usb: fix hub-port pm_runtime_enable() vs runtime pm transitions

Commit 9262c19d14c4 "usb: disable port power control if not supported in
wHubCharacteristics" gated enabling runtime pm for usb_port devices on
whether the parent hub supports power control, which causes a
regression. The port must still be allowed to carry out runtime pm
callbacks and receive a -EAGAIN or -EBUSY result. Otherwise the
usb_port device will transition to the pm error state and trigger the
same for the child usb_device.

Prior to the offending commit usb_hub_create_port_device() arranged for
runtime pm to be disabled is dev_pm_qos_expose_flags() failed. Instead,
force the default state of PM_QOS_FLAG_NO_POWER_OFF flag to be set prior
to enabling runtime pm. If that policy can not be set then fail
registration.

Report: http://marc.info/?l=linux-usb&m=140290586301336&w=2
Fixes: 9262c19d14c4 ("usb: disable port power control if not supported in wHubCharacteristics")
Reported-by: Bjørn Mork <[e-mail]>
Reported-by: Alan Stern <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/core/hub.c

···
 		}
 	}
 	hdev->maxchild = i;
+	for (i = 0; i < hdev->maxchild; i++) {
+		struct usb_port *port_dev = hub->ports[i];
+
+		pm_runtime_put(&port_dev->dev);
+	}
+
 	mutex_unlock(&usb_port_peer_mutex);
 	if (ret < 0)
 		goto fail;

drivers/usb/core/hub.h

···
  * @dev: generic device interface
  * @port_owner: port's owner
  * @peer: related usb2 and usb3 ports (share the same connector)
+ * @req: default pm qos request for hubs without port power control
  * @connect_type: port's connect type
  * @location: opaque representation of platform connector location
  * @status_lock: synchronize port_event() vs usb_port_{suspend|resume}
···
 	struct device dev;
 	struct usb_dev_state *port_owner;
 	struct usb_port *peer;
+	struct dev_pm_qos_request *req;
 	enum usb_port_connect_type connect_type;
 	usb_port_location_t location;
 	struct mutex status_lock;

drivers/usb/core/port.c

···
 {
 	struct usb_port *port_dev = to_usb_port(dev);
 
+	kfree(port_dev->req);
 	kfree(port_dev);
 }
 
···
 	int retval;
 
 	port_dev = kzalloc(sizeof(*port_dev), GFP_KERNEL);
-	if (!port_dev) {
-		retval = -ENOMEM;
-		goto exit;
+	if (!port_dev)
+		return -ENOMEM;
+
+	port_dev->req = kzalloc(sizeof(*(port_dev->req)), GFP_KERNEL);
+	if (!port_dev->req) {
+		kfree(port_dev);
+		return -ENOMEM;
 	}
 
 	hub->ports[port1 - 1] = port_dev;
···
 			port1);
 	mutex_init(&port_dev->status_lock);
 	retval = device_register(&port_dev->dev);
-	if (retval)
-		goto error_register;
+	if (retval) {
+		put_device(&port_dev->dev);
+		return retval;
+	}
+
+	/* Set default policy of port-poweroff disabled. */
+	retval = dev_pm_qos_add_request(&port_dev->dev, port_dev->req,
+			DEV_PM_QOS_FLAGS, PM_QOS_FLAG_NO_POWER_OFF);
+	if (retval < 0) {
+		device_unregister(&port_dev->dev);
+		return retval;
+	}
 
 	find_and_link_peer(hub, port1);
 
+	/*
+	 * Enable runtime pm and hold a refernce that hub_configure()
+	 * will drop once the PM_QOS_NO_POWER_OFF flag state has been set
+	 * and the hub has been fully registered (hdev->maxchild set).
+	 */
 	pm_runtime_set_active(&port_dev->dev);
+	pm_runtime_get_noresume(&port_dev->dev);
+	pm_runtime_enable(&port_dev->dev);
+	device_enable_async_suspend(&port_dev->dev);
 
 	/*
-	 * Do not enable port runtime pm if the hub does not support
-	 * power switching.  Also, userspace must have final say of
-	 * whether a port is permitted to power-off.  Do not enable
-	 * runtime pm if we fail to expose pm_qos_no_power_off.
+	 * Keep hidden the ability to enable port-poweroff if the hub
+	 * does not support power switching.
 	 */
-	if (hub_is_port_power_switchable(hub)
-			&& dev_pm_qos_expose_flags(&port_dev->dev,
-			PM_QOS_FLAG_NO_POWER_OFF) == 0)
-		pm_runtime_enable(&port_dev->dev);
+	if (!hub_is_port_power_switchable(hub))
+		return 0;
 
-	device_enable_async_suspend(&port_dev->dev);
+	/* Attempt to let userspace take over the policy. */
+	retval = dev_pm_qos_expose_flags(&port_dev->dev,
+			PM_QOS_FLAG_NO_POWER_OFF);
+	if (retval < 0) {
+		dev_warn(&port_dev->dev, "failed to expose pm_qos_no_poweroff\n");
+		return 0;
+	}
+
+	/* Userspace owns the policy, drop the kernel 'no_poweroff' request. */
+	retval = dev_pm_qos_remove_request(port_dev->req);
+	if (retval >= 0) {
+		kfree(port_dev->req);
+		port_dev->req = NULL;
+	}
 	return 0;
-
-error_register:
-	put_device(&port_dev->dev);
-exit:
-	return retval;
 }
 
 void usb_hub_remove_port_device(struct usb_hub *hub, int port1)