linux-kernel - Re: [RFC,PATCH 1/3] Add a common struct clk

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Message-ID: <4D5B5835.4070407@codeaurora.org>
Date:	Tue, 15 Feb 2011 20:53:09 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Russell King <linux@....linux.org.uk>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dima Zavin <dmitriyz@...gle.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC,PATCH 1/3] Add a common struct clk

On 02/14/2011 09:33 PM, Saravana Kannan wrote:
> Russell, A question for you further down this email. Please take a look.

Russell, Ping... I will snip the rest of the discussion so that it's 
easy to spot the question.

>>>> +int clk_set_parent(struct clk *clk, struct clk *parent)
>>>> +{
>>>> + if (clk->ops->set_parent)
>>>> + return clk->ops->set_parent(clk, parent);
>>>
>>> I'm not sure on this one. If the prepare ops for a clock also calls the
>>> prepare ops on the parent, shouldn't we prevent changing the parent
>>> while the prepare/unprepare is going on?
>>
>> Again, this is related to set_rate during enable/disable or
>> prepare/unprepare;
>> we don't have defined semantics for this at present.
>
> After thinking about this the past couple of days, this looks like a
> location where the locking is more necessary than inside set rate. I
> always saw the parent clock as the clock that generates the clock signal
> from which this clock derives (divide, etc) it's clock signal from.
>
> Assuming Russell and/or the community agrees on the semantics of
> "parent", without the generic implementation grabbing the prepare_lock
> while setting the parent, there is no way for the specific clock driver
> implementations to cleanly ensure correctness. The only option for them
> would be to peek into the generic clock struct and grab the prepare lock
> -- to me that would be an ugly hack and/or layering violation that would
> cause problems later on.
>
> Russell/All,
>
> What's the meaning of a parent clock? Do you agree with my definition --
> "the parent clock is the clock that generates the clock signal from
> which the child clock derives (divide, etc) it's clock signal from."? Or
> is it open to interpretation by each implementation?

Thanks,
Saravana

-- 
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