Hi,

I'm working on enhancing a driver for a Network Controller that
supports "Checksum Offloads".
So I'm offloading TCP/UDP checksum computation in the network driver
using NETIF_F_HW_CSUM on
linux kernel version 4.19.23 aarch64 for hikey android platform. The
Network Controller does not support scatter-gather (SG) DMA.
Hence I'm not enabling the NETIF_IF_SG feature.
I see that GSO for TCP is enabled by default in the kernel 4.19.23
When running iperf TCP traffic I observed that the TCP checksum is not
offloaded for the majority
of the TCP packets. Most of the skbs received in the output path in
the driver have skb->ip_summed
set to CHECKSUM_NONE.
The csum is offloaded only for the initial TCP connection establishment packets.
For UDP I do not observe this problem.
It appears that a decision was taken not to offload TCP csum (during GSO)
if the network driver does not support SG :

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 02c638a643ea..9c065ac72e87 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -3098,8 +3098,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
  if (nskb->len == len + doffset)
  goto perform_csum_check;

- if (!sg && !nskb->remcsum_offload) {
- nskb->ip_summed = CHECKSUM_NONE;
+ if (!sg) {
+ if (!nskb->remcsum_offload)
+ nskb->ip_summed = CHECKSUM_NONE;
  SKB_GSO_CB(nskb)->csum =
  skb_copy_and_csum_bits(head_skb, offset,
        skb_put(nskb, len),

The above is a code snippet from the actual commit :

commit 7fbeffed77c130ecf64e8a2f7f9d6d63a9d60a19