[PATCH 3/3 v5] IB/srp: Reduce number of BUSY conditions

[Bart Van Assche <bvanassche-HInyCGIudOg@public.gmane.org>]

As proposed by the SRP (draft) standard, ib_srp reserves one ring element for
SRP_TSK_MGMT requests. 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_NORMAL_REQ_SQ_SIZE (62) by reducing the number of BUSY responses reported
by ib_srp to the SCSI mid-layer.

Signed-off-by: Bart Van Assche <bvanassche-HInyCGIudOg@public.gmane.org>
Cc: Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>
Cc: David Dillow <dave-i1Mk8JYDVaaSihdK6806/g@public.gmane.org>

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

diff --git a/drivers/infiniband/ulp/srp/ib_srp.c b/drivers/infiniband/ulp/srp/ib_srp.c
index da62b57..fba39f0 100644
--- a/drivers/infiniband/ulp/srp/ib_srp.c
+++ b/drivers/infiniband/ulp/srp/ib_srp.c
@@ -1376,8 +1376,13 @@ 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_TASK_MGMT_SQ_SIZE,
+				      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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