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

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Date:   Mon, 17 Aug 2020 20:29:33 +0200
From:   Christian Lamparter <chunkeey@...il.com>
To:     Kalle Valo <kvalo@...eaurora.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Lee Jones <lee.jones@...aro.org>, 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

On 2020-08-17 14:59, 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.

Yes, this is much better than just adding __maybe_unused :).

To be on the safe side (and to get rid of a & in tx.c:666), the "3 - 
idx" should be something like "return (3 - idx) & 
CARL9170_TX_STATUS_QUEUE". I think its also possible to just use clamp_t
(or min_t since the u8 has no negative values) or make use of a switch 
statement [analogues what was done in ath10k commit: 91493e8e10 "ath10k: 
fix recent bandwidth conversion bug" (just to be clear. Yes this ath10k 
commit has nothing to do with queues, but it is a nice, atomic 
switch-case static inline function example).]

> 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.
Seems to be "sort of". I compiled both, the current vanilla carl9170 and
with Lee Jones' patch on Debian's current gcc 10.2.0 (Debian 10.2.0-5)
and gnu ld 2.35.

The ar9170_qmap symbol is only present in the object file if ar9170_qmap 
is used by the source. In the final module file (carl9170.ko), there are 
two ar9170_qmap symboles in the module's .rodata section (one is coming 
from main.o code and the other from tx.o).

(The use of __maybe_unused didn't make any difference. Same overall 
section and file sizes).

Cheers,
Christian