Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

                USB device persistence during system suspend

                   Alan Stern 

                 September 2, 2006 (Updated May 29, 2007)

        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?

Setting CONFIG_USB_PERSIST will cause the kernel to work around these

issues.  It enables a mode in which the core USB device data

structures are allowed 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.

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, just as it would without the CONFIG_USB_PERSIST option.

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 even when CONFIG_USB_PERSIST is set, the "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, 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 CONFIG_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 option 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: Using CONFIG_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.  Serial numbers and other strings are

not compared.  In many cases it wouldn't help if they were, because

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 "persist" feature can be extremely useful.  Make the most

of it.