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+USB device persistence during system suspend
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:Author: Alan Stern <stern@rowland.harvard.edu>
+:Date: September 2, 2006 (Updated February 25, 2008)
+
+
+What is the problem?
+====================
+
+According to the USB specification, when a USB bus is suspended the
+bus must continue to supply suspend current (around 1-5 mA).  This
+is so that devices can maintain their internal state and hubs can
+detect connect-change events (devices being plugged in or unplugged).
+The technical term is "power session".
+
+If a USB device's power session is interrupted then the system is
+required to behave as though the device has been unplugged.  It's a
+conservative approach; in the absence of suspend current the computer
+has no way to know what has actually happened.  Perhaps the same
+device is still attached or perhaps it was removed and a different
+device plugged into the port.  The system must assume the worst.
+
+By default, Linux behaves according to the spec.  If a USB host
+controller loses power during a system suspend, then when the system
+wakes up all the devices attached to that controller are treated as
+though they had disconnected.  This is always safe and it is the
+"officially correct" thing to do.
+
+For many sorts of devices this behavior doesn't matter in the least.
+If the kernel wants to believe that your USB keyboard was unplugged
+while the system was asleep and a new keyboard was plugged in when the
+system woke up, who cares?  It'll still work the same when you type on
+it.
+
+Unfortunately problems _can_ arise, particularly with mass-storage
+devices.  The effect is exactly the same as if the device really had
+been unplugged while the system was suspended.  If you had a mounted
+filesystem on the device, you're out of luck -- everything in that
+filesystem is now inaccessible.  This is especially annoying if your
+root filesystem was located on the device, since your system will
+instantly crash.
+
+Loss of power isn't the only mechanism to worry about.  Anything that
+interrupts a power session will have the same effect.  For example,
+even though suspend current may have been maintained while the system
+was asleep, on many systems during the initial stages of wakeup the
+firmware (i.e., the BIOS) resets the motherboard's USB host
+controllers.  Result: all the power sessions are destroyed and again
+it's as though you had unplugged all the USB devices.  Yes, it's
+entirely the BIOS's fault, but that doesn't do _you_ any good unless
+you can convince the BIOS supplier to fix the problem (lots of luck!).
+
+On many systems the USB host controllers will get reset after a
+suspend-to-RAM.  On almost all systems, no suspend current is
+available during hibernation (also known as swsusp or suspend-to-disk).
+You can check the kernel log after resuming to see if either of these
+has happened; look for lines saying "root hub lost power or was reset".
+
+In practice, people are forced to unmount any filesystems on a USB
+device before suspending.  If the root filesystem is on a USB device,
+the system can't be suspended at all.  (All right, it _can_ be
+suspended -- but it will crash as soon as it wakes up, which isn't
+much better.)
+
+
+What is the solution?
+=====================
+
+The kernel includes a feature called USB-persist.  It tries to work
+around these issues by allowing the core USB device data structures to
+persist across a power-session disruption.
+
+It works like this.  If the kernel sees that a USB host controller is
+not in the expected state during resume (i.e., if the controller was
+reset or otherwise had lost power) then it applies a persistence check
+to each of the USB devices below that controller for which the
+"persist" attribute is set.  It doesn't try to resume the device; that
+can't work once the power session is gone.  Instead it issues a USB
+port reset and then re-enumerates the device.  (This is exactly the
+same thing that happens whenever a USB device is reset.)  If the
+re-enumeration shows that the device now attached to that port has the
+same descriptors as before, including the Vendor and Product IDs, then
+the kernel continues to use the same device structure.  In effect, the
+kernel treats the device as though it had merely been reset instead of
+unplugged.
+
+The same thing happens if the host controller is in the expected state
+but a USB device was unplugged and then replugged, or if a USB device
+fails to carry out a normal resume.
+
+If no device is now attached to the port, or if the descriptors are
+different from what the kernel remembers, then the treatment is what
+you would expect.  The kernel destroys the old device structure and
+behaves as though the old device had been unplugged and a new device
+plugged in.
+
+The end result is that the USB device remains available and usable.
+Filesystem mounts and memory mappings are unaffected, and the world is
+now a good and happy place.
+
+Note that the "USB-persist" feature will be applied only to those
+devices for which it is enabled.  You can enable the feature by doing
+(as root)::
+
+	echo 1 >/sys/bus/usb/devices/.../power/persist
+
+where the "..." should be filled in the with the device's ID.  Disable
+the feature by writing 0 instead of 1.  For hubs the feature is
+automatically and permanently enabled and the power/persist file
+doesn't even exist, so you only have to worry about setting it for
+devices where it really matters.
+
+
+Is this the best solution?
+==========================
+
+Perhaps not.  Arguably, keeping track of mounted filesystems and
+memory mappings across device disconnects should be handled by a
+centralized Logical Volume Manager.  Such a solution would allow you
+to plug in a USB flash device, create a persistent volume associated
+with it, unplug the flash device, plug it back in later, and still
+have the same persistent volume associated with the device.  As such
+it would be more far-reaching than USB-persist.
+
+On the other hand, writing a persistent volume manager would be a big
+job and using it would require significant input from the user.  This
+solution is much quicker and easier -- and it exists now, a giant
+point in its favor!
+
+Furthermore, the USB-persist feature applies to _all_ USB devices, not
+just mass-storage devices.  It might turn out to be equally useful for
+other device types, such as network interfaces.
+
+
+WARNING: USB-persist can be dangerous!!
+=======================================
+
+When recovering an interrupted power session the kernel does its best
+to make sure the USB device hasn't been changed; that is, the same
+device is still plugged into the port as before.  But the checks
+aren't guaranteed to be 100% accurate.
+
+If you replace one USB device with another of the same type (same
+manufacturer, same IDs, and so on) there's an excellent chance the
+kernel won't detect the change.  The serial number string and other
+descriptors are compared with the kernel's stored values, but this
+might not help since manufacturers frequently omit serial numbers
+entirely in their devices.
+
+Furthermore it's quite possible to leave a USB device exactly the same
+while changing its media.  If you replace the flash memory card in a
+USB card reader while the system is asleep, the kernel will have no
+way to know you did it.  The kernel will assume that nothing has
+happened and will continue to use the partition tables, inodes, and
+memory mappings for the old card.
+
+If the kernel gets fooled in this way, it's almost certain to cause
+data corruption and to crash your system.  You'll have no one to blame
+but yourself.
+
+For those devices with avoid_reset_quirk attribute being set, persist
+maybe fail because they may morph after reset.
+
+YOU HAVE BEEN WARNED!  USE AT YOUR OWN RISK!
+
+That having been said, most of the time there shouldn't be any trouble
+at all.  The USB-persist feature can be extremely useful.  Make the
+most of it.