[PATCH net 2/3] xsk: document checksum offload handling in copy mode

[Nick Simpson]

In copy mode, XDP_TXMD_FLAGS_CHECKSUM requests are now left to the
device only for the TCP or UDP checksum of an unfragmented IPv4 or IPv6
packet, not IP-in-IP or GRE encapsulated, on an Ethernet device; other
requests are completed in software before transmission. On devices that
take skbs without a linear part (IFF_TX_SKB_NO_LINEAR), every request is
still left to the device. Say so in the description of the flag.

Assisted-by: LLM
Signed-off-by: Nick Simpson <nick.simo@xxxxxxxxx>
---
 Documentation/networking/xsk-tx-metadata.rst | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/xsk-tx-metadata.rst b/Documentation/networking/xsk-tx-metadata.rst
index f240930ca1df..028564dba43d 100644
--- a/Documentation/networking/xsk-tx-metadata.rst
+++ b/Documentation/networking/xsk-tx-metadata.rst
@@ -50,7 +50,12 @@ The flags field enables the particular offload:
 - ``XDP_TXMD_FLAGS_CHECKSUM``: requests the device to calculate L4
   checksum. ``request.csum_start`` specifies byte offset of where the
   checksumming should start and ``request.csum_offset`` specifies byte offset
-  where the device should store the computed checksum.
+  where the device should store the computed checksum. In copy mode
+  (``XDP_COPY``), only the TCP or UDP checksum of an unfragmented IPv4 or
+  IPv6 packet, not IP-in-IP or GRE encapsulated, on an Ethernet device is
+  left to the device; other requests are completed in software before the
+  packet is transmitted. On devices that take skbs without a linear part
+  (``IFF_TX_SKB_NO_LINEAR``), every request is still left to the device.
 - ``XDP_TXMD_FLAGS_LAUNCH_TIME``: requests the device to schedule the
   packet for transmission at a pre-determined time called launch time. The
   value of launch time is indicated by ``request.launch_time`` field of
-- 
2.53.0