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

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Date:	Thu, 14 Mar 2013 22:48:20 -0700
From:	Bill Huang <bilhuang@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Peter De Schrijver <pdeschrijver@...dia.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 1/1] clk: Add notifier support in
 clk_prepare_enable/clk_disable_unprepare

On Fri, 2013-03-15 at 13:22 +0800, Stephen Warren wrote:
> On 03/14/2013 07:20 PM, Bill Huang wrote:
> > On Fri, 2013-03-15 at 01:54 +0800, Stephen Warren wrote:
> >> On 03/14/2013 03:28 AM, Bill Huang wrote:
> >>> On Thu, 2013-03-14 at 17:21 +0800, Peter De Schrijver wrote:
> >>>> On Thu, Mar 14, 2013 at 03:15:11AM +0100, Bill Huang wrote:
> >>>>
> >>>>> I don't think deferring will work either, considering the usage of DVFS,
> >>>>> device voltage is tightly coupled with frequency, when clock rate is
> >>>>> about to increase, we have to boost voltage first and we can lower the
> >>>>> voltage after the clock rate has decreased. All the above sequence have
> >>>>> to be guaranteed or you might crash, so deferring not only make thing
> >>>>> complicated in controlling the order but also hurt performance.
> >>>>
> >>>> But we could use notifiers in clk_prepare/clk_unprepare to set the voltage no?
> >>>> As clk_prepare/clk_unprepare have to be called before clk_enable or after
> >>>> clk_disable, the voltage can be raised to a safe level, before the clock
> >>>> becomes active.
> >>>
> >>> Thanks Peter, actually I'm just about to propose my v2 RFC which add
> >>> notifier in clk_prepare/clk_unprepare.
> >>
> >> Can't clk_set_rate() be called while the clock is prepared, or even
> >> enabled? I don't see how your proposal would work.
> >
> > I think it works with just a little sacrifice on saving more power but
> > that's related minor. Taking clk_prepare as an indicator on that clock
> > will be enabled later, so we can raise the voltage to a safe level
> > (according to the current rate or maybe default rate when clk_prepare is
> > called, some time late when clk_set_rate() is called we can adjust again
> > according to the requested rate change)
> 
> Is clk_set_rate() only legal to call in non-atomic contexts then? The
> header file doesn't say, although I guess since many other functions
> explicitly say they can't, then by omission it can...

I think clk_set_rate can only be called in non-atomic contexts since
there are existing non-atomic clock notifier hooked in it.


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