USB: add USB-Persist facility

This patch (as886) adds the controversial USB-persist facility,
allowing USB devices to persist across a power loss during system
suspend.

The facility is controlled by a new Kconfig option (with appropriate
warnings about the potential dangers); when the option is off the
behavior will remain the same as it is now.  But when the option is
on, people will be able to use suspend-to-disk and keep their USB
filesystems intact -- something particularly valuable for small
machines where the root filesystem is on a USB device!

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

1 files changed, 22 insertions, 0 deletions

diff --git a/drivers/usb/core/Kconfig b/drivers/usb/core/Kconfig
index 346fc030c929..5113ef4cb7f6 100644
--- a/drivers/usb/core/Kconfig
+++ b/drivers/usb/core/Kconfig
@@ -86,6 +86,28 @@ config USB_SUSPEND
 
           If you are unsure about this, say N here.
 
+config USB_PERSIST
+        bool "USB device persistence during system suspend (DANGEROUS)"
+        depends on USB && PM && EXPERIMENTAL
+        default n
+        help
+          If you say Y here, USB device data structures will remain
+          persistent across system suspend, even if the USB bus loses
+          power.  (This includes software-suspend, also known as swsusp,
+          or suspend-to-disk.)  The devices will reappear as if by magic
+          when the system wakes up, with no need to unmount USB filesystems,
+          rmmod host-controller drivers, or do anything else.
+
+                WARNING: This option can be dangerous!
+
+          If a USB device is replaced by another of the same type while
+          the system is asleep, there's a good chance the kernel won't
+          detect the change.  Likewise if the media in a USB storage
+          device is replaced.  When this happens it's almost certain to
+          cause data corruption and maybe even crash your system.
+
+          If you are unsure, say N here.
+
 config USB_OTG
         bool
         depends on USB && EXPERIMENTAL