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

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Message-ID: <1363151317.3311.9.camel@bilhuang-vm1>
Date:	Tue, 12 Mar 2013 22:08:37 -0700
From:	Bill Huang <bilhuang@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"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 Wed, 2013-03-13 at 12:42 +0800, Stephen Warren wrote:
> On 03/12/2013 07:47 PM, 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.
> 
> I believe the point Russell is making is not that the idea behind this
> patch is wrong, but simply that the function where you put the hooks is
> wrong. The hooks should at least be in clk_enable/clk_disable and not
> clk_prepare_enable/clk_disable_unprepare, since any driver is free to
> call clk_prepare separately from clk_enable. The hooks should be
> implemented in the lowest-level common function that all
> driver-accessible paths call through.

Thanks, I know the point, but unfortunately there is no good choice for
hooking this since those low level functions clk_enable/clk_disable will
be called in interrupt context so it is not possible to send notify. We
might need to come out a better approach if we can think of any.
Currently I still think this is acceptable (Having all the drivers which
are using our interested clocks call these function to enable/disable
clock in their runtime_pm calls) though it's not perfect. 

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