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    SubjectRe: [PATCH v2] ftgmac100: Disable HW checksum generation on AST2500
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    On Fri, 2019-10-18 at 10:14 +1100, Benjamin Herrenschmidt wrote:
    >
    > I don't understand what you are saying. You reported a problem with
    > IPV6 checksums generation. The HW doesn't support it. What's "not a
    > matter of unsupported csum" ?
    >
    > Your patch uses a *deprecated* bit to tell the network stack to only do
    > HW checksum generation on IPV4.
    >
    > This bit is deprecated for a reason, again, see skbuff.h. The right
    > approach, *which the driver already does*, is to tell the stack that we
    > support HW checksuming using NETIF_F_HW_CSUM, and then, in the transmit
    > handler, to call skb_checksum_help() to have the SW calculate the
    > checksum if it's not a supported type.
    >
    > This is exactly what ftgmac100_prep_tx_csum() does. It only enables HW
    > checksum generation on supported types and uses skb_checksum_help()
    > otherwise, supported types being protocol ETH_P_IP and IP protocol
    > being raw IP, TCP and UDP.
    >
    > So this *should* have fallen back to SW for IPV6. So either something
    > in my code there is making an incorrect assumption, or something is
    > broken in skb_checksum_help() for IPV6 (which I somewhat doubt) or
    > something else I can't think of, but setting a *deprecated* flag is
    > definitely not the right answer, neither is completely disabling HW
    > checksumming.
    >
    > So can you investigate what's going on a bit more closely please ? I
    > can try myself, though I have very little experience with IPV6 and
    > probably won't have time before next week.

    I did get that piece of information from Aspeed: The HW checksum
    generation is supported if:

    - The length of UDP header is always 20 bytes.
    - The length of TCP and IP header have 4 * N bytes (N is 5 to 15).

    Now these afaik are also the protocol limits, so it *should* work.

    Or am I missing something or some funky encaspulation/header format
    that can be used under some circumstances ?

    Cheers,
    Ben.


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