Re: [PATCH net v4 2/3] net: ethernet: cortina: Handle large frames

[Vladimir Oltean]

On Thu, Nov 09, 2023 at 10:03:13AM +0100, Linus Walleij wrote:
> 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: 4d5ae32f5e1e ("net: ethernet: Add a driver for Gemini gigabit ethernet")
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---

Reviewed-by: Vladimir Oltean <olteanv@xxxxxxxxx>