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

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Date:   Fri, 31 Dec 2021 14:33:41 +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 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.

>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>

Kind regards
Uffe

> ---
>  include/asm-generic/vmlinux.lds.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 42f3866bca69..50d494d94d6c 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -362,7 +362,8 @@
>         BRANCH_PROFILE()                                                \
>         TRACE_PRINTKS()                                                 \
>         BPF_RAW_TP()                                                    \
> -       TRACEPOINT_STR()
> +       TRACEPOINT_STR()                                                \
> +       DTPM_TABLE()
>
>  /*
>   * Data section helpers
> @@ -723,7 +724,6 @@
>         ACPI_PROBE_TABLE(irqchip)                                       \
>         ACPI_PROBE_TABLE(timer)                                         \
>         THERMAL_TABLE(governor)                                         \
> -       DTPM_TABLE()                                                    \
>         EARLYCON_TABLE()                                                \
>         LSM_TABLE()                                                     \
>         EARLY_LSM_TABLE()                                               \
> --
> 2.25.1
>