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

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Message-ID: <5142B027.4040403@wwwdotorg.org>
Date:	Thu, 14 Mar 2013 23:22:47 -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 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...
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