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Subject[RFC] USB device persistence across suspend-to-disk
Lots of people have asked why USB mass-storage devices don't survive
suspend-to-disk (swsusp). The answer is technical and not too
interesting; what matters is that people have been forced to unmount all
filesystems on USB devices before doing suspend-to-disk. On some systems
this is necessary even for suspend-to-RAM.

This patch adds the capability for USB devices to persist across such
suspends. You can even suspend with your root fs on a USB flash device!
(I think -- I haven't actually tried it. But it _should_ work.)

It's all explained in Documentation/usb/persist.txt, added by the patch.
The "persist" mode is turned off by default because it can be dangerous.

The patch is based on 2.6.18-rc5-mm1. If people like it, I'll submit it
to Greg KH for inclusion in the USB development tree.

Alan Stern

Index: mm/drivers/usb/core/generic.c
--- mm.orig/drivers/usb/core/generic.c
+++ mm/drivers/usb/core/generic.c
@@ -194,6 +194,8 @@ static int generic_suspend(struct usb_de

static int generic_resume(struct usb_device *udev)
+ if (udev->do_persist)
+ return usb_reset_persistent_device(udev);
return usb_port_resume(udev);

Index: mm/drivers/usb/core/hub.c
--- mm.orig/drivers/usb/core/hub.c
+++ mm/drivers/usb/core/hub.c
@@ -61,6 +61,18 @@ module_param (blinkenlights, bool, S_IRU
MODULE_PARM_DESC (blinkenlights, "true to cycle leds on hubs");

+ * Support persistence of devices across power-loss during suspend.
+ *
+ * WARNING: This option can corrupt filesystems and crash the kernel
+ * if a mass-storage device or its media are switched while the system
+ * is asleep and the controller is unpowered.
+ */
+static int persist;
+module_param(persist, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(persist, "Make devices persist across suspend-to-disk "
* As of 2.6.10 we introduce a new USB device initialization scheme which
* closely resembles the way Windows works. Hopefully it will be compatible
* with a wider range of devices than the old scheme. However some previously
@@ -540,6 +552,12 @@ static void hub_pre_reset(struct usb_int
struct usb_device *hdev = hub->hdev;
int port1;

+ /* If this is part of a "persistent-device" reset then we should
+ * do nothing, especially not disconnect the children.
+ */
+ if (hdev->do_persist)
+ return;
for (port1 = 1; port1 <= hdev->maxchild; ++port1) {
if (hdev->children[port1 - 1]) {
usb_disconnect(&hdev->children[port1 - 1]);
@@ -1056,6 +1074,9 @@ void usb_set_device_state(struct usb_dev
* is resumed and Vbus power has been interrupted or the controller
* has been reset. The routine marks all the children of the root hub
* as NOTATTACHED and marks logical connect-change events on their ports.
+ *
+ * But if the "persist" module parameter is set, instead the routine
+ * begins the power-session recovery procedure at the root hub.
void usb_root_hub_lost_power(struct usb_device *rhdev)
@@ -1064,6 +1085,13 @@ void usb_root_hub_lost_power(struct usb_
unsigned long flags;

dev_warn(&rhdev->dev, "root hub lost power or was reset\n");
+ /* Start the "persistent device" mechanism */
+ if (persist) {
+ usb_reset_persistent_device(rhdev);
+ return;
+ }
spin_lock_irqsave(&device_state_lock, flags);
hub = hdev_to_hub(rhdev);
for (port1 = 1; port1 <= rhdev->maxchild; ++port1) {
@@ -1077,6 +1105,90 @@ void usb_root_hub_lost_power(struct usb_

+ * usb_reset_persistent_device - reset device following power-session loss
+ * @udev: device to be reset instead of resumed
+ *
+ * If a host controller doesn't maintain VBUS suspend current during a
+ * system sleep or is reset when the system wakes up, all the USB
+ * power sessions below it will be broken. This is especially troublesome
+ * for mass-storage devices containing mounted filesystems, since the
+ * device will appear to have disconnected and all the memory mappings
+ * to it will be lost.
+ *
+ * As an alternative, this routine attempts to recover power sessions for
+ * devices that are still present by resetting them instead of resuming
+ * them. If all goes well, the devices will appear to persist across the
+ * the interruption of the power sessions.
+ *
+ * This facility is inherently dangerous. Although usb_reset_device()
+ * makes every effort to insure that the same device is present after the
+ * reset as before, it cannot provide a 100% guarantee. Furthermore it's
+ * quite possible for a device to remain unaltered but its media to be
+ * changed. If the user replaces a flash memory card while the system is
+ * asleep, he will have only himself to blame when the filesystem on the
+ * new card is corrupted and the system crashes.
+ */
+int usb_reset_persistent_device(struct usb_device *udev)
+ struct usb_host_config *config;
+ int i;
+ /* Root hubs don't need to be (and can't be) reset */
+ if (!udev->parent)
+ goto mark_children;
+ /* The parent hub's port connect-change status might be set */
+ clear_port_feature(udev->parent, udev->portnum,
+ /* Can't reset it if it's marked as suspended */
+ usb_set_device_state(udev, USB_STATE_ADDRESS);
+ i = usb_reset_device(udev);
+ if (i < 0)
+ return i;
+ /* Let the interface drivers know the device has been reset.
+ * The call to the post_reset() method essentially replaces the
+ * call to resume().
+ */
+ config = udev->actconfig;
+ if (config) {
+ struct usb_interface *cintf;
+ struct usb_driver *drv;
+ for (i = 0; i < config->desc.bNumInterfaces; ++i) {
+ cintf = config->interface[i];
+ /* We can't lock the interface because we already
+ * hold the pm_mutex. Luckily it doesn't matter
+ * since no other tasks will be running during a
+ * system resume.
+ */
+ if (device_is_registered(&cintf->dev) &&
+ cintf->dev.driver &&
+ !is_active(cintf)) {
+ drv = to_usb_driver(cintf->dev.driver);
+ if (drv->pre_reset && drv->post_reset) {
+ (drv->pre_reset)(cintf);
+ (drv->post_reset)(cintf);
+ mark_active(cintf);
+ }
+ }
+ }
+ }
+ /* Mark the child devices for reset */
+ for (i = 0; i < udev->maxchild; ++i) {
+ if (udev->children[i])
+ udev->children[i]->do_persist = 1;
+ }
+ udev->do_persist = 0;
+ return 0;
#endif /* CONFIG_PM */

static void choose_address(struct usb_device *udev)
@@ -2845,6 +2957,8 @@ static int config_descriptors_changed(st
unsigned len = 0;
struct usb_config_descriptor *buf;

+ /* FIXME: Compare the Vendor, Product, and Serial string descriptors */
for (index = 0; index < udev->descriptor.bNumConfigurations; index++) {
if (len < le16_to_cpu(udev->config[index].desc.wTotalLength))
len = le16_to_cpu(udev->config[index].desc.wTotalLength);
Index: mm/drivers/usb/core/usb.h
--- mm.orig/drivers/usb/core/usb.h
+++ mm/drivers/usb/core/usb.h
@@ -36,6 +36,7 @@ extern int usb_suspend_both(struct usb_d
extern int usb_resume_both(struct usb_device *udev);
extern int usb_port_suspend(struct usb_device *dev);
extern int usb_port_resume(struct usb_device *dev);
+extern int usb_reset_persistent_device(struct usb_device *udev);


Index: mm/include/linux/usb.h
--- mm.orig/include/linux/usb.h
+++ mm/include/linux/usb.h
@@ -355,7 +355,8 @@ struct usb_device {
unsigned short bus_mA; /* Current available from the bus */
u8 portnum; /* Parent port number (origin 1) */

- int have_langid; /* whether string_langid is valid */
+ unsigned do_persist:1; /* Needs persistent-device reset */
+ unsigned have_langid:1; /* whether string_langid is valid */
int string_langid; /* language ID for strings */

/* static strings from the device */
Index: mm/Documentation/kernel-parameters.txt
--- mm.orig/Documentation/kernel-parameters.txt
+++ mm/Documentation/kernel-parameters.txt
@@ -1235,6 +1235,16 @@ and is between 256 and 4096 characters.
Format: { 0 | 1 }
See arch/parisc/kernel/pdc_chassis.c

+ persist= [USB] Enable USB device persistence across power loss
+ during system suspend.
+ Format: { 0 | 1 }
+ See also Documentation/usb/persist.txt
+ WARNING!! If a USB mass-storage device or its media
+ are changed while the system is suspended, the kernel
+ may not realize what has happened. If this option is
+ enabled then filesystem corruption and a system crash
+ may result.
pf. [PARIDE]
See Documentation/paride.txt.

Index: mm/Documentation/usb/persist.txt
--- /dev/null
+++ mm/Documentation/usb/persist.txt
@@ -0,0 +1,154 @@
+ USB device persistence during system suspend
+ Alan Stern <>
+ September 2, 2006
+ What is the problem?
+According to the USB specification, when a USB bus is suspended the
+bus must continue to supply suspend current (around 1-5 mA). This
+is so that devices can maintain their internal state and hubs can
+detect connect-change events (devices being plugged in or unplugged).
+The technical term is "power session".
+If a USB device's power session is interrupted then the system is
+required to behave as though the device has been unplugged. It's a
+conservative approach; in the absence of suspend current the computer
+has no way to know what has actually happened. Perhaps the same
+device is still attached or perhaps it was removed and a different
+device plugged into the port. The system must assume the worst.
+By default, Linux behaves according to the spec. If a USB host
+controller loses power during a system suspend, then when the system
+wakes up all the devices attached to that controller are treated as
+though they had disconnected. This is always safe and it is the
+"officially correct" thing to do.
+For many sorts of devices this behavior doesn't matter in the least.
+If the kernel wants to believe that your USB keyboard was unplugged
+while the system was asleep and a new keyboard was plugged in when the
+system woke up, who cares? It'll still work the same when you type on
+Unfortunately problems _can_ arise, particularly with mass-storage
+devices. The effect is exactly the same as if the device really had
+been unplugged while the system was suspended. If you had a mounted
+filesystem on the device, you're out of luck -- everything in that
+filesystem is now inaccessible. This is especially annoying if your
+root filesystem was located on the device, since your system will
+instantly crash. :-(
+Loss of power isn't the only mechanism to worry about. Anything that
+interrupts a power session will have the same effect. For example,
+even though suspend current may have been maintained while the system
+was asleep, on many systems during the initial stages of wakeup the
+firmware (i.e., the BIOS) resets the motherboard's USB host
+controllers. Result: all the power sessions are destroyed and again
+it's as though you had unplugged all the USB devices. Yes, it's
+entirely the BIOS's fault, but that doesn't do _you_ any good unless
+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".
+In practice, people are forced to unmount any filesystems on a USB
+device before suspending. If the root filesystem is on a USB device,
+the system can't be suspended at all. (All right, it _can_ be
+suspended -- but it will crash as soon as it wakes up, which isn't
+much better.)
+ What is the solution?
+Setting the "persist=y" module parameter for usbcore will cause the
+kernel to work around these issues. If usbcore is build into the
+main kernel instead of as a separate module, you can put
+"usbcore.persist=1" on the boot command line. You can also change the
+kernel's behavior on the fly using sysfs: Type
+ echo y >/sys/module/usbcore/parameters/persist
+to turn the option on, and replace the 'y' with an 'n' to turn it off.
+The "persist" option enables a mode in which the core USB device data
+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 followed by a re-enumeration.
+(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
+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 "persist=y" option.
+The end result is that the USB device remains available and usable.
+Mounts and memory mappings are unaffected, and the world is now a good
+and happy place.
+ Is this the best solution?
+Perhaps not. Arguably, keeping track of mounted filesystems and
+memory mappings across device disconnects should be handled by a
+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 usbcore's "persist=y" option.
+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 "persist" option 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 "persist=y" 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
+device is still plugged into the port as before. But the checks
+aren't guaranteed to be 100% accurate.
+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
+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 USB card reader while the system is asleep, the kernel will have
+no way to know you did it. The kernel will assume that nothing has
+happened and will continue to use the partition tables and memory
+mappings for the old card.
+If the kernel gets fooled in this way, it's almost certain to cause
+filesystem corruption and to crash your system. You'll have no one to
+blame but yourself.
+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.
Index: mm/drivers/usb/core/message.c
--- mm.orig/drivers/usb/core/message.c
+++ mm/drivers/usb/core/message.c
@@ -764,7 +764,7 @@ int usb_string(struct usb_device *dev, i
err = -EINVAL;
goto errout;
} else {
- dev->have_langid = -1;
+ dev->have_langid = 1;
dev->string_langid = tbuf[2] | (tbuf[3]<< 8);
/* always use the first langid listed */
dev_dbg (&dev->dev, "default language 0x%04x\n",
Index: mm/Documentation/power/swsusp.txt
--- mm.orig/Documentation/power/swsusp.txt
+++ mm/Documentation/power/swsusp.txt
@@ -405,6 +405,9 @@ safest thing is to unmount all filesyste
Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
before suspending; then remount them after resuming.

+There is a work-around for this problem. For more information, see
Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
compiled with the similar configuration files. Anyway I found that
suspend to disk (and resume) is much slower on 2.6.16 compared to
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