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From:   Christian Lamparter <chunkeey@gmail.com>
To:     Kalle Valo <kvalo@codeaurora.org>
Cc:     linux-wireless@vger.kernel.org
Subject: Re: [PATCH] carl9170: fix enum compare splat
Date:   Mon, 10 Jun 2019 13:45:31 +0200
Message-ID: <1601416.I41mb68Wgz@debian64>
In-Reply-To: <87pnnlncll.fsf@codeaurora.org>
References: <20190608144947.744-1-chunkeey@gmail.com> <87pnnlncll.fsf@codeaurora.org>
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On Monday, June 10, 2019 9:06:30 AM CEST Kalle Valo wrote:
> Christian Lamparter <chunkeey@gmail.com> writes:
>=20
> > This patch fixes a noisy warning triggered by -Wenum-compare
> >
> > |main.c:1390:31: warning: comparison between =E2=80=98enum nl80211_ac=
=E2=80=99
> > |	and =E2=80=98enum ar9170_txq=E2=80=99 [-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.
>=20
> 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"=20
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-O=
ptions

> > --- 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_h=
w *hw,
> >  	int ret;
> > =20
> >  	BUILD_BUG_ON(ARRAY_SIZE(ar9170_qmap) !=3D __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);
>=20
> 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.