RDMA/srp: Honor the max_send_sge device attribute

Instead of assuming that max_send_sge >= 3, restrict the number of scatter
gather elements to what is supported by the RDMA adapter.

Link: https://lore.kernel.org/r/20190930231707.48259-8-bvanassche@acm.org
Cc: Honggang LI <honli@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -552,6 +552,7 @@ static int srp_create_ch_ib(struct srp_rdma_ch *ch)
 {
 	struct srp_target_port *target = ch->target;
 	struct srp_device *dev = target->srp_host->srp_dev;
+	const struct ib_device_attr *attr = &dev->dev->attrs;
 	struct ib_qp_init_attr *init_attr;
 	struct ib_cq *recv_cq, *send_cq;
 	struct ib_qp *qp;
@@ -583,12 +584,14 @@ static int srp_create_ch_ib(struct srp_rdma_ch *ch)
 	init_attr->cap.max_send_wr     = m * target->queue_size;
 	init_attr->cap.max_recv_wr     = target->queue_size + 1;
 	init_attr->cap.max_recv_sge    = 1;
-	init_attr->cap.max_send_sge    = SRP_MAX_SGE;
+	init_attr->cap.max_send_sge    = min(SRP_MAX_SGE, attr->max_send_sge);
 	init_attr->sq_sig_type         = IB_SIGNAL_REQ_WR;
 	init_attr->qp_type             = IB_QPT_RC;
 	init_attr->send_cq             = send_cq;
 	init_attr->recv_cq             = recv_cq;
 
+	ch->max_imm_sge = min(init_attr->cap.max_send_sge - 1U, 255U);
+
 	if (target->using_rdma_cm) {
 		ret = rdma_create_qp(ch->rdma_cm.cm_id, dev->pd, init_attr);
 		qp = ch->rdma_cm.cm_id->qp;
@@ -1838,7 +1841,7 @@ static int srp_map_data(struct scsi_cmnd *scmnd, struct srp_rdma_ch *ch,
 		return -EIO;
 
 	if (ch->use_imm_data &&
-	    count <= SRP_MAX_IMM_SGE &&
+	    count <= ch->max_imm_sge &&
 	    SRP_IMM_DATA_OFFSET + data_len <= ch->max_it_iu_len &&
 	    scmnd->sc_data_direction == DMA_TO_DEVICE) {
 		struct srp_imm_buf *buf;

drivers/infiniband/ulp/srp/ib_srp.h

@@ -161,6 +161,7 @@ struct srp_rdma_ch {
 	};
 	uint32_t		max_it_iu_len;
 	uint32_t		max_ti_iu_len;
+	u8			max_imm_sge;
 	bool			use_imm_data;
 
 	/* Everything above this point is used in the hot path of