RDMA/core: Document confusing code

While looking into Coverity ID 1351047 I ran into the following
piece of code at
drivers/infiniband/core/verbs.c:496:

ret = rdma_addr_find_l2_eth_by_grh(&dgid, &sgid,
                                   ah_attr->dmac,
                                   wc->wc_flags & IB_WC_WITH_VLAN ?
                                   NULL : &vlan_id,
                                   &if_index, &hoplimit);

The issue here is that the position of arguments in the call to
rdma_addr_find_l2_eth_by_grh() function do not match the order of
the parameters:

&dgid is passed to sgid
&sgid is passed to dgid

This is the function prototype:

int rdma_addr_find_l2_eth_by_grh(const union ib_gid *sgid,
 				 const union ib_gid *dgid,
 				 u8 *dmac, u16 *vlan_id, int *if_index,
 				 int *hoplimit)

My question here is if this is intentional?

Answer:
Yes. ib_init_ah_from_wc() creates ah from the incoming packet.
Incoming packet has dgid of the receiver node on which this code is
getting executed and sgid contains the GID of the sender.

When resolving mac address of destination, you use arrived dgid as
sgid and use sgid as dgid because sgid contains destinations GID whom to
respond to.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/drivers/infiniband/core/verbs.c b/drivers/infiniband/core/verbs.c
index c973a83c898b..47ee1f83c9a9 100644
--- a/drivers/infiniband/core/verbs.c
+++ b/drivers/infiniband/core/verbs.c
@@ -452,6 +452,19 @@ int ib_get_gids_from_rdma_hdr(const union rdma_network_hdr *hdr,
 }
 EXPORT_SYMBOL(ib_get_gids_from_rdma_hdr);
 
+/*
+ * This function creates ah from the incoming packet.
+ * Incoming packet has dgid of the receiver node on which this code is
+ * getting executed and, sgid contains the GID of the sender.
+ *
+ * When resolving mac address of destination, the arrived dgid is used
+ * as sgid and, sgid is used as dgid because sgid contains destinations
+ * GID whom to respond to.
+ *
+ * This is why when calling rdma_addr_find_l2_eth_by_grh() function, the
+ * position of arguments dgid and sgid do not match the order of the
+ * parameters.
+ */
 int ib_init_ah_from_wc(struct ib_device *device, u8 port_num,
 		       const struct ib_wc *wc, const struct ib_grh *grh,
 		       struct rdma_ah_attr *ah_attr)