nvme: add capability to connect to an administrative controller

Add capability to connect to an administrative controller by
preventing ioq creation for admin-controllers.

Add a nvme_admin_ctrl() to check if a controller's CNTRLTYPE indicates
that it is an administrative controller and override ctrl->queue_count to
1 for admin controllers, so that only the admin queue and no I/O queues
are created for an administrative controller.  This override is done in
nvme_init_ctrl_finish() after ctrl->cntrltype has been initialized in
nvme_init_identify() so nvme_admin_ctrl() will work correctly.
Doing this override in generic code (nvme_init_ctrl_finish) makes it
transport agnostic and will work properly for nvme/tcp as well as for
nvme/rdma.

Suggested-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Kamaljit Singh <kamaljit.singh1@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Author: Kamaljit Singh

drivers/nvme/host/core.c

@@ -3158,6 +3158,11 @@ static inline bool nvme_discovery_ctrl(struct nvme_ctrl *ctrl)
 	return ctrl->opts && ctrl->opts->discovery_nqn;
 }
 
+static inline bool nvme_admin_ctrl(struct nvme_ctrl *ctrl)
+{
+	return ctrl->cntrltype == NVME_CTRL_ADMIN;
+}
+
 static bool nvme_validate_cntlid(struct nvme_subsystem *subsys,
 		struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
 {
@@ -3670,6 +3675,17 @@ int nvme_init_ctrl_finish(struct nvme_ctrl *ctrl, bool was_suspended)
 	if (ret)
 		return ret;
 
+	if (nvme_admin_ctrl(ctrl)) {
+		/*
+		 * An admin controller has one admin queue, but no I/O queues.
+		 * Override queue_count so it only creates an admin queue.
+		 */
+		dev_dbg(ctrl->device,
+			"Subsystem %s is an administrative controller",
+			ctrl->subsys->subnqn);
+		ctrl->queue_count = 1;
+	}
+
 	ret = nvme_configure_apst(ctrl);
 	if (ret < 0)
 		return ret;