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

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Message-ID: <556eca9c-4ce8-1c79-cc6d-08d0ec603bd4@linaro.org>
Date:   Fri, 7 Jan 2022 14:15:52 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Ulf Hansson <ulf.hansson@...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 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.


>> 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
>>


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