Linux Checksum offloading

For bridging and non-udp/tcp packet, no so called offloading support in tx path
For Fragmented packets, no checksum offloading support in tx path
For UDP/TCP routing packet (incremental), no checksum offloading support in tx path. at the same time, no ip_summed flag change
Only Linux local output UDP/TCP’s checksum can be offloaded in tx path
For L2TP related tunnels, only inner UDP/TCP can be offloaded, outer UDP/TCP is done by linux itself in tx path.

Below variables in skb is very important for checksum offloading:

Linux Checksum

union {

    __wsum                       csum;
    struct {
        __u16  csum_start;
        __u16  csum_offset;
   };
};

Variables/Direction	skb→ip_summed	skb→csum or csum_start/offset
Input	CHECKSUM_NONE 0	not valid	SW need do L4 payload + pseudo header verification
	CHECKSUM_UNNECESSARY 1	not valid	HW already do L4 payload + pseudo header verification
	CHECKSUM_COMPLETE 2	csum valid (stored with L4 payload checksum)	SW need to do skb→csum + pseudo header verification
Output	CHECKSUM_NONE 0	not valid	SW already do necessary L4 checksum calculation
	CHECKSUM_PARTIAL 3	csum_start/offset valid	SW already do pseudo header and HW need to further calculate its payload based on