UBUNTU: (fix) nvme: only require 1 interrupt vector, not 2+

BugLink: http://bugs.launchpad.net/bugs/1651602

Change the required minimum number of MSI/MSIX interrupt vectors from
2 to 1; the admin queue and first I/O queue actually share vector 0.
Also change the max_qid assignment, to equal the number of vectors.

Since the number of vectors is set to the number of cpus, on a system
with only 1 cpu the previous code is guaranteed to fail, since there
will only be 1 vector configured.

The max_qid, while 0-based, also needs to be 1 larger than the number
of (1-based) vectors, due to the admin queue and first I/O queue
sharing the first vector.  So the 0-based max_qid is set to equal the
1-based number of vectors, resulting in the correct number of total
queues, which is 1 more than the total number of vectors, due to
the admin queue and first I/O queue sharing vector 0.

Fixes: 96fce9e4 ("NVMe: only setup MSIX once")
Signed-off-by: Dan Streetman <dan.streetman@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>

drivers/nvme/host/pci.c

@@ -1613,7 +1613,7 @@ static int nvme_setup_io_queues(struct nvme_dev *dev)
 	struct pci_dev *pdev = to_pci_dev(dev->dev);
 	int result, i, vecs, nr_io_queues, size;
 
-	nr_io_queues = dev->max_qid + 1;
+	nr_io_queues = dev->max_qid;
 	result = nvme_set_queue_count(&dev->ctrl, &nr_io_queues);
 	if (result < 0)
 		return result;
@@ -1653,7 +1653,7 @@ static int nvme_setup_io_queues(struct nvme_dev *dev)
 		adminq->q_db = dev->dbs;
 	}
 
-	dev->max_qid = nr_io_queues - 1;
+	dev->max_qid = nr_io_queues;
 
 	/* Free previously allocated queues that are no longer usable */
 	nvme_free_queues(dev, nr_io_queues + 1);
@@ -1839,13 +1839,13 @@ static int nvme_pci_enable(struct nvme_dev *dev)
 		}
 	}
 
-	if (vecs < 2) {
-		dev_err(dev->ctrl.device, "Failed to get enough MSI/MSIX interrupts\n");
+	if (vecs < 1) {
+		dev_err(dev->ctrl.device, "Failed to get any MSI/MSIX interrupts\n");
 		result = -ENOSPC;
 		goto disable;
 	}
 
-	dev->max_qid = vecs - 1;
+	dev->max_qid = vecs;
 
 	cap = lo_hi_readq(dev->bar + NVME_REG_CAP);