Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.
1 USB device persistence during system suspend 2 3 Alan Stern <firstname.lastname@example.org> 4 5 September 2, 2006 (Updated February 25, 2008) 6 7 8 What is the problem? 9 10 According to the USB specification, when a USB bus is suspended the 11 bus must continue to supply suspend current (around 1-5 mA). This 12 is so that devices can maintain their internal state and hubs can 13 detect connect-change events (devices being plugged in or unplugged). 14 The technical term is "power session". 15 16 If a USB device's power session is interrupted then the system is 17 required to behave as though the device has been unplugged. It's a 18 conservative approach; in the absence of suspend current the computer 19 has no way to know what has actually happened. Perhaps the same 20 device is still attached or perhaps it was removed and a different 21 device plugged into the port. The system must assume the worst. 22 23 By default, Linux behaves according to the spec. If a USB host 24 controller loses power during a system suspend, then when the system 25 wakes up all the devices attached to that controller are treated as 26 though they had disconnected. This is always safe and it is the 27 "officially correct" thing to do. 28 29 For many sorts of devices this behavior doesn't matter in the least. 30 If the kernel wants to believe that your USB keyboard was unplugged 31 while the system was asleep and a new keyboard was plugged in when the 32 system woke up, who cares? It'll still work the same when you type on 33 it. 34 35 Unfortunately problems _can_ arise, particularly with mass-storage 36 devices. The effect is exactly the same as if the device really had 37 been unplugged while the system was suspended. If you had a mounted 38 filesystem on the device, you're out of luck -- everything in that 39 filesystem is now inaccessible. This is especially annoying if your 40 root filesystem was located on the device, since your system will 41 instantly crash. 42 43 Loss of power isn't the only mechanism to worry about. Anything that 44 interrupts a power session will have the same effect. For example, 45 even though suspend current may have been maintained while the system 46 was asleep, on many systems during the initial stages of wakeup the 47 firmware (i.e., the BIOS) resets the motherboard's USB host 48 controllers. Result: all the power sessions are destroyed and again 49 it's as though you had unplugged all the USB devices. Yes, it's 50 entirely the BIOS's fault, but that doesn't do _you_ any good unless 51 you can convince the BIOS supplier to fix the problem (lots of luck!). 52 53 On many systems the USB host controllers will get reset after a 54 suspend-to-RAM. On almost all systems, no suspend current is 55 available during hibernation (also known as swsusp or suspend-to-disk). 56 You can check the kernel log after resuming to see if either of these 57 has happened; look for lines saying "root hub lost power or was reset". 58 59 In practice, people are forced to unmount any filesystems on a USB 60 device before suspending. If the root filesystem is on a USB device, 61 the system can't be suspended at all. (All right, it _can_ be 62 suspended -- but it will crash as soon as it wakes up, which isn't 63 much better.) 64 65 66 What is the solution? 67 68 The kernel includes a feature called USB-persist. It tries to work 69 around these issues by allowing the core USB device data structures to 70 persist across a power-session disruption. 71 72 It works like this. If the kernel sees that a USB host controller is 73 not in the expected state during resume (i.e., if the controller was 74 reset or otherwise had lost power) then it applies a persistence check 75 to each of the USB devices below that controller for which the 76 "persist" attribute is set. It doesn't try to resume the device; that 77 can't work once the power session is gone. Instead it issues a USB 78 port reset and then re-enumerates the device. (This is exactly the 79 same thing that happens whenever a USB device is reset.) If the 80 re-enumeration shows that the device now attached to that port has the 81 same descriptors as before, including the Vendor and Product IDs, then 82 the kernel continues to use the same device structure. In effect, the 83 kernel treats the device as though it had merely been reset instead of 84 unplugged. 85 86 The same thing happens if the host controller is in the expected state 87 but a USB device was unplugged and then replugged, or if a USB device 88 fails to carry out a normal resume. 89 90 If no device is now attached to the port, or if the descriptors are 91 different from what the kernel remembers, then the treatment is what 92 you would expect. The kernel destroys the old device structure and 93 behaves as though the old device had been unplugged and a new device 94 plugged in. 95 96 The end result is that the USB device remains available and usable. 97 Filesystem mounts and memory mappings are unaffected, and the world is 98 now a good and happy place. 99 100 Note that the "USB-persist" feature will be applied only to those 101 devices for which it is enabled. You can enable the feature by doing 102 (as root): 103 104 echo 1 >/sys/bus/usb/devices/.../power/persist 105 106 where the "..." should be filled in the with the device's ID. Disable 107 the feature by writing 0 instead of 1. For hubs the feature is 108 automatically and permanently enabled and the power/persist file 109 doesn't even exist, so you only have to worry about setting it for 110 devices where it really matters. 111 112 113 Is this the best solution? 114 115 Perhaps not. Arguably, keeping track of mounted filesystems and 116 memory mappings across device disconnects should be handled by a 117 centralized Logical Volume Manager. Such a solution would allow you 118 to plug in a USB flash device, create a persistent volume associated 119 with it, unplug the flash device, plug it back in later, and still 120 have the same persistent volume associated with the device. As such 121 it would be more far-reaching than USB-persist. 122 123 On the other hand, writing a persistent volume manager would be a big 124 job and using it would require significant input from the user. This 125 solution is much quicker and easier -- and it exists now, a giant 126 point in its favor! 127 128 Furthermore, the USB-persist feature applies to _all_ USB devices, not 129 just mass-storage devices. It might turn out to be equally useful for 130 other device types, such as network interfaces. 131 132 133 WARNING: USB-persist can be dangerous!! 134 135 When recovering an interrupted power session the kernel does its best 136 to make sure the USB device hasn't been changed; that is, the same 137 device is still plugged into the port as before. But the checks 138 aren't guaranteed to be 100% accurate. 139 140 If you replace one USB device with another of the same type (same 141 manufacturer, same IDs, and so on) there's an excellent chance the 142 kernel won't detect the change. The serial number string and other 143 descriptors are compared with the kernel's stored values, but this 144 might not help since manufacturers frequently omit serial numbers 145 entirely in their devices. 146 147 Furthermore it's quite possible to leave a USB device exactly the same 148 while changing its media. If you replace the flash memory card in a 149 USB card reader while the system is asleep, the kernel will have no 150 way to know you did it. The kernel will assume that nothing has 151 happened and will continue to use the partition tables, inodes, and 152 memory mappings for the old card. 153 154 If the kernel gets fooled in this way, it's almost certain to cause 155 data corruption and to crash your system. You'll have no one to blame 156 but yourself. 157 158 For those devices with avoid_reset_quirk attribute being set, persist 159 maybe fail because they may morph after reset. 160 161 YOU HAVE BEEN WARNED! USE AT YOUR OWN RISK! 162 163 That having been said, most of the time there shouldn't be any trouble 164 at all. The USB-persist feature can be extremely useful. Make the 165 most of it.