linux-kernel - Re: [PATCH v5 1/6] powercap/drivers/dtpm: Move dtpm table from init to data section

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Message-ID: <CAPDyKFpWtVFoqhFrhMQOgHUjggvg_GPYwmeK-jkphQpA7KQZRw@mail.gmail.com>
Date:   Fri, 7 Jan 2022 15:49:56 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, lukasz.luba@....com, robh@...nel.org,
        heiko@...ech.de, arnd@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v5 1/6] powercap/drivers/dtpm: Move dtpm table from init
 to data section

On Fri, 7 Jan 2022 at 14:15, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> On 31/12/2021 14:33, Ulf Hansson wrote:
> > On Sat, 18 Dec 2021 at 14:00, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
> >>
> >> The dtpm table is used to let the different dtpm backends to register
> >> their setup callbacks in a single place and preventing to export
> >> multiple functions all around the kernel. That allows the dtpm code to
> >> be self-encapsulated.
> >
> > Well, that's not entirely true. The dtpm code and its backends (or
> > ops, whatever we call them) are already maintained from a single
> > place, the /drivers/powercap/* directory. I assume we intend to keep
> > it like this going forward too, right?
> >
> > That is also what patch4 with the devfreq backend continues to conform to.
> >
> >>
> >> The dtpm hierarchy will be passed as a parameter by a platform
> >> specific code and that will lead to the creation of the different dtpm
> >> nodes.
> >>
> >> The function creating the hierarchy could be called from a module at
> >> init time or when it is loaded. However, at this moment the table is
> >> already freed as it belongs to the init section and the creation will
> >> lead to a invalid memory access.
> >>
> >> Fix this by moving the table to the data section.
> >
> > With the above said, I find it a bit odd to put a table in the data
> > section like this. Especially, since the only remaining argument for
> > why, is to avoid exporting functions, which isn't needed anyway.
> >
> > I mean, it would be silly if we should continue to put subsystem
> > specific tables in here, to just let them contain a set of subsystem
> > specific callbacks.
>
> So I tried to change the approach and right now I was not able to find
> an alternative keeping the code self-encapsulate and without introducing
> cyclic dependencies.
>
> I suggest to keep the patch as it is and double check if it makes sense
> to change it after adding more dtpm backends
>
> Alternatively I can copy the table to a dynamically allocated table.

I am not sure I understand the problem. You don't need a "table of
callbacks" at all, at least to start with.

Instead, what you need is to make a call to a function, or actually
one call per supported dtpm type from dtpm_setup_dt() (introduced in
patch2).

For CPUs, you would simply call dtpm_cpu_setup() (introduced in
patch3) from dtpm_setup_dt(), rather than walking the dtpm table an
invoking the ->setup() callback.

Did that make sense to you?

Going forward, when we decide to introduce the option to add/remove
support for dtpm types dynamically, you can then convert to a
dynamically allocated table.

[...]

Kind regards
Uffe