[Bug 285837] rtwn with RTL8821AU and h/w checksum offload seems to treat 0 for IPv4+UDP as invalid

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 01 Apr 2025 20:27:25 UTC 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=285837

            Bug ID: 285837
           Summary: rtwn with RTL8821AU and h/w checksum offload seems to
                    treat 0 for IPv4+UDP as invalid
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: wireless
          Assignee: wireless@FreeBSD.org
          Reporter: jrtc27@freebsd.org

When talking to a DHCP server that leaves the UDP checksum as 0, I see
r12a_check_frame_checksum report R12A_RXDW1_CKSUM and R12A_RXDW1_CKSUM_ERR both
being set. When talking to a DHCP server that sets the UDP checksum to non-zero
I see R12A_RXDW1_CKSUM being set but not R12A_RXDW1_CKSUM_ERR. Further
investigation is surely needed, but it appears as if RTL8821AU does not
correctly handle the optional nature of IPv4+UDP's checksum (and, as pure
speculation, may also not handle all-ones correctly as the other related edge
case?).

Neither Linux nor OpenBSD appears to use the checksum offloading in their
equivalent drivers, only letting the NIC do FCS checking.

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