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

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Message-ID: <CAKwvOdnhzwg0OswLc+RwgX0=Z4beF6EEO09w8Ezdhg-YyXQeJg@mail.gmail.com>
Date:   Fri, 8 Jul 2022 16:47:59 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Justin Stitt <justinstitt@...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 4:27 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.

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