ibmvnic: Inspect header requirements before using scrq direct

Previously, the TX header requirement for standard frames was ignored.
This requirement is a bitstring sent from the VIOS which maps to the
type of header information needed during TX. If no header information,
is needed then send subcrq direct can be used (which can be more
performant).

This bitstring was previously ignored for standard packets (AKA non LSO,
non CSO) due to the belief that the bitstring was over-cautionary. It
turns out that there are some configurations where the backing device
does need header information for transmission of standard packets. If
the information is not supplied then this causes continuous "Adapter
error" transport events. Therefore, this bitstring should be respected
and observed before considering the use of send subcrq direct.

Fixes: 74839f7 ("ibmvnic: Introduce send sub-crq direct")
Signed-off-by: Nick Child <nnac123@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241001163200.1802522-2-nnac123@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/ibm/ibmvnic.c

@@ -2472,9 +2472,11 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
 	/* if we are going to send_subcrq_direct this then we need to
 	 * update the checksum before copying the data into ltb. Essentially
 	 * these packets force disable CSO so that we can guarantee that
-	 * FW does not need header info and we can send direct.
+	 * FW does not need header info and we can send direct. Also, vnic
+	 * server must be able to xmit standard packets without header data
 	 */
-	if (!skb_is_gso(skb) && !ind_bufp->index && !netdev_xmit_more()) {
+	if (*hdrs == 0 && !skb_is_gso(skb) &&
+	    !ind_bufp->index && !netdev_xmit_more()) {
 		use_scrq_send_direct = true;
 		if (skb->ip_summed == CHECKSUM_PARTIAL &&
 		    skb_checksum_help(skb))