usb: expose usb port's pm qos flags to user space

This patch is to expose usb port's pm qos flags(pm_qos_no_power_off,
pm_qos_remote_wakeup) to user space. User can set pm_qos_no_power_off
flag to prohibit the port from being powered off.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/port.c

@@ -67,6 +67,7 @@ static void usb_port_device_release(struct device *dev)
 {
 	struct usb_port *port_dev = to_usb_port(dev);
 
+	dev_pm_qos_hide_flags(dev);
 	usb_acpi_unregister_power_resources(dev);
 	kfree(port_dev);
 }
@@ -176,8 +177,16 @@ int usb_hub_create_port_device(struct usb_hub *hub, int port1)
 		goto error_register;
 
 	pm_runtime_set_active(&port_dev->dev);
+
+	/* It would be dangerous if user space couldn't
+	 * prevent usb device from being powered off. So don't
+	 * enable port runtime pm if failed to expose port's pm qos.
+	 */
+	if (!dev_pm_qos_expose_flags(&port_dev->dev,
+			PM_QOS_FLAG_NO_POWER_OFF))
 		pm_runtime_enable(&port_dev->dev);
 
+
 	retval = usb_acpi_register_power_resources(&port_dev->dev);
 	if (retval && retval != -ENODEV)
 		dev_warn(&port_dev->dev, "the port can't register its ACPI power resource.\n");