Can someone help me understand the reason for this code in ib_isert.c?

[Nicholas A. Bellinger]

/*
* FIXME: Use devattr.max_sge - 2 for max_send_sge as
* work-around for RDMA_READ..
*/
attr.cap.max_send_sge = device->dev_attr.max_sge - 2;
It's not clear from the comment what this is a work-around for, and I wasn't able
to figure it out from looking at logs.

I believe this refers to some IBTA spec corner case which comes into
play with the max_sges advertized by mlx4, Eli, can you shed some
light (IBTA pointer) on that? is this the case (i.e dev_attr.max_sge
isn't always achievable) with mlx5 too?

It's for ConnectX-2 reporting max_sge=31 during development, while the
largest max_send_sge for stable large block RDMA reads was (is..?) 29
SGEs.

In trying to get isert working with ocrdma I ran into a problem where the code
thought there was only 1 send SGE available when in fact the device has 3.
Then I found the above work-around, which explained the problem.

Nic, any reason for attr.cap.max_send_sge = 1 not to work OK?

IIRC, pre fastreg code supported max_send_sge=1 by limiting the transfer
size per RDMA read, and would issue subsequent RDMA read + offset from
completion up to the total se_cmd->data_length.
Not sure how this works with fastreg today. Sagi..?

While on fastreg mode, for RDMA reads we use only one SGE, talking to
Sagi he explained me that the problem with creating the QP with
max_send_sge = 1 has to do with other flows where two or even three SGEs
are needed, e.g when the iscsi/iser header (doesn't exist in RDMA ops)
is taken from one spot of the memory and the data (sense) taken from
another one?
Nic, we need to see what is the minimum needed by the code (two?) and
tweak that line to make sure it gets there as long as the device
supports it, SB, makes sense?
diff --git a/drivers/infiniband/ulp/isert/ib_isert.c
b/drivers/infiniband/ulp/isert/ib_isert.c
index 0bea577..24abbb3 100644
--- a/drivers/infiniband/ulp/isert/ib_isert.c
+++ b/drivers/infiniband/ulp/isert/ib_isert.c
@@ -115,9 +115,10 @@ isert_conn_setup_qp(struct isert_conn *isert_conn,
struct rdma_cm_id *cma_id,
attr.cap.max_recv_wr = ISERT_QP_MAX_RECV_DTOS;
/*
* FIXME: Use devattr.max_sge - 2 for max_send_sge as
- * work-around for RDMA_READ..
+ * work-around for RDMA_READ.. still need to make sure to
+ * have at least two SGEs for SCSI responses.
*/
- attr.cap.max_send_sge = device->dev_attr.max_sge - 2;
+ attr.cap.max_send_sge = max(2, device->dev_attr.max_sge - 2);
isert_conn->max_sge = attr.cap.max_send_sge;
attr.cap.max_recv_sge = 1;

After a quick audit, the minimum max_send_sge=2 patch looks correct for
the hardcoded tx_desc->num_sge cases in isert_put_login_tx(),
isert_put_response(), isert_put_reject() and isert_put_text_rsp().

For the RDMA read path with non fast-reg, isert_build_rdma_wr() is still
correctly limiting the SGEs per operation based upon the negotiated
isert_conn->max_sge set above in isert_conn_setup_qp().

Chris, please confirm on ocrdma with Or's patch, and I'll include his
patch into target-pending/queue for now, and move into master once you
give a proper Tested-By.

Thanks!

--nab