Alan Stern <stern@rowland.harvard.edu>
- September 2, 2006 (Updated May 29, 2007)
+ September 2, 2006 (Updated February 25, 2008)
What is the problem?
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.
+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
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, just as it would without the CONFIG_USB_PERSIST option.
+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 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):
+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, so you only have to worry about
-setting it for devices where it really matters.
+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?
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.
+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 option applies to _all_ USB devices, not
+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: Using CONFIG_USB_PERSIST can be dangerous!!
+ 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
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.
+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
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.
+at all. The USB-persist feature can be extremely useful. Make the
+most of it.
git.openpandora.org / pandora-kernel.git / blobdiff