[PATCH 3/8] mbuf: fix Tx checksum offload examples

[David Marchand <david.marchand@redhat.com>]

Mandate use of rte_eth_tx_prepare() in the mbuf Tx checksum offload
examples.

Remove unneeded resetting of checksums to align with the mbuf
API doxygen.

Clarify the case when requesting "inner" checksum offload with
lack of outer L4 checksum offload.

Fixes: f00dcb7b0a74 ("mbuf: fix Tx checksum offload API doc")
Fixes: 609dd68ef14f ("mbuf: enhance the API documentation of offload flags")
Cc: stable@dpdk.org

Signed-off-by: David Marchand <david.marchand@redhat.com>
---
 doc/guides/prog_guide/mbuf_lib.rst | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/doc/guides/prog_guide/mbuf_lib.rst b/doc/guides/prog_guide/mbuf_lib.rst
index 049357c755..4e285c0aab 100644
--- a/doc/guides/prog_guide/mbuf_lib.rst
+++ b/doc/guides/prog_guide/mbuf_lib.rst
@@ -126,6 +126,9 @@ processing to the hardware if it supports it. For instance, the
 RTE_MBUF_F_TX_IP_CKSUM flag allows to offload the computation of the IPv4
 checksum.
 
+Support for such processing by the hardware is advertised through RTE_ETH_TX_OFFLOAD_* capabilities.
+Please note that a call to ``rte_eth_tx_prepare`` is needed to handle driver specific requirements such as resetting some checksum fields.
+
 The following examples explain how to configure different TX offloads on
 a vxlan-encapsulated tcp packet:
 ``out_eth/out_ip/out_udp/vxlan/in_eth/in_ip/in_tcp/payload``
@@ -135,7 +138,6 @@ a vxlan-encapsulated tcp packet:
     mb->l2_len = len(out_eth)
     mb->l3_len = len(out_ip)
     mb->ol_flags |= RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_IP_CSUM
-    set out_ip checksum to 0 in the packet
 
   This is supported on hardware advertising RTE_ETH_TX_OFFLOAD_IPV4_CKSUM.
 
@@ -144,8 +146,6 @@ a vxlan-encapsulated tcp packet:
     mb->l2_len = len(out_eth)
     mb->l3_len = len(out_ip)
     mb->ol_flags |= RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_IP_CSUM | RTE_MBUF_F_TX_UDP_CKSUM
-    set out_ip checksum to 0 in the packet
-    set out_udp checksum to pseudo header using rte_ipv4_phdr_cksum()
 
   This is supported on hardware advertising RTE_ETH_TX_OFFLOAD_IPV4_CKSUM
   and RTE_ETH_TX_OFFLOAD_UDP_CKSUM.
@@ -155,7 +155,6 @@ a vxlan-encapsulated tcp packet:
     mb->l2_len = len(out_eth + out_ip + out_udp + vxlan + in_eth)
     mb->l3_len = len(in_ip)
     mb->ol_flags |= RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_IP_CSUM
-    set in_ip checksum to 0 in the packet
 
   This is similar to case 1), but l2_len is different. It is supported
   on hardware advertising RTE_ETH_TX_OFFLOAD_IPV4_CKSUM.
@@ -166,8 +165,6 @@ a vxlan-encapsulated tcp packet:
     mb->l2_len = len(out_eth + out_ip + out_udp + vxlan + in_eth)
     mb->l3_len = len(in_ip)
     mb->ol_flags |= RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_IP_CSUM | RTE_MBUF_F_TX_TCP_CKSUM
-    set in_ip checksum to 0 in the packet
-    set in_tcp checksum to pseudo header using rte_ipv4_phdr_cksum()
 
   This is similar to case 2), but l2_len is different. It is supported
   on hardware advertising RTE_ETH_TX_OFFLOAD_IPV4_CKSUM and
@@ -181,9 +178,6 @@ a vxlan-encapsulated tcp packet:
     mb->l4_len = len(in_tcp)
     mb->ol_flags |= RTE_MBUF_F_TX_IPV4 | RTE_MBUF_F_TX_IP_CKSUM | RTE_MBUF_F_TX_TCP_CKSUM |
       RTE_MBUF_F_TX_TCP_SEG;
-    set in_ip checksum to 0 in the packet
-    set in_tcp checksum to pseudo header without including the IP
-      payload length using rte_ipv4_phdr_cksum()
 
   This is supported on hardware advertising RTE_ETH_TX_OFFLOAD_TCP_TSO.
   Note that it can only work if outer L4 checksum is 0.
@@ -196,12 +190,10 @@ a vxlan-encapsulated tcp packet:
     mb->l3_len = len(in_ip)
     mb->ol_flags |= RTE_MBUF_F_TX_OUTER_IPV4 | RTE_MBUF_F_TX_OUTER_IP_CKSUM  | \
       RTE_MBUF_F_TX_IP_CKSUM |  RTE_MBUF_F_TX_TCP_CKSUM;
-    set out_ip checksum to 0 in the packet
-    set in_ip checksum to 0 in the packet
-    set in_tcp checksum to pseudo header using rte_ipv4_phdr_cksum()
 
   This is supported on hardware advertising RTE_ETH_TX_OFFLOAD_IPV4_CKSUM,
   RTE_ETH_TX_OFFLOAD_UDP_CKSUM and RTE_ETH_TX_OFFLOAD_OUTER_IPV4_CKSUM.
+  Note that it can only work if outer L4 checksum is 0.
 
 The list of flags and their precise meaning is described in the mbuf API
 documentation (rte_mbuf.h). Also refer to the testpmd source code
-- 
2.44.0