[PATCH net-next v1 1/1] net: usb: lan78xx: annotate checksum assignment to silence sparse warnings

[PATCH net-next v1 1/1] net: usb: lan78xx: annotate checksum assignment to silence sparse warnings

[Oleksij Rempel]

sparse warns about suspicious type casts in checksum assignment:

drivers/net/usb/lan78xx.c:3865:29: warning: cast to restricted __be16
drivers/net/usb/lan78xx.c:3865:27: warning: incorrect type in assignment
    (different base types) expected restricted __wsum [usertype] csum
got unsigned short [usertype]

Use __force to annotate the casts from u16 to __be16 to __wsum.
This makes the type conversion explicit and silences the warnings.
The code is otherwise correct; this change only clarifies intent.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/net/usb/lan78xx.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c
index f00284c9ad34..565b9847e2ab 100644
--- a/drivers/net/usb/lan78xx.c
+++ b/drivers/net/usb/lan78xx.c
@@ -3819,7 +3819,10 @@ static void lan78xx_rx_csum_offload(struct lan78xx_net *dev,
 	     !(dev->net->features & NETIF_F_HW_VLAN_CTAG_RX))) {
 		skb->ip_summed = CHECKSUM_NONE;
 	} else {
-		skb->csum = ntohs((u16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_));
+		__be16 csum_raw;
+
+		csum_raw = (__force __be16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_);
+		skb->csum = (__force __wsum)ntohs(csum_raw);
 		skb->ip_summed = CHECKSUM_COMPLETE;
 	}
 }
-- 
2.39.5

Re: [PATCH net-next v1 1/1] net: usb: lan78xx: annotate checksum assignment to silence sparse warnings

[Jakub Kicinski]

On Fri, 20 Jun 2025 10:46:18 +0200 Oleksij Rempel wrote:
> -		skb->csum = ntohs((u16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_));
> +		__be16 csum_raw;
> +
> +		csum_raw = (__force __be16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_);
> +		skb->csum = (__force __wsum)ntohs(csum_raw);

You can avoid the __force __be16 if you switch the variable to be u16
and then htons instead of ntohs
-- 
pw-bot: cr

Re: [PATCH net-next v1 1/1] net: usb: lan78xx: annotate checksum assignment to silence sparse warnings

[Simon Horman]

On Mon, Jun 23, 2025 at 04:49:13PM -0700, Jakub Kicinski wrote:
> On Fri, 20 Jun 2025 10:46:18 +0200 Oleksij Rempel wrote:
> > -		skb->csum = ntohs((u16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_));
> > +		__be16 csum_raw;
> > +
> > +		csum_raw = (__force __be16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_);
> > +		skb->csum = (__force __wsum)ntohs(csum_raw);
> 
> You can avoid the __force __be16 if you switch the variable to be u16
> and then htons instead of ntohs

Ok, now I am confused.
What is the endian of these integers?