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[net-next,v3] docs: net: dsa: describe issues with checksum offload

Message ID 20220416052737.25509-1-luizluca@gmail.com (mailing list archive)
State Accepted
Commit a997157e42e3119b13c644549a3d8381a1d825d6
Delegated to: Netdev Maintainers
Headers show
Series [net-next,v3] docs: net: dsa: describe issues with checksum offload | expand

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netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
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netdev/module_param success Was 0 now: 0
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netdev/verify_fixes success No Fixes tag
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Commit Message

Luiz Angelo Daros de Luca April 16, 2022, 5:27 a.m. UTC 
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>
---
 Documentation/networking/dsa/dsa.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

Comments

Florian Fainelli April 16, 2022, 7:29 p.m. UTC | #1 
On 4/15/2022 10:27 PM, Luiz Angelo Daros de Luca wrote:
> 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>
patchwork-bot+netdevbpf@kernel.org April 18, 2022, 12:30 p.m. UTC | #2 
Hello:

This patch was applied to netdev/net-next.git (master)
by David S. Miller <davem@davemloft.net>:

On Sat, 16 Apr 2022 02:27:37 -0300 you wrote:
> 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>
> ---
>  Documentation/networking/dsa/dsa.rst | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)

Here is the summary with links:
  - [net-next,v3] docs: net: dsa: describe issues with checksum offload
    https://git.kernel.org/netdev/net-next/c/a997157e42e3

You are awesome, thank you!
diff mbox series

Patch

diff --git a/Documentation/networking/dsa/dsa.rst b/Documentation/networking/dsa/dsa.rst
index ddc1dd039337..ed7fa76e7a40 100644
--- a/Documentation/networking/dsa/dsa.rst
+++ b/Documentation/networking/dsa/dsa.rst
@@ -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