Re: [PATCH] carl9170: fix enum compare splat

[Kalle Valo <kvalo@codeaurora.org>]

Christian Lamparter <chunkeey@gmail.com> writes:

> On Monday, June 10, 2019 9:06:30 AM CEST Kalle Valo wrote:
>> Christian Lamparter <chunkeey@gmail.com> writes:
>> 
>> > This patch fixes a noisy warning triggered by -Wenum-compare
>> >
>> > |main.c:1390:31: warning: comparison between ‘enum nl80211_ac’
>> > |	and ‘enum ar9170_txq’ [-Wenum-compare]
>> > |  BUILD_BUG_ON(NL80211_NUM_ACS > __AR9170_NUM_TXQ);
>> > |                               ^
>> > | [...]
>> >
>> > This is a little bit unfortunate, since the number of queues
>> > (hence NL80211_NUM_ACS) is a constant based on the IEEE 802.11
>> > (much like IEEE80211_NUM_ACS) and __AR9170_NUM_TXQ is more or
>> > less defined by the AR9170 hardware.
>> 
>> Is the warning enabled by default? TBH I'm not seeing how useful this
>> warning is for kernel development.
>
> It is included in the "-Wall" (which is coming from "KBUILD_CFLAGS" 
> in the main Makefile).
>
> I tried debian's gcc starting from 4.6 to the lastest 8.3. They all
> complain about it in various degrees.
>
> https://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Warning-Options.html#Warning-Options

Ok, odd that I haven't noticed this warning. Maybe I have been just
blind.

>> > --- a/drivers/net/wireless/ath/carl9170/main.c
>> > +++ b/drivers/net/wireless/ath/carl9170/main.c
>> > @@ -1387,7 +1387,7 @@ static int carl9170_op_conf_tx(struct ieee80211_hw *hw,
>> >  	int ret;
>> >  
>> >  	BUILD_BUG_ON(ARRAY_SIZE(ar9170_qmap) != __AR9170_NUM_TXQ);
>> > -	BUILD_BUG_ON(NL80211_NUM_ACS > __AR9170_NUM_TXQ);
>> > +	BUILD_BUG_ON((size_t)NL80211_NUM_ACS > (size_t)__AR9170_NUM_TXQ);
>> 
>> IMHO this just makes the code worse. Does it make sense to workaround
>> (stupid) compiler warnings like this?
>
> True. What's worse: This isn't really code. The BUILD_BUG_ON Macro is there
> to guard but it's getting compiled away. I could also just drop it.

Either way is fine for me. If the GCC (by default) emits a warning for
this we need to silence that warning, so in that respect I guess we
should apply this. Unless better solutions come up, of course :)

-- 
Kalle Valo