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: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index ff86355..43ce146 100644 (file)
--- a/drivers/usb/core/hub.c
+++ b/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);