netdev - Re: [PATCH 08/30] net: wireless: ath: carl9170: Mark 'ar9170_qmap' as __maybe

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Date:   Thu, 27 Aug 2020 10:54:21 +0300
From:   Kalle Valo <kvalo@...eaurora.org>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Christian Lamparter <chunkeey@...il.com>, davem@...emloft.net,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        Christian Lamparter <chunkeey@...glemail.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 08/30] net: wireless: ath: carl9170: Mark 'ar9170_qmap' as __maybe_unused

Lee Jones <lee.jones@...aro.org> writes:

> On Mon, 17 Aug 2020, Kalle Valo wrote:
>
>> Rasmus Villemoes <linux@...musvillemoes.dk> writes:
>> 
>> > On 14/08/2020 17.14, Christian Lamparter wrote:
>> >> On 2020-08-14 13:39, Lee Jones wrote:
>> >>> 'ar9170_qmap' is used in some source files which include carl9170.h,
>> >>> but not all of them.  Mark it as __maybe_unused to show that this is
>> >>> not only okay, it's expected.
>> >>>
>> >>> Fixes the following W=1 kernel build warning(s)
>> >> 
>> >> Is this W=1 really a "must" requirement? I find it strange having
>> >> __maybe_unused in header files as this "suggests" that the
>> >> definition is redundant.
>> >
>> > In this case it seems one could replace the table lookup with a
>> >
>> > static inline u8 ar9170_qmap(u8 idx) { return 3 - idx; }
>> >
>> > gcc doesn't warn about unused static inline functions (or one would have
>> > a million warnings to deal with). Just my $0.02.
>> 
>> Yeah, this is much better.
>> 
>> And I think that static variables should not even be in the header
>> files. Doesn't it mean that there's a local copy of the variable
>> everytime the .h file is included? Sure, in this case the overhead is
>> small (4 bytes per include) but still it's wrong.
>
> It happens a lot.
>
> As I stated before, the 2 viable options are to a) move it into the
> source files; ensuring code duplication, unnecessary maintenance
> burden and probably disparity over time, or b) create (or locate if
> there is one already) a special header file which is only to be
> included by the users.
>
> The later option gets really complicated if there are a variety of
> tables which are included by any given number of source file
> permutations.
>
> The accepted answer in all of the other subsystems I've worked with so
> far, is to use __maybe_unused.  It's simple, non-intrusive and doesn't
> rely on any functional changes.
>
>> Having a static inline
>> function would solve that problem as well the compiler warning.
>
> This time yes, but it's a hack that will only work with simple,
> linear data.  

To me __maybe_unused is a hack and a static inline function is a much
better solution.

> Try doing that with some of the other, more complicated
> tables, like mwifiex_sdio_sd8*.

Then the table should be moved to a .c file and the .h file should have
"extern const int foo[]"

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