net/smc: add comments for smc_link_{usable|sendable}

Add comments for both smc_link_sendable() and smc_link_usable()
to help better distinguish and use them.

No function changes.
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/smc/smc_core.h

@@ -409,7 +409,13 @@ static inline struct smc_connection *smc_lgr_find_conn(
 	return res;
 }
 
-/* returns true if the specified link is usable */
+/*
+ * Returns true if the specified link is usable.
+ *
+ * usable means the link is ready to receive RDMA messages, map memory
+ * on the link, etc. This doesn't ensure we are able to send RDMA messages
+ * on this link, if sending RDMA messages is needed, use smc_link_sendable()
+ */
 static inline bool smc_link_usable(struct smc_link *lnk)
 {
 	if (lnk->state == SMC_LNK_UNUSED || lnk->state == SMC_LNK_INACTIVE)
@@ -417,6 +423,15 @@ static inline bool smc_link_usable(struct smc_link *lnk)
 	return true;
 }
 
+/*
+ * Returns true if the specified link is ready to receive AND send RDMA
+ * messages.
+ *
+ * For the client side in first contact, the underlying QP may still in
+ * RESET or RTR when the link state is ACTIVATING, checks in smc_link_usable()
+ * is not strong enough. For those places that need to send any CDC or LLC
+ * messages, use smc_link_sendable(), otherwise, use smc_link_usable() instead
+ */
 static inline bool smc_link_sendable(struct smc_link *lnk)
 {
 	return smc_link_usable(lnk) &&