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 346fc03..5113ef4 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