USB: add USB-Persist facility

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

Documentation/usb/persist.txt.

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 <stern@rowland.harvard.edu>

		 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

it.

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

devices.

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.

YOU HAVE BEEN WARNED!  USE AT YOUR OWN RISK!

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.

	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);

	  If you are unsure about this, say N here.

config USB_PERSIST

	bool "USB device persistence during system suspend (DANGEROUS)"

	depends on USB && PM && EXPERIMENTAL

	default n

	help

	  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

	bool

	depends on USB && EXPERIMENTAL

	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. */

}

/* 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;

	}

	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

			mark_active(intf);

	} else {

	} else

		dev_warn(&intf->dev, "no resume for driver %s?\n",

				driver->name);

		mark_active(intf);

	}

done:

	// dev_dbg(&intf->dev, "%s: status %d\n", __FUNCTION__, status);

	if (status == 0)

		mark_active(intf);

	return status;

}

	if (status != 0) {

		while (--i >= 0) {

			intf = udev->actconfig->interface[i];

			usb_resume_interface(intf);

			usb_resume_interface(intf, 0);

		}

		/* Try another autosuspend when the interfaces aren't busy */

 		}

	} 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);

			usb_resume_interface(intf, udev->reset_resume);

		}

	}

 done:

	// dev_dbg(&udev->dev, "%s: status %d\n", __FUNCTION__, status);

	udev->reset_resume = 0;

	return status;

}

	if (!is_usb_device(dev))	/* Ignore PM for interfaces */

		return 0;

	udev = to_usb_device(dev);

	if (udev->autoresume_disabled)

	/* 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);

}