IB/srpt: Change default behavior from using SRQ to using RC

Although in the RC mode more resources are needed that mode has three
advantages over SRQ:
- It works with all RDMA adapters, even those that do not support
  SRQ.
- Posting WRs and polling WCs does not trigger lock contention
  because only one thread at a time accesses a WR or WC queue in
  non-SRQ mode.
- The end-to-end flow control mechanism is used.

>From the IB spec:

    C9-150.2.1: For QPs that are not associated with an SRQ, each HCA
    receive queue shall generate end-to-end flow control credits. If
    a QP is associated with an SRQ, the HCA receive queue shall not
    generate end-to-end flow control credits.

Add new configfs attributes that allow to configure which mode to use
(/sys/kernel/config/target/srpt/$GUID/$GUID/attrib/use_srq). Note:
only the attribute for port 1 is relevant on multi-port adapters.
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/srpt/ib_srpt.h

@@ -252,6 +252,7 @@ enum rdma_ch_state {
  * @free_list:     Head of list with free send I/O contexts.
  * @state:         channel state. See also enum rdma_ch_state.
  * @ioctx_ring:    Send ring.
+ * @ioctx_recv_ring: Receive I/O context ring.
  * @list:          Node for insertion in the srpt_device.rch_list list.
  * @cmd_wait_list: List of SCSI commands that arrived before the RTU event. This
  *                 list contains struct srpt_ioctx elements and is protected
@@ -281,6 +282,7 @@ struct srpt_rdma_ch {
 	struct list_head	free_list;
 	enum rdma_ch_state	state;
 	struct srpt_send_ioctx	**ioctx_ring;
+	struct srpt_recv_ioctx	**ioctx_recv_ring;
 	struct list_head	list;
 	struct list_head	cmd_wait_list;
 	struct se_session	*sess;
@@ -295,11 +297,13 @@ struct srpt_rdma_ch {
  * @srp_max_rdma_size: Maximum size of SRP RDMA transfers for new connections.
  * @srp_max_rsp_size: Maximum size of SRP response messages in bytes.
  * @srp_sq_size: Shared receive queue (SRQ) size.
+ * @use_srq: Whether or not to use SRQ.
  */
 struct srpt_port_attrib {
 	u32			srp_max_rdma_size;
 	u32			srp_max_rsp_size;
 	u32			srp_sq_size;
+	bool			use_srq;
 };
 
 /**
@@ -347,6 +351,7 @@ struct srpt_port {
  * @srq:           Per-HCA SRQ (shared receive queue).
  * @cm_id:         Connection identifier.
  * @srq_size:      SRQ size.
+ * @use_srq:       Whether or not to use SRQ.
  * @ioctx_ring:    Per-HCA SRQ.
  * @rch_list:      Per-device channel list -- see also srpt_rdma_ch.list.
  * @ch_releaseQ:   Enables waiting for removal from rch_list.
@@ -362,6 +367,7 @@ struct srpt_device {
 	struct ib_srq		*srq;
 	struct ib_cm_id		*cm_id;
 	int			srq_size;
+	bool			use_srq;
 	struct srpt_recv_ioctx	**ioctx_ring;
 	struct list_head	rch_list;
 	wait_queue_head_t	ch_releaseQ;