driver-api/usb/persist.rst

1 .. _usb-persist:
3 USB device persistence during system suspend
14 bus must continue to supply suspend current (around 1-5 mA).  This
16 detect connect-change events (devices being plugged in or unplugged).
21 conservative approach; in the absence of suspend current the computer
22 has no way to know what has actually happened.  Perhaps the same
27 controller loses power during a system suspend, then when the system
38 Unfortunately problems _can_ arise, particularly with mass-storage
41 filesystem on the device, you're out of luck -- everything in that
48 even though suspend current may have been maintained while the system
49 was asleep, on many systems during the initial stages of wakeup the
56 On many systems the USB host controllers will get reset after a
57 suspend-to-RAM.  On almost all systems, no suspend current is
58 available during hibernation (also known as swsusp or suspend-to-disk).
60 has happened; look for lines saying "root hub lost power or was reset".
65 suspended -- but it will crash as soon as it wakes up, which isn't
72 The kernel includes a feature called USB-persist.  It tries to work
74 persist across a power-session disruption.
77 not in the expected state during resume (i.e., if the controller was
78 reset or otherwise had lost power) then it applies a persistence check
82 port reset and then re-enumerates the device.  (This is exactly the
83 same thing that happens whenever a USB device is reset.)  If the
84 re-enumeration shows that the device now attached to that port has the
87 kernel treats the device as though it had merely been reset instead of
94 If no device is now attached to the port, or if the descriptors are
104 Note that the "USB-persist" feature will be applied only to those
126 it would be more far-reaching than USB-persist.
130 solution is much quicker and easier -- and it exists now, a giant
133 Furthermore, the USB-persist feature applies to _all_ USB devices, not
134 just mass-storage devices.  It might turn out to be equally useful for
138 WARNING: USB-persist can be dangerous!!
155 USB card reader while the system is asleep, the kernel will have no
161 data corruption and to crash your system.  You'll have no one to blame
165 maybe fail because they may morph after reset.
170 at all.  The USB-persist feature can be extremely useful.  Make the