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

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Date:	Tue, 10 Mar 2015 08:55:43 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Pi-Cheng Chen <pi-cheng.chen@...aro.org>
Cc:	Mike Turquette <mturquette@...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,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] clk: mediatek: Export CPU mux clocks for CPU frequency
 control

On Tue, Mar 10, 2015 at 09:53:19AM +0800, Pi-Cheng Chen wrote:
> On 5 March 2015 at 15:42, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >
> > 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
> 
> Hi Sascha,
> 
> Thanks for your suggestion. I've tried to take this approach, but there's some
> issues here.
> 
> Calling clk_set_rate() inside the set_rate callback of cpumux will cause
> an infinite recursive calling in the clock framework:
> mux.set_rate() -> pll.set_rate() -> mux.set_rate -> ...

I don't understand why setting the PLL rate should call into the mux
set_rate. Are you sure you call clk_set_rate for the mux parent clk?
I think the general approach should work, drivers/clk/sirf/clk-common.c
does something similar in cpu_clk_set_rate(). If you like you can send
me your work in progress state privatly, I'll have a look then.

> 
> I've also tries to update pll register settings in the set_rate()
> callback of cpumux,
> but the PLL clock information will not be correctly updated in this case.

No, that won't work.

Sascha


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