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

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Date:	Fri, 10 Jun 2016 08:10:52 -0500
From:	Rob Herring <robh@...nel.org>
To:	Doug Anderson <dianders@...omium.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

On Wed, Jun 08, 2016 at 01:52:20PM -0700, Doug Anderson wrote:
> 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?

Yes, please just re-word it to make it clear there is a connection. With 
that,

Acked-by: Rob Herring <robh@...nel.org>