linux-kernel - Re: [PATCH] amd-xgbe: fix clang -Wformat warnings

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Date:   Fri, 8 Jul 2022 17:09:24 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] amd-xgbe: fix clang -Wformat warnings

On Fri, Jul 8, 2022 at 4:48 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Fri, Jul 8, 2022 at 4:27 PM Justin Stitt <justinstitt@...gle.com> wrote:
> >
> > When building with Clang we encounter the following warning:
> > | drivers/net/ethernet/amd/xgbe/xgbe-dcb.c:234:42: error: format specifies
> > | type 'unsigned char' but the argument has type '__u16' (aka 'unsigned
> > | short') [-Werror,-Wformat] pfc->pfc_cap, pfc->pfc_en, pfc->mbc,
> > | pfc->delay);
> >
> > pfc->pfc_cap , pfc->pfc_cn, pfc->mbc are all of type `u8` while pfc->delay is
> > of type `u16`. The correct format specifiers `%hh[u|x]` were used for
> > the first three but not for pfc->delay, which is causing the warning
> > above.
> >
> > Variadic functions (printf-like) undergo default argument promotion.
> > Documentation/core-api/printk-formats.rst specifically recommends using
> > the promoted-to-type's format flag. In this case `%d` (or `%x` to
> > maintain hex representation) should be used since both u8's and u16's
> > are fully representable by an int.
> >
> > Moreover, C11 6.3.1.1 states:
> > (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf) `If an int
> > can represent all values of the original type ..., the value is
> > converted to an int; otherwise, it is converted to an unsigned int.
> > These are called the integer promotions.`
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/378
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
>
> Thanks for the patch, this fixes an instance of -Wformat I observe for
> x86_64 allmodconfig.  I thought you had already fixed this file up?
> https://lore.kernel.org/llvm/20220607191119.20686-1-jstitt007@gmail.com/
> which was merged. I'm guessing that was from a defconfig, while this
> was from an allmodconfig?
> Seems like there's a bunch of config ifdef'ery around the definition
> of netif_dbg.

Definitely different across configs but ALSO files are
ever-so-slightly different:
amd/xgbe/xgbe-drv.c      _v.s_      amd/xgbe/xgbe-dcb.c

>
> Either way, thanks for the patches and
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> > ---
> > For clarification, the first three parameters given to netif_dbg did NOT
> > cause a -Wformat warning. I changed them simply to follow what the
> > standard and documentation recommend.
> >
> >  drivers/net/ethernet/amd/xgbe/xgbe-dcb.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-dcb.c b/drivers/net/ethernet/amd/xgbe/xgbe-dcb.c
> > index 895d35639129..c68ace804e37 100644
> > --- a/drivers/net/ethernet/amd/xgbe/xgbe-dcb.c
> > +++ b/drivers/net/ethernet/amd/xgbe/xgbe-dcb.c
> > @@ -230,7 +230,7 @@ static int xgbe_dcb_ieee_setpfc(struct net_device *netdev,
> >         struct xgbe_prv_data *pdata = netdev_priv(netdev);
> >
> >         netif_dbg(pdata, drv, netdev,
> > -                 "cap=%hhu, en=%#hhx, mbc=%hhu, delay=%hhu\n",
> > +                 "cap=%d, en=%#x, mbc=%d, delay=%d\n",
> >                   pfc->pfc_cap, pfc->pfc_en, pfc->mbc, pfc->delay);
> >
> >         /* Check PFC for supported number of traffic classes */
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers