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 11:54:03 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Bill Huang <bilhuang@...dia.com>
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 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.

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