linux-kernel - Re: [PATCH v7 2/6] ACPI / processor_idle: Add support for Low Power Idle(LPI) states

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Message-ID: <CAJZ5v0iEKnWVwVatpCZOi7CsrZrcEmhEVRaEparfUdKZneP7-A@mail.gmail.com>
Date:	Mon, 4 Jul 2016 16:11:50 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Vikas Sajjan <vikas.cha.sajjan@....com>,
	Sunil <sunil.vl@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	PrashanthPrakash <pprakash@...eaurora.org>,
	Al Stone <al.stone@...aro.org>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v7 2/6] ACPI / processor_idle: Add support for Low Power
 Idle(LPI) states

On Mon, Jul 4, 2016 at 3:42 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>
>
> On 04/07/16 14:17, Rafael J. Wysocki wrote:
>>
>> On Monday, July 04, 2016 02:00:03 PM Sudeep Holla wrote:
>>>
>>>
>>> On 01/07/16 14:07, Daniel Lezcano wrote:
>>>>
>>>> On 06/28/2016 03:55 PM, Sudeep Holla wrote:
>>>>>
>>>>> ACPI 6.0 introduced an optional object _LPI that provides an alternate
>>>>> method to describe Low Power Idle states. It defines the local power
>>>>> states for each node in a hierarchical processor topology. The OSPM can
>>>>> use _LPI object to select a local power state for each level of
>>>>> processor
>>>>> hierarchy in the system. They used to produce a composite power state
>>>>> request that is presented to the platform by the OSPM.
>>>>>
>>>>> Since multiple processors affect the idle state for any non-leaf
>>>>> hierarchy
>>>>> node, coordination of idle state requests between the processors is
>>>>> required. ACPI supports two different coordination schemes: Platform
>>>>> coordinated and  OS initiated.
>>>>>
>>>>> This patch adds initial support for Platform coordination scheme of
>>>>> LPI.
>>>>>
>>>>> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
>>>>> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
>>>>> ---
>>>>
>>>>
>>>> Hi Sudeep,
>>>>
>>>> I looked at the acpi processor idle code sometime ago and from my POV,
>>>> it was awful, unnecessary complex and very difficult to maintain. The
>>>> usage of flags all over the places is significant of the lack of control
>>>> of the written code.
>>>>
>>>
>>> So you have any specific things in mind ? That's too broad and I know
>>> it's not so clean, but it's so for legacy reasons. I would leave that
>>> to Rafael to decide as it takes lots of testing to clean up these code.
>>
>>
>> The cleanup needs to be done at one point.
>>
>> Question is when to do it, before adding LPI support or after doing that
>> (and each option has its pros and cons IMO).
>>
>>>> Even if you are not responsible of this implementation, the current
>>>> situation forces you to add more awful code on top of that, which is
>>>> clearly against "making Linux better".
>>>>
>>>
>>> OK
>>
>>
>> So if there are cases in which you need to make the code more complex
>> because of the legacy stuff in there, I'd say it's better to clean it up
>> first.
>>
>
> I am not sure if Daniel was referring to anything specific. I have
> cleaned up in patch 1/6 for cstate. More cleanups can be done there but
> needs better understanding and reasoning for the current code which I
> don't have as they are mostly x86 related.
>
> Unless someone points me what they would like to change and how, I don't
> have much in my mind to do here. Yes it may not look as clean as other
> code in the kernel relatively, but without complete understanding of the
> history/reasoning for the current state of code I wouldn't touch
> anything I don't understand.
>
> I am open to make changes if there's something specific. Sorry I can't
> go ahead making changes the way I think based on some vague idea that
> the current code is not clean.

Fair enough.