Re: [PATCH net] net: amd-xgbe: support receiving packets with bad FCS

[James <aslan.jnn@gmail.com>]

Hi David, thank you for your feedback!

So on my test hardware (AMD Snowy Owl SoC), it is the former: FCS
validation is actually disabled, and packets with bad FCS are processed as
valid packets.

Concretely, with DCRCC=1 (the current behavior):

  - `xgbe_dev_read()` treats the packet as valid: the skb is passed up the
    stack normally (e.g. `tcpdump -Q in` will see the packets with
    bad FCS).
  - The RX CRC error counter is not incremented, so `ethtool -S` shows no
    error.
  - Since CST=1 (CRC stripping) is also set, the FCS bytes are stripped
    and are not visible in the captured frame from user-space, thus
    user space can't tell if the packet got bad FCS or not).

I verified this empirically: if the link partner sends two identical
packets, one with a valid FCS and one with a completely inverted FCS, both
arrive identically in `tcpdump -Q in` with no way to distinguish them, and
`ethtool -S` shows zero CRC errors.

With the patch applied (e.g. setting DCRCC=0), the bad-FCS packet is
dropped and RX CRC counter will get incremented.

Regards,
James

On Wed, Aug 19, 2026 at 12:43:07PM +0100, David Laight <david.laight.linux@gmail.com> wrote:
> What actually happens to packets with bad FCS?
> If FCS validation is actually disabled they'd get processed as valid packets.
> That would be a serious bug.
> OTOH the rx status could contain an 'fcs error' bit that causes the packet
> be discarded and the software counts an error.
> That would match the historic behaviour of many ethernet chips.