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

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Date:	Thu, 5 Mar 2015 15:53:35 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	Pi-Cheng Chen <pi-cheng.chen@...aro.org>,
	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
Subject: Re: [PATCH] clk: mediatek: Export CPU mux clocks for CPU frequency control

On 5 March 2015 at 14:49, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> The sequence to change the CPU frequency on the Mediatek SoCs is like this:
>
> - Change CPU from CPU PLL to another clock source (intermediate source)
> - Change CPU PLL frequency
> - wait until PLL has settled
> - switch back to CPU PLL

This should be the case for most of the intermediate-freq users..

> The frequency of the intermediate source is irrelevant, the important
> thing is that the CPU is switched to this source while the CPU PLL is
> reconfigured.

Right.

> In Pi-Chengs patches the switch to th eintermediate clock is done like:
>
>         rate = clk_get_rate(intermediate_clk);
>         clk_set_rate(cpu_clk, rate);
>
> Now the clk framework does the switch not because it's told to switch
> to another parent, but only because the other parent happens to be the
> only provider for that rate. That's rubbish, when the parent must be
> changed, then it should be done explicitly.
> What if the CPU PLL and the intermediate clk happen to have the same
> rate? Then the clk_set_rate above simply does nothing, no parent is
> changed and the following rate change of the CPU PLL just crashes the
> system.

The problem is that the code is common across platforms that need to
reparent or just change rate for intermediate clocks. And the best we
can do is clk_set_rate() and so probably the clk driver need to take care
of this somehow and make sure we don't result in a crash like you just
demonstrated.
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