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

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Date:   Fri,  8 Jul 2022 16:26:53 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] amd-xgbe: fix clang -Wformat warnings

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>
---
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