NVMe: Fix admin queue ring wrap

BugLink: http://bugs.launchpad.net/bugs/1531539

The tag set queue depth needs to be one less than the h/w queue depth
so we don't wrap the circular buffer. This conforms to the specification
defined "Full Queue" condition.
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit e3e9d50c)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/nvme/host/pci.c

@@ -1271,7 +1271,12 @@ static int nvme_alloc_admin_tags(struct nvme_dev *dev)
 	if (!dev->ctrl.admin_q) {
 		dev->admin_tagset.ops = &nvme_mq_admin_ops;
 		dev->admin_tagset.nr_hw_queues = 1;
-		dev->admin_tagset.queue_depth = NVME_AQ_BLKMQ_DEPTH;
+
+		/*
+		 * Subtract one to leave an empty queue entry for 'Full Queue'
+		 * condition. See NVM-Express 1.2 specification, section 4.1.2.
+		 */
+		dev->admin_tagset.queue_depth = NVME_AQ_BLKMQ_DEPTH - 1;
 		dev->admin_tagset.timeout = ADMIN_TIMEOUT;
 		dev->admin_tagset.numa_node = dev_to_node(dev->dev);
 		dev->admin_tagset.cmd_size = nvme_cmd_size(dev);