serial/8250_pci: init-quirk msi support for kt serial controller

The semantics of UPF_IIR_ONCE (once per serial irq) are only guaranteed
if the kt irq is not shared (once per serial isr in the shared case ==
potentially unwanted reads of the IIR).
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Acked-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/tty/serial/8250_pci.c

@@ -1118,6 +1118,18 @@ pci_xr17c154_setup(struct serial_private *priv,
 	return pci_default_setup(priv, board, port, idx);
 }
 
+static int try_enable_msi(struct pci_dev *dev)
+{
+	/* use msi if available, but fallback to legacy otherwise */
+	pci_enable_msi(dev);
+	return 0;
+}
+
+static void disable_msi(struct pci_dev *dev)
+{
+	pci_disable_msi(dev);
+}
+
 #define PCI_VENDOR_ID_SBSMODULARIO	0x124B
 #define PCI_SUBVENDOR_ID_SBSMODULARIO	0x124B
 #define PCI_DEVICE_ID_OCTPRO		0x0001
@@ -1233,7 +1245,9 @@ static struct pci_serial_quirk pci_serial_quirks[] __refdata = {
 		.device		= PCI_DEVICE_ID_INTEL_PATSBURG_KT,
 		.subvendor	= PCI_ANY_ID,
 		.subdevice	= PCI_ANY_ID,
+		.init		= try_enable_msi,
 		.setup		= kt_serial_setup,
+		.exit		= disable_msi,
 	},
 	/*
 	 * ITE