xprtrdma: Reduce max_frwr_depth

Some devices advertise a large max_fast_reg_page_list_len
capability, but perform optimally when MRs are significantly smaller
than that depth -- probably when the MR itself is no larger than a
page.

By default, the RDMA R/W core API uses max_sge_rd as the maximum
page depth for MRs. For some devices, the value of max_sge_rd is
1, which is also not optimal. Thus, when max_sge_rd is larger than
1, use that value. Otherwise use the value of the
max_fast_reg_page_list_len attribute.

I've tested this with CX-3 Pro, FastLinq, and CX-5 devices. It
reproducibly improves the throughput of large I/Os by several
percent.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/frwr_ops.c

@@ -193,10 +193,17 @@ frwr_op_open(struct rpcrdma_ia *ia, struct rpcrdma_ep *ep,
 	if (attrs->device_cap_flags & IB_DEVICE_SG_GAPS_REG)
 		ia->ri_mrtype = IB_MR_TYPE_SG_GAPS;
 
-	ia->ri_max_frwr_depth =
-			min_t(unsigned int, RPCRDMA_MAX_DATA_SEGS,
-			      attrs->max_fast_reg_page_list_len);
-	dprintk("RPC:       %s: device's max FR page list len = %u\n",
+	/* Quirk: Some devices advertise a large max_fast_reg_page_list_len
+	 * capability, but perform optimally when the MRs are not larger
+	 * than a page.
+	 */
+	if (attrs->max_sge_rd > 1)
+		ia->ri_max_frwr_depth = attrs->max_sge_rd;
+	else
+		ia->ri_max_frwr_depth = attrs->max_fast_reg_page_list_len;
+	if (ia->ri_max_frwr_depth > RPCRDMA_MAX_DATA_SEGS)
+		ia->ri_max_frwr_depth = RPCRDMA_MAX_DATA_SEGS;
+	dprintk("RPC:       %s: max FR page list depth = %u\n",
 		__func__, ia->ri_max_frwr_depth);
 
 	/* Add room for frwr register and invalidate WRs.