linux-kernel - Re: [PATCH v5 00/23] Introduce runtime modifiable Energy Model

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Message-ID: <0640a9bf-b864-45ef-ab39-14b0e85ff9ad@arm.com>
Date:   Wed, 13 Dec 2023 12:34:10 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     rui.zhang@...el.com, amit.kucheria@...durent.com,
        linux-pm@...r.kernel.org, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
        qyousef@...alina.io, wvw@...gle.com, linux-kernel@...r.kernel.org,
        rafael@...nel.org
Subject: Re: [PATCH v5 00/23] Introduce runtime modifiable Energy Model

On 13/12/2023 10:23, Lukasz Luba wrote:
> Hi Dietmar,
> 
> Thank you for the review, I will go one-by-one to respond
> your comments in patches as well. First comments are below.
> 
> On 12/12/23 18:48, Dietmar Eggemann wrote:
>> On 29/11/2023 12:08, Lukasz Luba wrote:

[...]

>>> Changelog:
>>> v5:
>>> - removed 2 tables design
>>> - have only one table (runtime_table) used also in thermal (Wei, Rafael)
>>
>> Until v4 you had 2 EM's, the static and the modifiable (runtime). Now in
>> v5 this changed to only have one, the modifiable. IMHO it would be
>> better to change the existing table to be modifiable rather than staring
>> with two EM's and then removing the static one. I assume you end up with
>> way less code changes and the patch-set will become easier to digest for
>> reviewers.
> 
> The patches are structured in this way following Daniel's recommendation
> I got when I was adding similar big changes to EM in 2020 (support all
> devices in kernel). The approach is as follows:
> 0. Do some basic clean-up/refactoring if needed for a new feature, to
>    re-use some code if possible in future
> 1. Introduce new feature next to the existing one
> 2. Add API and all needed infrastructure (structures, fields) for
>    drivers
> 3. Re-wire the existing drivers/frameworks to the new feature via new
>    API; ideally keep 1 patch per driver so the maintainer can easily
>    grasp the changes and ACK it, because it will go via different tree
>    (Rafael's tree); in case of some code clash in the driver's code
>    during merge - it will be a single driver so easier to handle
> 4. when all drivers and frameworks are wired up with the new feature
>    remove the old feature (structures, fields, APIs, etc)
> 5. Update the documentation with new latest state of desing
> 
> In this approach the patches are less convoluted. Because if I remove
> the old feature and add new in a single patch (e.g. the main structure)
> that patch will have to modify all drivers to still compile. It
> would be a big messy patch for this re-design.
> 
> I can see in some later comment from Rafael that he is OK with current
> patch set structure.

OK, in case Rafael and Daniel prefer this, then it's fine.

I just find it weird that we now have

70 struct em_perf_domain {
71         struct em_perf_table __rcu *runtime_table;
                                       ^^^^^^^^^^^^^

as the only EM table.