linux-kernel - Re: [PATCH v7 3/8] cpufreq: kirkwood: Remove use of the clk provider API

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Date:	Tue, 26 Aug 2014 22:04:05 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Andrew Lunn <andrew@...n.ch>
Cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>, Tomasz Figa <t.figa@...sung.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 3/8] cpufreq: kirkwood: Remove use of the clk provider API

On Tue, Aug 26, 2014 at 5:35 PM, Andrew Lunn <andrew@...n.ch> wrote:
>> One final thought I have had is that it might be a good idea to have a
>> mux clock which represents the clock signal that feeds into the cpu. It
>> seems that a mux is exactly what is going on here with cpuclk rate and
>> ddrclk rate.
>
> I did think of this when i implemented the cpufreq driver. What makes
> it hard is that this bit is mixed in the same 32 bit register as the
> gate clocks. It would mean two clock providers sharing the same
> register, sharing a spinlock, etc. And the gating provider is shared
> with all mvebu platforms, dove, kirkword, orion5x, and four different
> armada SoCs. So i'm pushing complexity into this shared code, which
> none of the others need. Does the standard mux clock do what is
> needed? Or would i have to write a new clock provider?

Well I think that the mux-clock type should suffice. Both the standard
gate and mux can have a spinlock passed in at registration-time, so it
could be a shared spinlock. The standard mux clock expects a bitfield
in a register, similar to the single-bit approach taken by the gate
clock. So I think it could do just fine.

>
>> I even wonder if it is even appropriate to model this transition with a
>> clock enable operation? Maybe it is only a multiplex operation, or
>> perhaps a combination of enabling the powersave clock and changing the
>> parent input to the cpu?
>>
>> My idea is instead of relying on a cpufreq driver to parse the state of
>> your clocks and understand the multiplexing, you can use the clock
>> framework for that. In fact that might help you get one step closer to
>> using the cpufreq-cpu0.c/cpufreq-generic.c implementation.
>
> So you want the whole disabling of interrupt delivery to the cpu,
> flipping the mux, wait for interrupt and re-enabling of interrupt
> delivery to the cpu inside the clock provider? That is way past a
> simple mux clock.

No way! I said, "one step closer" for a reason. The interrupt stuff is
clearly out of scope.

Regards,
Mike

>
>        Andrew
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