USB: set hub's default autosuspend delay as 0

This patch sets hub device's default autosuspend delay as 0 to
speedup bus suspend, see comments in code for details.
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hub.c

@@ -1614,6 +1614,41 @@ static int hub_probe(struct usb_interface *intf, const struct usb_device_id *id)
 	desc = intf->cur_altsetting;
 	hdev = interface_to_usbdev(intf);
 
+	/*
+	 * Set default autosuspend delay as 0 to speedup bus suspend,
+	 * based on the below considerations:
+	 *
+	 * - Unlike other drivers, the hub driver does not rely on the
+	 *   autosuspend delay to provide enough time to handle a wakeup
+	 *   event, and the submitted status URB is just to check future
+	 *   change on hub downstream ports, so it is safe to do it.
+	 *
+	 * - The patch might cause one or more auto supend/resume for
+	 *   below very rare devices when they are plugged into hub
+	 *   first time:
+	 *
+	 *   	devices having trouble initializing, and disconnect
+	 *   	themselves from the bus and then reconnect a second
+	 *   	or so later
+	 *
+	 *   	devices just for downloading firmware, and disconnects
+	 *   	themselves after completing it
+	 *
+	 *   For these quite rare devices, their drivers may change the
+	 *   autosuspend delay of their parent hub in the probe() to one
+	 *   appropriate value to avoid the subtle problem if someone
+	 *   does care it.
+	 *
+	 * - The patch may cause one or more auto suspend/resume on
+	 *   hub during running 'lsusb', but it is probably too
+	 *   infrequent to worry about.
+	 *
+	 * - Change autosuspend delay of hub can avoid unnecessary auto
+	 *   suspend timer for hub, also may decrease power consumption
+	 *   of USB bus.
+	 */
+	pm_runtime_set_autosuspend_delay(&hdev->dev, 0);
+
 	/* Hubs have proper suspend/resume support. */
 	usb_enable_autosuspend(hdev);