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 13:38:51 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Bill Huang <bilhuang@...dia.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"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 03/15/2013 06:33 AM, Ulf Hansson wrote:
> On 15 March 2013 13:06, Bill Huang <bilhuang@...dia.com> wrote:
>> On Fri, 2013-03-15 at 18:08 +0800, Ulf Hansson wrote:
...
>>> Some prerequisites; I think am in favor of using the clk API to
>>> trigger DVFS changes and then I agree on that clk_prepare|unprepare
>>> needs to be possible to track from a DVFS perspective. clk_set_rate is
>>> not enough.
>>>
>>> So if we decide to do the above (using the clk API to trigger DVFS
>>> changes), I believe we should discuss two possible solutions;
>>> - clk notifiers or..
>>> - dvfs clock type.
>>>
>>> I am trying to make up my mind of what I think is the best solution.
>>> Have you considered "dvfs clock type"?
>>> I put some comments about this for "[PATCH 2/5] clk: notifier handler
>>> for dynamic voltage scaling" recently as well.
>>>
>>> What could the advantages/disadvantages be between the two options?
>>
>> I personally prefer clk notifiers since that's easy and all the existing
>> device drivers don't need to be modified, a new clock or API might be
>> more thoroughly considered (and hence maybe more graceful) but that
>> means we need more time to cook and many drivers need to plug into that
>> API when it comes out, a lot of test/verification or maybe chaos
>> follows, I'm not sure will that be a little overkill.
> 
> I guess you did not fully got what I meant with "dvfs clock type". It
> will not affect the clock API. But instead the dvfs is handled by
> implementing a specific clk hw type. So the same thing is accomplished
> as with clk notifiers, no changes should be needed to device drivers.
> 
> The difference is only that no notifiers will be needed, and all the
> dvfs stuff will be handled in the clk hw instead. It will mean that we
> will bundle dvfs stuff into the clock drivers, instead of separating
> the code outside the clock drivers. But, on the other hand no
> notifiers will be needed.

The advantage here is that I assume that a notifier would continually
have to check whether the clock being modified was one that the DVFS
notifier cared about. By integrating the CVFS logic into the clk_hw
itself, it'll only ever get executed for clocks that really care about
DVFS. Presumably, the code that implements the clk_hw could also use
some common DVFS library as part of the implementation, and still share
code. Or perhaps, what about putting DVFS "ops" into a clk_hw alongside
any other existing ops, and having the clock core call them whenever
appropriate?
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