Re: [PATCH iwl-next v6] ice: Add E830 checksum offload support

[Simon Horman <horms@kernel.org>]

On Wed, Dec 18, 2024 at 04:11:45AM -0500, Paul Greenwalt wrote:
> E830 supports raw receive and generic transmit checksum offloads.
> 
> Raw receive checksum support is provided by hardware calculating the
> checksum over the whole packet, regardless of type. The calculated
> checksum is provided to driver in the Rx flex descriptor. Then the driver
> assigns the checksum to skb->csum and sets skb->ip_summed to
> CHECKSUM_COMPLETE.
> 
> Generic transmit checksum support is provided by hardware calculating the
> checksum given two offsets: the start offset to begin checksum calculation,
> and the offset to insert the calculated checksum in the packet. Support is
> advertised to the stack using NETIF_F_HW_CSUM feature.
> 
> E830 has the following limitations when both generic transmit checksum
> offload and TCP Segmentation Offload (TSO) are enabled:
> 
> 1. Inner packet header modification is not supported. This restriction
>    includes the inability to alter TCP flags, such as the push flag. As a
>    result, this limitation can impact the receiver's ability to coalesce
>    packets, potentially degrading network throughput.
> 2. The Maximum Segment Size (MSS) is limited to 1023 bytes, which prevents
>    support of Maximum Transmission Unit (MTU) greater than 1063 bytes.
> 
> Therefore NETIF_F_HW_CSUM and NETIF_F_ALL_TSO features are mutually
> exclusive. NETIF_F_HW_CSUM hardware feature support is indicated but is not
> enabled by default. Instead, IP checksums and NETIF_F_ALL_TSO are the
> defaults. Enforcement of mutual exclusivity of NETIF_F_HW_CSUM and
> NETIF_F_ALL_TSO is done in ice_set_features(). Mutual exclusivity
> of IP checksums and NETIF_F_HW_CSUM is handled by netdev_fix_features().
> 
> When NETIF_F_HW_CSUM is requested the provided skb->csum_start and
> skb->csum_offset are passed to hardware in the Tx context descriptor
> generic checksum (GCS) parameters. Hardware calculates the 1's complement
> from skb->csum_start to the end of the packet, and inserts the result in
> the packet at skb->csum_offset.
> 
> Co-developed-by: Alice Michael <alice.michael@intel.com>
> Signed-off-by: Alice Michael <alice.michael@intel.com>
> Co-developed-by: Eric Joyner <eric.joyner@intel.com>
> Signed-off-by: Eric Joyner <eric.joyner@intel.com>
> Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>

Reviewed-by: Simon Horman <horms@kernel.org>