RDS/IB: Disable flow control in sysctl and explain why

Backwards compatibility with rds 3.0 causes protocol-
based flow control to be disabled as a side-effect.

I don't want to pull out FC support from the IB transport
but I do want to document and keep the sysctl consistent
if possible.
Signed-off-by: NAndy Grover <andy.grover@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/rds/ib_sysctl.c

@@ -53,7 +53,17 @@ unsigned long rds_ib_sysctl_max_unsig_bytes = (16 << 20);
 static unsigned long rds_ib_sysctl_max_unsig_bytes_min = 1;
 static unsigned long rds_ib_sysctl_max_unsig_bytes_max = ~0UL;
 
-unsigned int rds_ib_sysctl_flow_control = 1;
+/*
+ * This sysctl does nothing.
+ *
+ * Backwards compatibility with RDS 3.0 wire protocol
+ * disables initial FC credit exchange.
+ * If it's ever possible to drop 3.0 support,
+ * setting this to 1 and moving init/refill of send/recv
+ * rings from ib_cm_connect_complete() back into ib_setup_qp()
+ * will cause credits to be added before protocol negotiation.
+ */
+unsigned int rds_ib_sysctl_flow_control = 0;
 
 ctl_table rds_ib_sysctl_table[] = {
 	{