diff options
Diffstat (limited to 'Documentation/usb/persist.txt')
| -rw-r--r-- | Documentation/usb/persist.txt | 38 |
1 files changed, 25 insertions, 13 deletions
diff --git a/Documentation/usb/persist.txt b/Documentation/usb/persist.txt index 6dcd5f884795..df54d645cbb5 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 March 27, 2007) + September 2, 2006 (Updated May 29, 2007) What is the problem? @@ -52,9 +52,9 @@ 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 suspend-to-disk (also known as swsusp). 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". +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, @@ -71,15 +71,16 @@ 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. 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. +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 @@ -91,6 +92,17 @@ 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? |