USB device persistence during system suspend
  
  		   Alan Stern <stern@rowland.harvard.edu>
  
  		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.