NVMe: Always use MSI/MSI-x interrupts

Multiple users have reported device initialization failure due the driver
not receiving legacy PCI interrupts. This is not unique to any particular
controller, but has been observed on multiple platforms.

There have been no issues reported or observed when with message signaled
interrupts, so this patch attempts to use MSI-x during initialization,
falling back to MSI. If that fails, legacy would become the default.

The setup_io_queues error handling had to change as a result: the admin
queue's msix_entry used to be initialized to the legacy IRQ. The case
where nr_io_queues is 0 would fail request_irq when setting up the admin
queue's interrupt since re-enabling MSI-x fails with 0 vectors, leaving
the admin queue's msix_entry invalid. Instead, return success immediately.

Reported-by: Tim Muhlemmer <[e-mail]>
Reported-by: Jon Derrick <[e-mail]>
Signed-off-by: Keith Busch <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/nvme/host/pci.c

···
 	if (result > 0) {
 		dev_err(dev->ctrl.device,
 			"Could not set queue count (%d)\n", result);
-		nr_io_queues = 0;
-		result = 0;
+		return 0;
 	}
 
 	if (dev->cmb && NVME_CMB_SQS(dev->cmbsz)) {
···
 	 * If we enable msix early due to not intx, disable it again before
 	 * setting up the full range we need.
 	 */
-	if (!pdev->irq)
+	if (pdev->msi_enabled)
+		pci_disable_msi(pdev);
+	else if (pdev->msix_enabled)
 		pci_disable_msix(pdev);
 
 	for (i = 0; i < nr_io_queues; i++)
···
 	if (pci_enable_device_mem(pdev))
 		return result;
 
-	dev->entry[0].vector = pdev->irq;
 	pci_set_master(pdev);
 
 	if (dma_set_mask_and_coherent(dev->dev, DMA_BIT_MASK(64)) &&
···
 	}
 
 	/*
-	 * Some devices don't advertse INTx interrupts, pre-enable a single
-	 * MSIX vec for setup. We'll adjust this later.
+	 * Some devices and/or platforms don't advertise or work with INTx
+	 * interrupts. Pre-enable a single MSIX or MSI vec for setup. We'll
+	 * adjust this later.
 	 */
-	if (!pdev->irq) {
-		result = pci_enable_msix(pdev, dev->entry, 1);
-		if (result < 0)
-			goto disable;
+	if (pci_enable_msix(pdev, dev->entry, 1)) {
+		pci_enable_msi(pdev);
+		dev->entry[0].vector = pdev->irq;
+	}
+
+	if (!dev->entry[0].vector) {
+		result = -ENODEV;
+		goto disable;
 	}
 
 	cap = lo_hi_readq(dev->bar + NVME_REG_CAP);