ahci: only try to use multi-MSI mode if there is more than 1 port

We should only try to allocate multiple MSI or MSI-X vectors if the device
actually has multiple ports.  Otherwise pci_alloc_irq_vectors will return
a single vector due to n_ports = 1, in which case we shouldn't set the
AHCI_HFLAG_MULTI_MSI flag.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: 0b9e2988 ("ahci: use pci_alloc_irq_vectors")
Reported-by: Emmanuel Benisty <benisty.e@gmail.com>
Tested-by: Emmanuel Benisty <benisty.e@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/ahci.c

@@ -1418,6 +1418,7 @@ static int ahci_init_msi(struct pci_dev *pdev, unsigned int n_ports,
 	 * Message mode could be enforced. In this case assume that advantage
 	 * of multipe MSIs is negated and use single MSI mode instead.
 	 */
+	if (n_ports > 1) {
 		nvec = pci_alloc_irq_vectors(pdev, n_ports, INT_MAX,
 				PCI_IRQ_MSIX | PCI_IRQ_MSI);
 		if (nvec > 0) {
@@ -1428,12 +1429,14 @@ static int ahci_init_msi(struct pci_dev *pdev, unsigned int n_ports,
 			}
 
 			/*
-		 * Fallback to single MSI mode if the controller enforced MRSM
-		 * mode.
+			 * Fallback to single MSI mode if the controller
+			 * enforced MRSM mode.
 			 */
-		printk(KERN_INFO "ahci: MRSM is on, fallback to single MSI\n");
+			printk(KERN_INFO
+				"ahci: MRSM is on, fallback to single MSI\n");
 			pci_free_irq_vectors(pdev);
 		}
+	}
 
 	/*
 	 * -ENOSPC indicated we don't have enough vectors.  Don't bother trying