diff options
Diffstat (limited to 'Documentation/usb')
| -rw-r--r-- | Documentation/usb/persist.txt | 35 |
1 files changed, 19 insertions, 16 deletions
diff --git a/Documentation/usb/persist.txt b/Documentation/usb/persist.txt index df54d645cbb..bea58dbd30f 100644 --- a/Documentation/usb/persist.txt +++ b/Documentation/usb/persist.txt @@ -2,7 +2,7 @@ Alan Stern <stern@rowland.harvard.edu> - September 2, 2006 (Updated May 29, 2007) + September 2, 2006 (Updated February 25, 2008) What is the problem? @@ -65,9 +65,10 @@ 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. +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 @@ -80,28 +81,30 @@ 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. +unplugged. The same thing happens if the host controller is in the +expected state but a USB device was unplugged and then replugged. 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? @@ -112,19 +115,19 @@ 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. +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 @@ -152,5 +155,5 @@ 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. +at all. The USB-persist feature can be extremely useful. Make the +most of it. |