linux-kernel - Re: [PATCH] clk: mediatek: Export CPU mux clocks for CPU frequency control

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Date:	Tue, 10 Mar 2015 16:59:12 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Sascha Hauer" <s.hauer@...gutronix.de>
Cc:	"Pi-Cheng Chen" <pi-cheng.chen@...aro.org>,
	"Stephen Boyd" <sboyd@...eaurora.org>,
	"Matthias Brugger" <matthias.bgg@...il.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>,
	"Henry Chen" <henryc.chen@...iatek.com>,
	"James Liao" <jamesjj.liao@...iatek.com>,
	"Chen Fan" <fan.chen@...iatek.com>,
	"Eddie Huang" <eddie.huang@...iatek.com>,
	"Joe.C" <yingjoe.chen@...iatek.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"Linaro Kernel Mailman List" <linaro-kernel@...ts.linaro.org>,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] clk: mediatek: Export CPU mux clocks for CPU frequency control

Quoting Viresh Kumar (2015-03-05 00:59:50)
> On 5 March 2015 at 13:12, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> > We have clk_set_parent for changing the parent and clk_set_rate to
> > change the rate. Use the former for changing the parent and the latter
> > for changing the rate. What you are interested in is changing the
> > parent, so use clk_set_parent for this and not abuse a side effect
> > of clk_set_rate.
> 
> clk_set_rate() for CPUs clock is responsible to change clock rate
> of the CPU. Whether it plays with PLLs or muxes, its not that relevant.

Agreed.

> 
> > My suggestion is to take another approach. Implement clk_set_rate for
> > these muxes and in the set_rate hook:
> >
> > - switch mux to intermediate PLL parent
> > - call clk_set_rate() for the real parent PLL
> > - switch mux back to real parent PLL
> >
> > This way the things happening behind the scenes are completely transparent
> > to the cpufreq driver and you can use cpufreq-dt as is without changes.
> 
> CPUFreq wants to change to intermediate frequency by itself against
> some magic change behind the scene. The major requirement for that
> comes from the fact that we want to send PRE/POST freq notifiers on
> which loops-per-jiffie depends.

I assume you are saying that you want to update loops-per-jiffie while
at an intermediate frequency. Why? This operation should not take very
long.

Imagine a (hypothetical?) processor that changes frequency in many small
steps until it converges to the target rate. Would you want to update
lpj for every step?

Regards,
Mike
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