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docs: net: dsa: describe issues with checksum offload

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DSA tags before IP header (categories 1 and 2) or after the payload (3)
might introduce offload checksum issues.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Luiz Angelo Daros de Luca 2022-04-16 02:27:37 -03:00 committed by David S. Miller
parent 2a38de067b
commit a997157e42
1 changed files with 17 additions and 0 deletions
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17
Documentation/networking/dsa/dsa.rst
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@ -193,6 +193,23 @@ protocol. If not all packets are of equal size, the tagger can implement the
default behavior by specifying the correct offset incurred by each individual
RX packet. Tail taggers do not cause issues to the flow dissector.
Checksum offload should work with category 1 and 2 taggers when the DSA master
driver declares NETIF_F_HW_CSUM in vlan_features and looks at csum_start and
csum_offset. For those cases, DSA will shift the checksum start and offset by
the tag size. If the DSA master driver still uses the legacy NETIF_F_IP_CSUM
or NETIF_F_IPV6_CSUM in vlan_features, the offload might only work if the
offload hardware already expects that specific tag (perhaps due to matching
vendors). DSA slaves inherit those flags from the master port, and it is up to
the driver to correctly fall back to software checksum when the IP header is not
where the hardware expects. If that check is ineffective, the packets might go
to the network without a proper checksum (the checksum field will have the
pseudo IP header sum). For category 3, when the offload hardware does not
already expect the switch tag in use, the checksum must be calculated before any
tag is inserted (i.e. inside the tagger). Otherwise, the DSA master would
include the tail tag in the (software or hardware) checksum calculation. Then,
when the tag gets stripped by the switch during transmission, it will leave an
incorrect IP checksum in place.
Due to various reasons (most common being category 1 taggers being associated
with DSA-unaware masters, mangling what the master perceives as MAC DA), the
tagging protocol may require the DSA master to operate in promiscuous mode, to