Re: [PATCH] carl9170: fix enum compare splat

[Christian Lamparter <chunkeey@gmail.com>]

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

> > --- 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.