebner <heiko@...ech.de>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	Ziyuan Xu <xzy.xu@...k-chips.com>,
	Brian Norris <briannorris@...omium.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/11] Documentation: mmc: sdhci-of-arasan: Add ability to
 export card clock

Rob,

On Wed, Jun 8, 2016 at 1:19 PM, Rob Herring <robh@...nel.org> wrote:
> On Tue, Jun 07, 2016 at 03:44:39PM -0700, Douglas Anderson wrote:
>> Some SD/eMMC PHYs (like the PHY from Arasan that is designed to work
>> with arasan,sdhci-5.1) need to know the card clock in order to function
>> properly.  Let's expose this clock using a standard device tree
>> mechanism so that the PHY can get access to and query the card clock.
>
> Need to know the clock freq or need the clock? The former doesn't need
> to be in DT.

The PHY needs to know the card clock frequency.  This clock clock
frequency is known nowhere else in the system except the SDHCI driver
since the SDHCI component takes its input clock and applies some
dividers to get the card clock.  In silicon, the SDHCI component and
the PHY are separate and there is a physical clock signal going from
the SDHCI component to the PHY component, so modeling this as a clock
in in the device tree seems sane, doesn't it?

-Doug