linux/Documentation/usb/persist.txt
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   1                USB device persistence during system suspend
   2
   3                   Alan Stern <stern@rowland.harvard.edu>
   4
   5                September 2, 2006 (Updated February 25, 2008)
   6
   7
   8        What is the problem?
   9
  10According to the USB specification, when a USB bus is suspended the
  11bus must continue to supply suspend current (around 1-5 mA).  This
  12is so that devices can maintain their internal state and hubs can
  13detect connect-change events (devices being plugged in or unplugged).
  14The technical term is "power session".
  15
  16If a USB device's power session is interrupted then the system is
  17required to behave as though the device has been unplugged.  It's a
  18conservative approach; in the absence of suspend current the computer
  19has no way to know what has actually happened.  Perhaps the same
  20device is still attached or perhaps it was removed and a different
  21device plugged into the port.  The system must assume the worst.
  22
  23By default, Linux behaves according to the spec.  If a USB host
  24controller loses power during a system suspend, then when the system
  25wakes up all the devices attached to that controller are treated as
  26though they had disconnected.  This is always safe and it is the
  27"officially correct" thing to do.
  28
  29For many sorts of devices this behavior doesn't matter in the least.
  30If the kernel wants to believe that your USB keyboard was unplugged
  31while the system was asleep and a new keyboard was plugged in when the
  32system woke up, who cares?  It'll still work the same when you type on
  33it.
  34
  35Unfortunately problems _can_ arise, particularly with mass-storage
  36devices.  The effect is exactly the same as if the device really had
  37been unplugged while the system was suspended.  If you had a mounted
  38filesystem on the device, you're out of luck -- everything in that
  39filesystem is now inaccessible.  This is especially annoying if your
  40root filesystem was located on the device, since your system will
  41instantly crash.
  42
  43Loss of power isn't the only mechanism to worry about.  Anything that
  44interrupts a power session will have the same effect.  For example,
  45even though suspend current may have been maintained while the system
  46was asleep, on many systems during the initial stages of wakeup the
  47firmware (i.e., the BIOS) resets the motherboard's USB host
  48controllers.  Result: all the power sessions are destroyed and again
  49it's as though you had unplugged all the USB devices.  Yes, it's
  50entirely the BIOS's fault, but that doesn't do _you_ any good unless
  51you can convince the BIOS supplier to fix the problem (lots of luck!).
  52
  53On many systems the USB host controllers will get reset after a
  54suspend-to-RAM.  On almost all systems, no suspend current is
  55available during hibernation (also known as swsusp or suspend-to-disk).
  56You can check the kernel log after resuming to see if either of these
  57has happened; look for lines saying "root hub lost power or was reset".
  58
  59In practice, people are forced to unmount any filesystems on a USB
  60device before suspending.  If the root filesystem is on a USB device,
  61the system can't be suspended at all.  (All right, it _can_ be
  62suspended -- but it will crash as soon as it wakes up, which isn't
  63much better.)
  64
  65
  66        What is the solution?
  67
  68The kernel includes a feature called USB-persist.  It tries to work
  69around these issues by allowing the core USB device data structures to
  70persist across a power-session disruption.
  71
  72It works like this.  If the kernel sees that a USB host controller is
  73not in the expected state during resume (i.e., if the controller was
  74reset or otherwise had lost power) then it applies a persistence check
  75to each of the USB devices below that controller for which the
  76"persist" attribute is set.  It doesn't try to resume the device; that
  77can't work once the power session is gone.  Instead it issues a USB
  78port reset and then re-enumerates the device.  (This is exactly the
  79same thing that happens whenever a USB device is reset.)  If the
  80re-enumeration shows that the device now attached to that port has the
  81same descriptors as before, including the Vendor and Product IDs, then
  82the kernel continues to use the same device structure.  In effect, the
  83kernel treats the device as though it had merely been reset instead of
  84unplugged.
  85
  86The same thing happens if the host controller is in the expected state
  87but a USB device was unplugged and then replugged, or if a USB device
  88fails to carry out a normal resume.
  89
  90If no device is now attached to the port, or if the descriptors are
  91different from what the kernel remembers, then the treatment is what
  92you would expect.  The kernel destroys the old device structure and
  93behaves as though the old device had been unplugged and a new device
  94plugged in.
  95
  96The end result is that the USB device remains available and usable.
  97Filesystem mounts and memory mappings are unaffected, and the world is
  98now a good and happy place.
  99
 100Note that the "USB-persist" feature will be applied only to those
 101devices for which it is enabled.  You can enable the feature by doing
 102(as root):
 103
 104        echo 1 >/sys/bus/usb/devices/.../power/persist
 105
 106where the "..." should be filled in the with the device's ID.  Disable
 107the feature by writing 0 instead of 1.  For hubs the feature is
 108automatically and permanently enabled and the power/persist file
 109doesn't even exist, so you only have to worry about setting it for
 110devices where it really matters.
 111
 112
 113        Is this the best solution?
 114
 115Perhaps not.  Arguably, keeping track of mounted filesystems and
 116memory mappings across device disconnects should be handled by a
 117centralized Logical Volume Manager.  Such a solution would allow you
 118to plug in a USB flash device, create a persistent volume associated
 119with it, unplug the flash device, plug it back in later, and still
 120have the same persistent volume associated with the device.  As such
 121it would be more far-reaching than USB-persist.
 122
 123On the other hand, writing a persistent volume manager would be a big
 124job and using it would require significant input from the user.  This
 125solution is much quicker and easier -- and it exists now, a giant
 126point in its favor!
 127
 128Furthermore, the USB-persist feature applies to _all_ USB devices, not
 129just mass-storage devices.  It might turn out to be equally useful for
 130other device types, such as network interfaces.
 131
 132
 133        WARNING: USB-persist can be dangerous!!
 134
 135When recovering an interrupted power session the kernel does its best
 136to make sure the USB device hasn't been changed; that is, the same
 137device is still plugged into the port as before.  But the checks
 138aren't guaranteed to be 100% accurate.
 139
 140If you replace one USB device with another of the same type (same
 141manufacturer, same IDs, and so on) there's an excellent chance the
 142kernel won't detect the change.  The serial number string and other
 143descriptors are compared with the kernel's stored values, but this
 144might not help since manufacturers frequently omit serial numbers
 145entirely in their devices.
 146
 147Furthermore it's quite possible to leave a USB device exactly the same
 148while changing its media.  If you replace the flash memory card in a
 149USB card reader while the system is asleep, the kernel will have no
 150way to know you did it.  The kernel will assume that nothing has
 151happened and will continue to use the partition tables, inodes, and
 152memory mappings for the old card.
 153
 154If the kernel gets fooled in this way, it's almost certain to cause
 155data corruption and to crash your system.  You'll have no one to blame
 156but yourself.
 157
 158YOU HAVE BEEN WARNED!  USE AT YOUR OWN RISK!
 159
 160That having been said, most of the time there shouldn't be any trouble
 161at all.  The USB-persist feature can be extremely useful.  Make the
 162most of it.
 163
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