net: ag71xx: align the IP header behind DSA tags

The receive buffer offset includes NET_IP_ALIGN to get the IP header
aligned behind a 14 byte Ethernet header. Most ag71xx devices use DSA
switches whose tag sits in front of the EtherType, which moves the IP
header by another 2 bytes. Every access to the IP and TCP headers then
traps and gets emulated by the unaligned access handler, around 6 times
per forwarded frame.

Take the headroom needed by the DSA tagger into account when choosing
the offset.

TP-Link Archer C7 v2 (qca8k), median of 3 runs, Mbit/s:

                       before   after
  routed up               743     855
  routed down             660     749
  local receive           554     572

The unaligned instruction counter no longer moves while forwarding.

Assisted-by: LLM
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20261014001334.132962-1-rosenp@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

0 files changed, 0 insertions, 0 deletions