[PATCH net] qca_spi: Avoid unsequenced accesses to jiffies in qcaspi_netdev_tx_timeout()

[PATCH net] qca_spi: Avoid unsequenced accesses to jiffies in qcaspi_netdev_tx_timeout()

[Nathan Chancellor]

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

  drivers/net/ethernet/qualcomm/qca_spi.c:805:16: error: unsequenced accesses to volatile qualified 'jiffies' [-Werror,-Wunsequenced]
    805 |                     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: 291ab06ecf67 ("net: qualcomm: new Ethernet over SPI driver for QCA7000")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2197
Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/net/ethernet/qualcomm/qca_spi.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/qualcomm/qca_spi.c b/drivers/net/ethernet/qualcomm/qca_spi.c
index 38a779f4b866..3ea745d4aef7 100644
--- a/drivers/net/ethernet/qualcomm/qca_spi.c
+++ b/drivers/net/ethernet/qualcomm/qca_spi.c
@@ -800,9 +800,10 @@ static void
 qcaspi_netdev_tx_timeout(struct net_device *dev, unsigned int txqueue)
 {
 	struct qcaspi *qca = netdev_priv(dev);
+	unsigned long j = jiffies;
 
 	netdev_info(qca->net_dev, "Transmit timeout at %ld, latency %ld\n",
-		    jiffies, jiffies - dev_trans_start(dev));
+		    j, j - dev_trans_start(dev));
 	qca->net_dev->stats.tx_errors++;
 	/* Trigger tx queue flush and QCA7000 reset */
 	set_bit(SPI_RESET, &qca->flags);

---
base-commit: aa40afa831ebfe45e40a23721711059f2ac584d4
change-id: 20261114-qca_spi-wunsequenced-a81e54616844

Best regards,
--  
Cheers,
Nathan