linux-kernel - Re: [PATCH v3 3/5] clk: introduce the common clock framework

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Message-ID: <alpine.DEB.2.00.1112011124221.6289@utopia.booyaka.com>
Date:	Thu, 1 Dec 2011 11:30:16 -0700 (MST)
From:	Paul Walmsley <paul@...an.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
cc:	"Turquette, Mike" <mturquette@...com>, linux@....linux.org.uk,
	linus.walleij@...ricsson.com, patches@...aro.org,
	shawn.guo@...escale.com, magnus.damm@...il.com,
	linux-kernel@...r.kernel.org, amit.kucheria@...aro.org,
	richard.zhao@...aro.org, grant.likely@...retlab.ca,
	dsaxena@...aro.org, eric.miao@...aro.org, sboyd@...cinc.com,
	skannan@...cinc.com, linaro-dev@...ts.linaro.org,
	jeremy.kerr@...onical.com, linux-omap@...r.kernel.org,
	tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
	arnd.bergmann@...aro.org
Subject: Re: [PATCH v3 3/5] clk: introduce the common clock framework

Hi Mark,

On Thu, 1 Dec 2011, Mark Brown wrote:

> On Wed, Nov 30, 2011 at 11:39:59PM -0700, Paul Walmsley wrote:
> 
> > Clock rate/parent-change notifiers are requirements for DVFS use-cases, 
> > and they must be paired with something like the 
> > clk_{allow,block}_rate_change() functions to work efficiently.  I intend 
> > to comment on this later; it's not a simple problem.  It might be worth 
> > noting that Tero and I implemented a simplified version of this for the 
> > N900.
> 
> I'm thinking that if we're going to have clk_{allow,block}_rate_change()
> we may as well make that the main interface to enable rate changes - if
> a device wants to change the clock rate it allows rate changes using
> that interface rather than by disabling the clocks.  I've got devices
> which can do glitch free updates of active clocks so having to disable
> would be a real restriction, and cpufreq would have issues with actually
> disabling the clock too I expect.

The intention behind the clk_{allow,block}_rate_change() proposal was to 
allow the current user of the clock to change its rate without having to 
call clk_{allow,block}_rate_change(), if that driver was the sole user of 
the clock.

So for example, if you had a driver that did:

c = clk_get(dev, clk_name);
clk_enable(c);
clk_set_rate(c, clk_rate);

and c was currently not enabled by any other driver on the system, and 
nothing else had called clk_block_rate_change(c), then the rate change 
would be allowed to proceed.  (modulo any notifier activity, etc.)  

So clk_{allow,block}_rate_change() was simply intended to allow or 
restrict other users of the same clock, not the current user.


- Paul
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