Re: [Intel-wired-lan] [PATCH net v2] ice: Fix missing 1's complement negation in GCS raw checksum

Jacob Keller
2026-05-05T00:10:37Z

On 5/1/2026 2:57 AM, Matt Fleming wrote:
> From: Matt Fleming <mfleming@cloudflare.com>
>
> Commit 905d1a220e8d ("ice: Add E830 checksum offload support") added
> Generic Checksum (GCS) support for E830 NICs but omitted the 1's
> complement negation (~) when converting the hardware raw_csum to
> skb->csum for CHECKSUM_COMPLETE.
>
> Without the negation, every CHECKSUM_COMPLETE packet fails the
> fast-path validation in nf_ip_checksum() and falls through to software
> checksumming via __skb_checksum_complete(), which triggers the
> rate-limited "hw csum failure" warning. Packets are still accepted
> (the software recheck passes) but hardware checksum offload is
> effectively disabled and the warning floods dmesg on systems running
> nf_conntrack on VLAN sub-interfaces.
>
> Multiple other drivers (idpf, ehea, iwlwifi, cassini, sunhme, enetc)
> also apply ~ for CHECKSUM_COMPLETE. The ice driver was the only in-tree
> user of csum_unfold() for CHECKSUM_COMPLETE that omitted it.
>

Hi,

Based on your patch description, I assume that you've tested this on
real hardware.

I dug a little through some of our internal changes history and sawe
that it looks like the hardware has a register setting in its
GL_RDPU_CNTRL register which determines whether the checksum value
reported is inverted or not. In E830 hardware, it is supposed to be off
(i.e. the checksum value reported already matches the expected setting.

Perhaps your device somehow got the GL_RDPU_CNTRL register set to the
wrong mode and that results in the swap being necessary. Hmm.

I'll ask the team to see if they can confirm this behavior.

Thanks,
Jake

> Fixes: 905d1a220e8d ("ice: Add E830 checksum offload support")
> Cc: stable@vger.kernel.org
> Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
> Reviewed-by: Simon Horman <horms@kernel.org>
> Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
> ---
> v2:
> - Add Cc: stable@vger.kernel.org (Aleksandr)
> - Pick up Reviewed-by tags from Aleksandr and Simon
> - No code changes
> v1: https://lore.kernel.org/netdev/20260408190214.1287708-1-matt@readmodwrite.com/
>
> drivers/net/ethernet/intel/ice/ice_txrx_lib.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> index e695a664e53d..c177579e0114 100644
> --- a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> +++ b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> @@ -92,7 +92,7 @@ static void ice_rx_gcs(struct sk_buff *skb,
> desc = (struct ice_32b_rx_flex_desc_nic *)rx_desc;
> skb->ip_summed = CHECKSUM_COMPLETE;
> csum = (__force u16)desc->raw_csum;
> - skb->csum = csum_unfold((__force __sum16)swab16(csum));
> + skb->csum = csum_unfold((__force __sum16)~swab16(csum));
> }
>
> /**