IB/srp: Reduce number of BUSY conditions

The SRP (draft) standard specifies that an SRP initiator must never queue more than (SRP request limit) - 1 unanswered SRP_CMD information units. This patch makes sure that the SCSI mid-layer never tries to queue more than (SRP request limit) - 1 SCSI commands to ib_srp. This improves performance for targets whose request limit is less than or equal to
SRP_SQ_REQ_SIZE - SRP_TSK_MGMT_RSV (62) by reducing the number of BUSY responses reported by ib_srp to the SCSI mid-layer.

---
 drivers/infiniband/ulp/srp/ib_srp.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

index cc6a713..93d6d0f 100644
@@ -1354,8 +1354,13 @@ static int srp_cm_handler(struct ib_cm_id *cm_id, struct ib_cm_event *event)
 			target->max_ti_iu_len = be32_to_cpu(rsp->max_ti_iu_len);
 			target->req_lim       = be32_to_cpu(rsp->req_lim_delta);
 
-			target->scsi_host->can_queue = min(target->req_lim,
-							   target->scsi_host->can_queue);
+			/*
+			 * Set can_queue such that the SCSI mid-layer
+			 * never causes __srp_get_tx_iu() to fail.
+			 */
+			target->scsi_host->can_queue
+				= min(target->req_lim - SRP_TSK_MGMT_RSV,
+				      target->scsi_host->can_queue);
 		} else {
 			shost_printk(KERN_WARNING, target->scsi_host,
 				    PFX "Unhandled RSP opcode %#x\n", opcode);

1.6.4.2

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Note: this patch still has the old (incorrect) patch description. Will post a patch with an updated description soon.

Bart.
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