		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.

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.
