Commit 0458d5b4 authored by Alan Stern's avatar Alan Stern Committed by Greg Kroah-Hartman
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USB: add USB-Persist facility

This patch (as886) adds the controversial USB-persist facility,
allowing USB devices to persist across a power loss during system

The facility is controlled by a new Kconfig option (with appropriate
warnings about the potential dangers); when the option is off the
behavior will remain the same as it is now.  But when the option is
on, people will be able to use suspend-to-disk and keep their USB
filesystems intact -- something particularly valuable for small
machines where the root filesystem is on a USB device!
Signed-off-by: default avatarAlan Stern <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent ce7cd137
......@@ -393,6 +393,9 @@ safest thing is to unmount all filesystems on removable media (such USB,
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
USB device persistence during system suspend
Alan Stern <>
September 2, 2006 (Updated March 27, 2007)
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 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.
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.
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.
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.
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 CONFIG_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
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!!
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, inodes, and
memory mappings for the old card.
If the kernel gets fooled in this way, it's almost certain to cause
data 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.
......@@ -1015,7 +1015,7 @@ static void hid_pre_reset(struct usb_interface *intf)
hid_suspend(intf, PMSG_ON);
static void hid_post_reset(struct usb_interface *intf)
static void hid_post_reset(struct usb_interface *intf, int reset_resume)
struct usb_device *dev = interface_to_usbdev (intf);
......@@ -86,6 +86,28 @@ config USB_SUSPEND
If you are unsure about this, say N here.
bool "USB device persistence during system suspend (DANGEROUS)"
depends on USB && PM && EXPERIMENTAL
default n
If you say Y here, USB device data structures will remain
persistent across system suspend, even if the USB bus loses
power. (This includes software-suspend, also known as swsusp,
or suspend-to-disk.) The devices will reappear as if by magic
when the system wakes up, with no need to unmount USB filesystems,
rmmod host-controller drivers, or do anything else.
WARNING: This option can be dangerous!
If a USB device is replaced by another of the same type while
the system is asleep, there's a good chance the kernel won't
detect the change. Likewise if the media in a USB storage
device is replaced. When this happens it's almost certain to
cause data corruption and maybe even crash your system.
If you are unsure, say N here.
config USB_OTG
depends on USB && EXPERIMENTAL
......@@ -824,8 +824,9 @@ static int usb_resume_device(struct usb_device *udev)
struct usb_device_driver *udriver;
int status = 0;
if (udev->state == USB_STATE_NOTATTACHED ||
udev->state != USB_STATE_SUSPENDED)
if (udev->state == USB_STATE_NOTATTACHED)
goto done;
if (udev->state != USB_STATE_SUSPENDED && !udev->reset_resume)
goto done;
/* Can't resume it if it doesn't have a driver. */
......@@ -882,7 +883,7 @@ done:
/* Caller has locked intf's usb_device's pm_mutex */
static int usb_resume_interface(struct usb_interface *intf)
static int usb_resume_interface(struct usb_interface *intf, int reset_resume)
struct usb_driver *driver;
int status = 0;
......@@ -902,21 +903,21 @@ static int usb_resume_interface(struct usb_interface *intf)
driver = to_usb_driver(intf->dev.driver);
if (driver->resume) {
if (reset_resume && driver->post_reset)
driver->post_reset(intf, reset_resume);
else if (driver->resume) {
status = driver->resume(intf);
if (status)
dev_err(&intf->dev, "%s error %d\n",
"resume", status);
} else {
} else
dev_warn(&intf->dev, "no resume for driver %s?\n",
// dev_dbg(&intf->dev, "%s: status %d\n", __FUNCTION__, status);
if (status == 0)
return status;
......@@ -1063,7 +1064,7 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
if (status != 0) {
while (--i >= 0) {
intf = udev->actconfig->interface[i];
usb_resume_interface(intf, 0);
/* Try another autosuspend when the interfaces aren't busy */
......@@ -1162,20 +1163,21 @@ static int usb_resume_both(struct usb_device *udev)
} else {
/* Needed only for setting udev->dev.power.power_state.event
* and for possible debugging message. */
/* Needed for setting udev->dev.power.power_state.event,
* for possible debugging message, and for reset_resume. */
status = usb_resume_device(udev);
if (status == 0 && udev->actconfig) {
for (i = 0; i < udev->actconfig->desc.bNumInterfaces; i++) {
intf = udev->actconfig->interface[i];
usb_resume_interface(intf, udev->reset_resume);
// dev_dbg(&udev->dev, "%s: status %d\n", __FUNCTION__, status);
udev->reset_resume = 0;
return status;
......@@ -1510,8 +1512,15 @@ static int usb_resume(struct device *dev)
if (!is_usb_device(dev)) /* Ignore PM for interfaces */
return 0;
udev = to_usb_device(dev);
if (udev->autoresume_disabled)
return -EPERM;
/* If autoresume is disabled then we also want to prevent resume
* during system wakeup. However, a "persistent-device" reset-resume
* after power loss counts as a wakeup event. So allow a
* reset-resume to occur if remote wakeup is enabled. */
if (udev->autoresume_disabled) {
if (!(udev->reset_resume && udev->do_remote_wakeup))
return -EPERM;
return usb_external_resume_device(udev);
......@@ -217,7 +217,10 @@ static int generic_resume(struct usb_device *udev)
int rc;
rc = usb_port_resume(udev);
if (udev->reset_resume)
rc = usb_reset_suspended_device(udev);
rc = usb_port_resume(udev);
/* Root hubs don't have upstream ports to resume or reset,
* so the line above won't do much for them. We have to
......@@ -553,45 +553,121 @@ static int hub_hub_status(struct usb_hub *hub,
static int hub_port_disable(struct usb_hub *hub, int port1, int set_state)
struct usb_device *hdev = hub->hdev;
int ret;
int ret = 0;
if (hdev->children[port1-1] && set_state) {
if (hdev->children[port1-1] && set_state)
ret = clear_port_feature(hdev, port1, USB_PORT_FEAT_ENABLE);
if (!hub->error)
ret = clear_port_feature(hdev, port1, USB_PORT_FEAT_ENABLE);
if (ret)
dev_err(hub->intfdev, "cannot disable port %d (err = %d)\n",
port1, ret);
port1, ret);
return ret;
* Disable a port and mark a logical connnect-change event, so that some
* time later khubd will disconnect() any existing usb_device on the port
* and will re-enumerate if there actually is a device attached.
static void hub_port_logical_disconnect(struct usb_hub *hub, int port1)
dev_dbg(hub->intfdev, "logical disconnect on port %d\n", port1);
hub_port_disable(hub, port1, 1);
/* caller has locked the hub device */
static void hub_pre_reset(struct usb_interface *intf)
/* FIXME let caller ask to power down the port:
* - some devices won't enumerate without a VBUS power cycle
* - SRP saves power that way
* - ... new call, TBD ...
* That's easy if this hub can switch power per-port, and
* khubd reactivates the port later (timer, SRP, etc).
* Powerdown must be optional, because of reset/DFU.
set_bit(port1, hub->change_bits);
static void disconnect_all_children(struct usb_hub *hub, int logical)
struct usb_hub *hub = usb_get_intfdata(intf);
struct usb_device *hdev = hub->hdev;
int port1;
for (port1 = 1; port1 <= hdev->maxchild; ++port1) {
if (hdev->children[port1 - 1]) {
usb_disconnect(&hdev->children[port1 - 1]);
if (hub->error == 0)
hub_port_disable(hub, port1, 0);
if (hdev->children[port1-1]) {
if (logical)
hub_port_logical_disconnect(hub, port1);
#define USB_PERSIST 1
/* For "persistent-device" resets we must mark the child devices for reset
* and turn off a possible connect-change status (so khubd won't disconnect
* them later).
static void mark_children_for_reset_resume(struct usb_hub *hub)
struct usb_device *hdev = hub->hdev;
int port1;
for (port1 = 1; port1 <= hdev->maxchild; ++port1) {
struct usb_device *child = hdev->children[port1-1];
if (child) {
child->reset_resume = 1;
clear_port_feature(hdev, port1,
#define USB_PERSIST 0
static inline void mark_children_for_reset_resume(struct usb_hub *hub)
{ }
/* caller has locked the hub device */
static void hub_pre_reset(struct usb_interface *intf)
struct usb_hub *hub = usb_get_intfdata(intf);
/* This routine doesn't run as part of a reset-resume, so it's safe
* to disconnect all the drivers below the hub.
disconnect_all_children(hub, 0);
/* caller has locked the hub device */
static void hub_post_reset(struct usb_interface *intf)
static void hub_post_reset(struct usb_interface *intf, int reset_resume)
struct usb_hub *hub = usb_get_intfdata(intf);
if (reset_resume) {
else {
/* Reset-resume doesn't call pre_reset, so we have to
* disconnect the children here. But we may not lock
* the child devices, so we have to do a "logical"
* disconnect.
disconnect_all_children(hub, 1);
......@@ -1053,33 +1129,64 @@ void usb_set_device_state(struct usb_device *udev,
#ifdef CONFIG_PM
* usb_reset_suspended_device - reset a suspended device instead of resuming it
* @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_suspended_device(struct usb_device *udev)
int rc = 0;
dev_dbg(&udev->dev, "usb %sresume\n", "reset-");
/* After we're done the device won't be suspended any more.
* In addition, the reset won't work if udev->state is SUSPENDED.
usb_set_device_state(udev, udev->actconfig
/* Root hubs don't need to be (and can't be) reset */
if (udev->parent)
rc = usb_reset_device(udev);
return rc;
* usb_root_hub_lost_power - called by HCD if the root hub lost Vbus power
* @rhdev: struct usb_device for the root hub
* The USB host controller driver calls this function when its root hub
* 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.
* has been reset. The routine marks @rhdev as having lost power. When
* the hub driver is resumed it will take notice; if CONFIG_USB_PERSIST
* is enabled then it will carry out power-session recovery, otherwise
* it will disconnect all the child devices.
void usb_root_hub_lost_power(struct usb_device *rhdev)
struct usb_hub *hub;
int port1;
unsigned long flags;
dev_warn(&rhdev->dev, "root hub lost power or was reset\n");
spin_lock_irqsave(&device_state_lock, flags);
hub = hdev_to_hub(rhdev);
for (port1 = 1; port1 <= rhdev->maxchild; ++port1) {
if (rhdev->children[port1 - 1]) {
rhdev->children[port1 - 1]);
set_bit(port1, hub->change_bits);
spin_unlock_irqrestore(&device_state_lock, flags);
rhdev->reset_resume = 1;
......@@ -1513,29 +1620,6 @@ static int hub_port_reset(struct usb_hub *hub, int port1,
return status;
* Disable a port and mark a logical connnect-change event, so that some
* time later khubd will disconnect() any existing usb_device on the port
* and will re-enumerate if there actually is a device attached.
static void hub_port_logical_disconnect(struct usb_hub *hub, int port1)
dev_dbg(hub->intfdev, "logical disconnect on port %d\n", port1);
hub_port_disable(hub, port1, 1);
/* FIXME let caller ask to power down the port:
* - some devices won't enumerate without a VBUS power cycle
* - SRP saves power that way
* - ... new call, TBD ...
* That's easy if this hub can switch power per-port, and
* khubd reactivates the port later (timer, SRP, etc).
* Powerdown must be optional, because of reset/DFU.
set_bit(port1, hub->change_bits);
#ifdef CONFIG_PM
......@@ -3018,7 +3102,7 @@ int usb_reset_composite_device(struct usb_device *udev,
cintf->dev.driver) {
drv = to_usb_driver(cintf->dev.driver);
if (drv->post_reset)
(drv->post_reset)(cintf, 0);
if (cintf != iface)
......@@ -36,6 +36,7 @@ extern void usb_host_cleanup(void);
extern void usb_autosuspend_work(struct work_struct *work);
extern int usb_port_suspend(struct usb_device *dev);
extern int usb_port_resume(struct usb_device *dev);
extern int usb_reset_suspended_device(struct usb_device *udev);
extern int usb_external_suspend_device(struct usb_device *udev,
pm_message_t msg);
extern int usb_external_resume_device(struct usb_device *udev);
......@@ -236,7 +236,7 @@ static void storage_pre_reset(struct usb_interface *iface)
static void storage_post_reset(struct usb_interface *iface)
static void storage_post_reset(struct usb_interface *iface, int reset_resume)
struct us_data *us = usb_get_intfdata(iface);
......@@ -249,7 +249,11 @@ static void storage_post_reset(struct usb_interface *iface)
/* FIXME: Notify the subdrivers that they need to reinitialize
* the device */
/* If this is a reset-resume then the pre_reset routine wasn't
* called, so we don't need to unlock the mutex. */
if (!reset_resume)
......@@ -403,6 +403,7 @@ struct usb_device {
unsigned auto_pm:1; /* autosuspend/resume in progress */
unsigned do_remote_wakeup:1; /* remote wakeup should be enabled */
unsigned reset_resume:1; /* needs reset instead of resume */
unsigned autosuspend_disabled:1; /* autosuspend and autoresume */
unsigned autoresume_disabled:1; /* disabled by the user */
......@@ -819,7 +820,10 @@ struct usbdrv_wrap {
* @pre_reset: Called by usb_reset_composite_device() when the device
* is about to be reset.
* @post_reset: Called by usb_reset_composite_device() after the device
* has been reset.
* has been reset, or in lieu of @resume following a reset-resume
* (i.e., the device is reset instead of being resumed, as might
* happen if power was lost). The second argument tells which is
* the reason.
* @id_table: USB drivers use ID table to support hotplugging.
* Export this with MODULE_DEVICE_TABLE(usb,...). This must be set
* or your driver's probe function will never get called.
......@@ -861,7 +865,7 @@ struct usb_driver {
int (*resume) (struct usb_interface *intf);
void (*pre_reset) (struct usb_interface *intf);
void (*post_reset) (struct usb_interface *intf);
void (*post_reset) (struct usb_interface *intf, int reset_resume);
const struct usb_device_id *id_table;