usb: Make USB persist default configurable

Commit 9214d1d8 set the USB persist flag as a default for all devices.
This might be desirable for some distributions, but it certainly has its
trade-offs... most importantly, it can significantly increase system
resume time, because the kernel blocks on resuming (and sometimes
resetting) USB devices before it unfreezes userspace.

This patch introduces a new config option CONFIG_USB_DEFAULT_PERSIST,
which allows distributions to make this decision on their own without
the need to carry a custom patch or revert the kernel's setting in
userspace.

[edited the Kconfig help text a bit - gregkh]
Signed-off-by: Julius Werner <jwerner@chromium.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/Kconfig

@@ -27,6 +27,22 @@ config USB_ANNOUNCE_NEW_DEVICES
 comment "Miscellaneous USB options"
 	depends on USB
 
+config USB_DEFAULT_PERSIST
+	bool "Enable USB persist by default"
+	depends on USB
+	default y
+	help
+	  Say N here if you don't want USB power session persistance
+	  enabled by default.  If you say N it will make suspended USB
+	  devices that lose power get reenumerated as if they had been
+	  unplugged, causing any mounted filesystems to be lost.  The
+	  persist feature can still be enabled for individual devices
+	  through the power/persist sysfs node. See
+	  Documentation/usb/persist.txt for more info.
+
+	  If you have any questions about this, say Y here, only say N
+	  if you know exactly what you are doing.
+
 config USB_DYNAMIC_MINORS
 	bool "Dynamic USB minor allocation"
 	depends on USB

drivers/usb/core/quirks.c

@@ -201,20 +201,14 @@ void usb_detect_quirks(struct usb_device *udev)
 		dev_dbg(&udev->dev, "USB quirks for this device: %x\n",
 			udev->quirks);
 
-	/* For the present, all devices default to USB-PERSIST enabled */
-#if 0		/* was: #ifdef CONFIG_PM */
-	/* Hubs are automatically enabled for USB-PERSIST */
-	if (udev->descriptor.bDeviceClass == USB_CLASS_HUB)
+#ifdef CONFIG_USB_DEFAULT_PERSIST
+	if (!(udev->quirks & USB_QUIRK_RESET))
 		udev->persist_enabled = 1;
-
 #else
-	/* In the absence of PM, we can safely enable USB-PERSIST
-	 * for all devices.  It will affect things like hub resets
-	 * and EMF-related port disables.
-	 */
-	if (!(udev->quirks & USB_QUIRK_RESET))
+	/* Hubs are automatically enabled for USB-PERSIST */
+	if (udev->descriptor.bDeviceClass == USB_CLASS_HUB)
 		udev->persist_enabled = 1;
-#endif	/* CONFIG_PM */
+#endif	/* CONFIG_USB_DEFAULT_PERSIST */
 }
 
 void usb_detect_interface_quirks(struct usb_device *udev)