linux-kernel - Re: [RFC 1/1] clk: Add notifier support in clk_prepare_enable/clk_disable

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Message-ID: <20130315165744.GK4977@n2100.arm.linux.org.uk>
Date:	Fri, 15 Mar 2013 16:57:44 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Bill Huang <bilhuang@...dia.com>
Cc:	"mturquette@...aro.org" <mturquette@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"patches@...aro.org" <patches@...aro.org>
Subject: Re: [RFC 1/1] clk: Add notifier support in
	clk_prepare_enable/clk_disable_unprepare

On Tue, Mar 12, 2013 at 06:47:53PM -0700, Bill Huang wrote:
> On Tue, 2013-03-12 at 21:40 +0800, Russell King - ARM Linux wrote:
> > On Tue, Mar 12, 2013 at 05:37:41AM -0700, Bill Huang wrote:
> > > Add the below four notifier events so drivers which are interested in
> > > knowing the clock status can act accordingly. This is extremely useful
> > > in some of the DVFS (Dynamic Voltage Frequency Scaling) design.
> > > 
> > > PRE_CLK_ENABLE
> > > POST_CLK_ENABLE
> > > PRE_CLK_DISABLE
> > > POST_CLK_DISABLE
> > > 
> > > Signed-off-by: Bill Huang <bilhuang@...dia.com>
> > 
> > NAK.  *Sigh* NO, this is the wrong level to be doing stuff like this.
> > 
> > The *ONLY* thing that clk_prepare_enable() and clk_prepare_disable() should
> > *EVER* be doing is calling clk_prepare(), clk_enable(), clk_disable() and
> > clk_unprepare().  Those two functions are *merely* helpers for drivers
> > who don't wish to make the individual calls.
> > 
> > Drivers are still completely free to call the individual functions, at
> > which point your proposal breaks horribly - and they _do_ call the
> > individual functions.
>
> I'm proposing to give device driver a choice when it knows that some
> driver might be interested in knowing its clock's enabled/disabled state
> change at runtime, this is very important for centralized DVFS core
> driver. It is not meant to be covering all cases especially for drivers
> which is not part of the DVFS, so we don't care if it is calling
> clk_enable/disable directly or not.

But you're not giving drivers a choice.  You're giving them an ultimatum.
Either they use clk_prepare_enable() which must only be called from non-
atomic contexts and have the notifiers, or if they need to use the
individual functions (which is what they _should_ be doing but people
are too lazy to properly convert stuff) they don't get the option of
the notifiers at all.

This sucks totally, design wise.

The whole point of clk_prepare_enable() is that it is a helper function
to _only_ do the clk_prepare() call followed by a clk_enable() call and
_nothing_ _else_ _what_ _so_ _ever_.
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