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