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

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Message-ID: <577A67BA.8060709@arm.com>
Date:	Mon, 4 Jul 2016 14:42:18 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Sudeep Holla <sudeep.holla@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	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@...r.kernel.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 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.

-- 
Regards,
Sudeep