[PATCH net] net: microchip: encx24j600: Avoid unsequenced accesses to jiffies in encx24j600_tx()

[PATCH net] net: microchip: encx24j600: Avoid unsequenced accesses to jiffies in encx24j600_tx()

[Nathan Chancellor]

After a recent improvement in clang to warn on unsequenced accesses to
volatile variables [1], an instance appears in encx24j600_tx(), which
can break the build with CONFIG_WERROR=y / W=e:

  drivers/net/ethernet/microchip/encx24j600.c:898:14: error: unsequenced accesses to volatile qualified 'jiffies' [-Werror,-Wunsequenced]
    898 |                   jiffies, jiffies - dev_trans_start(dev));
        |                   ~~~~~~~  ^

Per C11 5.1.2.3.2, accessing a volatile object is a side effect. Per C11
6.5.2, unsequenced side effects on the same scalar object are undefined
behavior. Per C11 6.5.2.2.10/12, the evaluation of arguments in a
function call is indeterminately sequenced, meaning any side effects in
the arguments are unsequenced.

Add a local variable to hold the value of jiffies before the function
call so that there are no unsequenced side effects, clearing up the
warning.

Cc: stable@vger.kernel.org
Fixes: 04fbfce7a222 ("net: Microchip encx24j600 driver")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2196
Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/net/ethernet/microchip/encx24j600.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/microchip/encx24j600.c b/drivers/net/ethernet/microchip/encx24j600.c
index b011bf5c2305..ebc067cb2c9d 100644
--- a/drivers/net/ethernet/microchip/encx24j600.c
+++ b/drivers/net/ethernet/microchip/encx24j600.c
@@ -893,9 +893,10 @@ static netdev_tx_t encx24j600_tx(struct sk_buff *skb, struct net_device *dev)
 static void encx24j600_tx_timeout(struct net_device *dev, unsigned int txqueue)
 {
 	struct encx24j600_priv *priv = netdev_priv(dev);
+	unsigned long j = jiffies;
 
 	netif_err(priv, tx_err, dev, "TX timeout at %ld, latency %ld\n",
-		  jiffies, jiffies - dev_trans_start(dev));
+		  j, j - dev_trans_start(dev));
 
 	dev->stats.tx_errors++;
 	netif_wake_queue(dev);

---
base-commit: aa40afa831ebfe45e40a23721711059f2ac584d4
change-id: 20261114-encx24j600-wunsequenced-d1c43cc36c2b

Best regards,
--  
Cheers,
Nathan