netdev - Confused about ip_summed member in sk

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Message-ID: <CAHUXu_WyYzuTOiz75VfhST6nL3gm0B49dDMjgkzEQ0m_h4Rh1g@mail.gmail.com>
Date:   Fri, 21 Oct 2022 14:29:26 +0800
From:   "J.J. Mars" <mars14850@...il.com>
To:     netdev@...r.kernel.org
Subject: Confused about ip_summed member in sk_buff

Hi everyone, I'm new here and I hope this mail won't disturb you :)

Recently I was working with something about ip_summed, and I'm really
confused about the question what does ip_summed exactly mean?
This member is defined with comment Driver fed us an IP checksum'. So
I guess it's about IP/L3 checksum status.
But the possible value of ip_summed like CHECKSUM_UNNECESSARY is about L4.

What confused me a lot is ip_summed seems to tell us the checksum of
IP/L3 layer is available from its name.
But it seems to tell us the checksum status of L4 layer from its value.

Besides, in ip_rcv() it seems the ip_summed is not used before
calculating the checksum of IP header.

So does ip_summed indicate the status of L3 checksum status or L4
checksum status?
If L4, why is it named like that?

Best regards, Mars