USB: don't send Set-Interface after reset

This patch (as1221) changes the way usbcore reinitializes a device
following a reset or a reset-resume.  Currently we call
usb_set_interface() for every interface in the active configuration;
this is to put the interface into the same altsetting as before the
reset and to make sure that the host's endpoint state matches the
device's endpoint state.

However, sending a Set-Interface request is a waste of time if an
interface was already in altsetting 0 before the reset, since it is
certainly in altsetting 0 afterward.  In addition, many devices can't
handle Set-Interface requests -- they crash when they receive them.

So instead, the patch adds code to check each interface.  If the
interface wasn't in altsetting 0 before the reset, we go head with the
Set-Interface request as before.  But if it was then we skip sending
the Set-Interface request, and we clear out the host-side endpoint
state by calling usb_disable_interface() followed by
usb_enable_interface().

The patch also adds a couple of new comments to explain what's going
on.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/hub.c

@@ -3393,10 +3393,10 @@ static int usb_reset_and_verify_device(struct usb_device *udev)
 		udev->descriptor = descriptor;	/* for disconnect() calls */
 		goto re_enumerate;
   	}
-  
+
+	/* Restore the device's previous configuration */
 	if (!udev->actconfig)
 		goto done;
-
 	ret = usb_control_msg(udev, usb_sndctrlpipe(udev, 0),
 			USB_REQ_SET_CONFIGURATION, 0,
 			udev->actconfig->desc.bConfigurationValue, 0,
@@ -3409,16 +3409,25 @@ static int usb_reset_and_verify_device(struct usb_device *udev)
   	}
 	usb_set_device_state(udev, USB_STATE_CONFIGURED);
 
+	/* Put interfaces back into the same altsettings as before.
+	 * Don't bother to send the Set-Interface request for interfaces
+	 * that were already in altsetting 0; besides being unnecessary,
+	 * many devices can't handle it.  Instead just reset the host-side
+	 * endpoint state.
+	 */
 	for (i = 0; i < udev->actconfig->desc.bNumInterfaces; i++) {
 		struct usb_interface *intf = udev->actconfig->interface[i];
 		struct usb_interface_descriptor *desc;
 
-		/* set_interface resets host side toggle even
-		 * for altsetting zero.  the interface may have no driver.
-		 */
 		desc = &intf->cur_altsetting->desc;
-		ret = usb_set_interface(udev, desc->bInterfaceNumber,
-			desc->bAlternateSetting);
+		if (desc->bAlternateSetting == 0) {
+			usb_disable_interface(udev, intf, true);
+			usb_enable_interface(udev, intf, true);
+			ret = 0;
+		} else {
+			ret = usb_set_interface(udev, desc->bInterfaceNumber,
+					desc->bAlternateSetting);
+		}
 		if (ret < 0) {
 			dev_err(&udev->dev, "failed to restore interface %d "
 				"altsetting %d (error=%d)\n",