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

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Date:	Fri, 15 Mar 2013 14:44:44 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Bill Huang <bilhuang@...dia.com>,
	"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 Fri, 15 Mar 2013, Russell King - ARM Linux wrote:

> 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_.

If people are too lazy and start abusing clk_prepare_enable() then this 
helper function becomes counter-productive and should simply be removed.
Same issue as with IS_ERR_OR_NULL().


Nicolas
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