nvmet: Add get_mdts op for controllers

Some transports, such as RDMA, would like to set the Maximum Data
Transfer Size (MDTS) according to device/port/ctrl characteristics.
This will enable the transport to set the optimal MDTS according to
controller needs and device capabilities. Add a new nvmet transport
op that is called during ctrl identification. This will not effect
transports that don't implement this option. The return value of the new
op is according to the NVMe spec definition for MDTS.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Israel Rukshin <israelr@mellanox.com>

drivers/nvme/target/admin-cmd.c

@@ -369,8 +369,12 @@ static void nvmet_execute_identify_ctrl(struct nvmet_req *req)
 	/* we support multiple ports, multiples hosts and ANA: */
 	id->cmic = (1 << 0) | (1 << 1) | (1 << 3);
 
-	/* no limit on data transfer sizes for now */
-	id->mdts = 0;
+	/* Limit MDTS according to transport capability */
+	if (ctrl->ops->get_mdts)
+		id->mdts = ctrl->ops->get_mdts(ctrl);
+	else
+		id->mdts = 0;
+
 	id->cntlid = cpu_to_le16(ctrl->cntlid);
 	id->ver = cpu_to_le32(ctrl->subsys->ver);
 

drivers/nvme/target/nvmet.h

@@ -289,6 +289,7 @@ struct nvmet_fabrics_ops {
 			struct nvmet_port *port, char *traddr);
 	u16 (*install_queue)(struct nvmet_sq *nvme_sq);
 	void (*discovery_chg)(struct nvmet_port *port);
+	u8 (*get_mdts)(const struct nvmet_ctrl *ctrl);
 };
 
 #define NVMET_MAX_INLINE_BIOVEC	8