mlx4: Expose correct max_sge_rd limit

mlx4 devices (ConnectX-2, ConnectX-3) has a limitation
where rdma read work queue entries cannot exceed 512 bytes.
A rdma_read wqe needs to fit in 512 bytes:
- wqe control segment (16 bytes)
- rdma segment (16 bytes)
- scatter elements (16 bytes each)

So max_sge_rd should be: (512 - 16 - 16) / 16 = 30.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

2 files changed, 12 insertions, 1 deletions

diff --git a/drivers/infiniband/hw/mlx4/main.c b/drivers/infiniband/hw/mlx4/main.c
index f567160a4a56..97d6878f9938 100644
--- a/drivers/infiniband/hw/mlx4/main.c
+++ b/drivers/infiniband/hw/mlx4/main.c
@@ -456,7 +456,7 @@ static int mlx4_ib_query_device(struct ib_device *ibdev,
         props->max_qp_wr           = dev->dev->caps.max_wqes - MLX4_IB_SQ_MAX_SPARE;
         props->max_sge             = min(dev->dev->caps.max_sq_sg,
                                          dev->dev->caps.max_rq_sg);
-        props->max_sge_rd = props->max_sge;
+        props->max_sge_rd          = MLX4_MAX_SGE_RD;
         props->max_cq              = dev->dev->quotas.cq;
         props->max_cqe             = dev->dev->caps.max_cqes;
         props->max_mr              = dev->dev->quotas.mpt;
diff --git a/include/linux/mlx4/device.h b/include/linux/mlx4/device.h
index 7501626ab529..d3133be12d92 100644
--- a/include/linux/mlx4/device.h
+++ b/include/linux/mlx4/device.h
@@ -427,6 +427,17 @@ enum {
 };
 
 enum {
+        /*
+         * Max wqe size for rdma read is 512 bytes, so this
+         * limits our max_sge_rd as the wqe needs to fit:
+         * - ctrl segment (16 bytes)
+         * - rdma segment (16 bytes)
+         * - scatter elements (16 bytes each)
+         */
+        MLX4_MAX_SGE_RD = (512 - 16 - 16) / 16
+};
+
+enum {
         MLX4_DEV_PMC_SUBTYPE_GUID_INFO   = 0x14,
         MLX4_DEV_PMC_SUBTYPE_PORT_INFO   = 0x15,
         MLX4_DEV_PMC_SUBTYPE_PKEY_TABLE  = 0x16,