linux-kernel - Re: [PATCH 1/5] clk: sun6i: Protect CPU clock

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Date:	Mon, 24 Feb 2014 21:30:05 -0300
From:	Emilio López <emilio@...pez.com.ar>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	devicetree@...r.kernel.org, Mike Turquette <mturquette@...aro.org>,
	Vinod Koul <vinod.koul@...el.com>,
	linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com,
	dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org, rjw@...k.pl
Subject: Re: [PATCH 1/5] clk: sun6i: Protect CPU clock

Hi Russell,

El 24/02/14 21:01, Russell King - ARM Linux escribió:
> Hi Emilio.
>
> On Mon, Feb 24, 2014 at 08:38:44PM -0300, Emilio López wrote:
>> Why is this so? Can't a clock be left enabled while nobody has a
>> reference to it? I have looked around in Documentation/ (rather quickly
>> I must say) and have not found any explicit mention that it is required
>> to keep a reference to the clock while it's enabled. I'd appreciate it
>> if you could explain this a bit more verbosely or point me to the
>> relevant documents.
>
> First up, if you have a requirement that a clock be enabled, then is it
> not unreasonable to ensure that the clock is referenced?

I was under the impression that the reference count was orthogonal to 
the clock status, but after getting that clarified, I can see your point.

> Secondly, what if we have code which scans the clocks in the system,
> shutting down those leaf clocks which appear to be unreferenced?

Indeed, that would break things.

> Thirdly, the API (as I designed it) says so:
>
> /**
>   * clk_put      - "free" the clock source
>   * @clk: clock source
>   *
>   * Note: drivers must ensure that all clk_enable calls made on this
>   * clock source are balanced by clk_disable calls prior to calling
>   * this function.
>   *
>   * clk_put should not be called from within interrupt context.
>   */
> void clk_put(struct clk *clk);
>
> which has been there since the API was first created - it's part of the
> contract between drivers using the API and implementers creating something
> which conforms to the API - which today means CCF.

That's enough of a reason on its own :) I should have checked clk.h

> The intention here is that while there are any users holding a clk_get()
> reference on a clock, the clock is assumed to be required for some
> device, and the struct clk may not be kfree'd, nor may its state be
> changed in an unpredictable way to those drivers holding a reference
> to it.

I understand now, thanks for the insight. I'll talk with Maxime and get 
this sorted out.

As a side note, should drivers/base/power/clock_ops.c be fixed too? I 
have added Rafael to Cc.

Cheers,

Emilio
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