The Gemini ethernet controller provides hardware checksumming
for frames up to 1514 bytes including ethernet headers but not
FCS.

If we start sending bigger frames (after first bumping up the MTU
on both interfaces sending and receiving the frames), truncated
packets start to appear on the target such as in this tcpdump
resulting from ping -s 1474:

23:34:17.241983 14:d6:4d:a8:3c:4f (oui Unknown) > bc:ae:c5:6b:a8:3d (oui Unknown),
ethertype IPv4 (0x0800), length 1514: truncated-ip - 2 bytes missing!
(tos 0x0, ttl 64, id 32653, offset 0, flags [DF], proto ICMP (1), length 1502)
OpenWrt.lan > Fecusia: ICMP echo request, id 1672, seq 50, length 1482

If we bypass the hardware checksumming and provide a software
fallback, everything starts working fine up to the max TX MTU
of 2047 bytes, for example ping -s2000 192.168.1.2:

00:44:29.587598 bc:ae:c5:6b:a8:3d (oui Unknown) > 14:d6:4d:a8:3c:4f (oui Unknown),
ethertype IPv4 (0x0800), length 2042:
(tos 0x0, ttl 64, id 51828, offset 0, flags [none], proto ICMP (1), length 2028)
Fecusia > OpenWrt.lan: ICMP echo reply, id 1683, seq 4, length 2008

The bit enabling to bypass hardware checksum (or any of the
"TSS" bits) are undocumented in the hardware reference manual.
The entire hardware checksum unit appears undocumented. The
conclusion that we need to use the "bypass" bit was found by
trial-and-error.

Since no hardware checksum will happen, we slot in a software
checksum fallback.

Check for the condition where we need to compute checksum on the
skb with either hardware or software using == CHECKSUM_PARTIAL instead
of != CHECKSUM_NONE which is an incomplete check according to
<linux/skbuff.h>.

On the D-Link DIR-685 router this fixes a bug on the conduit
interface to the RTL8366RB DSA switch: as the switch needs to add
space for its tag it increases the MTU on the conduit interface
to 1504 and that means that when the router sends packages
of 1500 bytes these get an extra 4 bytes of DSA tag and the
transfer fails because of the erroneous hardware checksumming,
affecting such basic functionality as the LuCI web interface.

Fixes: 4d5ae32f ("net: ethernet: Add a driver for Gemini gigabit ethernet")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20231109-gemini-largeframe-fix-v4-2-6e611528db08@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>