linux-kernel - Re: [PATCH v2] mmc: sdhci-msm: Boost controller core clock

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Message-ID: <20151118013827.GC32672@codeaurora.org>
Date:	Tue, 17 Nov 2015 17:38:27 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Bjorn Andersson <bjorn@...o.se>,
	"Ivan T. Ivanov" <ivan.ivanov@...aro.org>,
	Georgi Djakov <georgi.djakov@...aro.org>,
	Peter Griffin <peter.griffin@...aro.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2] mmc: sdhci-msm: Boost controller core clock

On 11/16, Ulf Hansson wrote:
> [...]
> 
> 
> >
> > In case you're wondering, the max frequency for sdc1 on 8974ac is
> > 400MHz. If it's just a plain 8974pro then the max frequency is
> > 200MHz. Otherwise, sdc2 maxes out at 200Mhz and sdc3 and sdc4 max
> > out at 100MHz.
> 
> When you say that sdc1 supports 400MHz, what does that mean? That it
> actually can cope with that clock rate when communicating with the MMC
> card?

I suspect there must be some internal divider in the sdc IP
itself so that it doesn't put out 400MHz on the bus, but I really
don't know. What I mean is that the clock going into the IP from
the clock controller is running at 400MHz, after it goes into the
IP it could be divided, etc. before exiting the SoC on some pin.

> 
> This makes me wonder how you deal with power management (DVFS).
> 
> For example when you have the possibility to gate this clock (at
> request inactivity) when the rate is set to 400 MHz and OPP is
> increased, how will then that clock gating affect the OPP?

Sorry I'm not really following the question here. The gate will
disable the clock in the clock controller, cutting the signal off
upstream of the sdc IP. When we do DVFS we'll stop considering
this clock as part of the overall power level for the voltage
associated with the frequency. When all other clocks that are
using the same voltage and are on and running at frequencies that
don't need that high of voltage we can reduce the voltage and
drop down to something lower.

-- 
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a Linux Foundation Collaborative Project
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