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

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Date:	Sun, 20 Feb 2011 13:13:09 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	Dima Zavin <dmitriyz@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC,PATCH 1/3] Add a common struct clk

On Mon, Feb 14, 2011 at 09:33:03PM -0800, Saravana Kannan wrote:
> 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?

Your definition seems sane - I'm not sure what use a parent clock which
had nothing to do with a child would be.

As for the locking issue, I've no idea on that at the moment.  I don't
think implementations should grab the prepare lock, I think that's
something the generic code should take care of for clk_set_rate(),
clk_set_parent() etc.
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