We had workarounds were the ethernet checksumming engine would be bypassed
for larger frames, this fixed devices using DSA, but regressed devices
where the ethernet was connected directly to a PHY.

The devices with a PHY connected directly can't handle large frames
either way, with or without bypass. Looking at the size of the frame
is probably just wrong.

Rework the workaround such that we just bypass the checksumming engine if
the ethertype inside the actual frame is something else than 0x0800
(IPv4) or 0x86dd (IPv6). These are the only frames the checksumming engine
can actually handle. VLAN framing (0x8100) also works fine.

We can't inspect skb->protocol because DSA frames will sometimes have a
custom ethertype despite skb->protocol is e.g. 0x0800.

After this both devices with direct ethernet attached such as D-Link
DNS-313 and devices with a DSA switch with a custom ethertype such as
D-Link DIR-685 work fine.

Fixes: d4d0c5b4d279 ("net: ethernet: cortina: Handle large frames")
Signed-off-by: Linus Walleij <redacted>
---
 drivers/net/ethernet/cortina/gemini.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index 6a7ea051391a..1400f19bf05b 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c

@@ -1143,7 +1143,9 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
        skb_frag_t *skb_frag;
        dma_addr_t mapping;
        unsigned short mtu;
+       u16 ethertype;
        void *buffer;
+       __be16 *p;

        mtu  = ETH_HLEN;
        mtu += netdev->mtu;

@@ -1158,7 +1160,24 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
                word3 |= mtu;
        }

-       if (skb->ip_summed == CHECKSUM_PARTIAL) {
+       /* Dig out the the ethertype actually in the buffer and not what the
+        * protocol claims to be. This is the raw data that the checksumming
+        * offload engine will have to deal with.
+        */
+       p = (__be16 *)(skb->data + 2 * ETH_ALEN);
+       ethertype = ntohs(*p);
+       if (ethertype == ETH_P_8021Q) {
+               p += 2; /* +2 sizeof(__be16) */
+               ethertype = ntohs(*p);
+       }