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

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Message-ID: <20110215083759.GA4152@n2100.arm.linux.org.uk>
Date:	Tue, 15 Feb 2011 08:37:59 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jeremy Kerr <jeremy.kerr@...onical.com>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	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 Tue, Feb 15, 2011 at 03:26:53PM +0800, Jeremy Kerr wrote:
> Hi Saravana,
> 
> > Sure, one could argue that in some archs for a certain set of clocks the
> > slow stuff in prepare/unprepare won't need to be done during set rate --
> > say, a simple clock that always runs off the same PLL but just has a
> > integer divider to change the rate.
> > 
> > In those cases, not grabbing the prepare_lock would make the code less
> > "locky".
> > 
> > > We
> > > may even want to disallow set_rate (and set_parent) when prepare_count is
> > > non- zero.
> > 
> > This is definitely not right.
> 
> Why is that? Consider two devices using one clock; one does some 
> initialisation based on the return value of clk_get_rate(), the other calls 
> clk_set_rate() some time later. Now the first device is incorrectly 
> initialised.

What about a clock sourced from a PLL which provides the dotclock for a
framebuffer device?  On every mode set, should the clk have to be disabled,
unprepared, rate set, re-prepared and re-enabled?

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