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+Documentation for userland software suspend interface
+	(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
+
+First, the warnings at the beginning of swsusp.txt still apply.
+
+Second, you should read the FAQ in swsusp.txt _now_ if you have not
+done it already.
+
+Now, to use the userland interface for software suspend you need special
+utilities that will read/write the system memory snapshot from/to the
+kernel.  Such utilities are available, for example, from
+<http://suspend.sourceforge.net>.  You may want to have a look at them if you
+are going to develop your own suspend/resume utilities.
+
+The interface consists of a character device providing the open(),
+release(), read(), and write() operations as well as several ioctl()
+commands defined in include/linux/suspend_ioctls.h .  The major and minor
+numbers of the device are, respectively, 10 and 231, and they can
+be read from /sys/class/misc/snapshot/dev.
+
+The device can be open either for reading or for writing.  If open for
+reading, it is considered to be in the suspend mode.  Otherwise it is
+assumed to be in the resume mode.  The device cannot be open for simultaneous
+reading and writing.  It is also impossible to have the device open more than
+once at a time.
+
+Even opening the device has side effects. Data structures are
+allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
+called.
+
+The ioctl() commands recognized by the device are:
+
+SNAPSHOT_FREEZE - freeze user space processes (the current process is
+	not frozen); this is required for SNAPSHOT_CREATE_IMAGE
+	and SNAPSHOT_ATOMIC_RESTORE to succeed
+
+SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
+
+SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the
+	last argument of ioctl() should be a pointer to an int variable,
+	the value of which will indicate whether the call returned after
+	creating the snapshot (1) or after restoring the system memory state
+	from it (0) (after resume the system finds itself finishing the
+	SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot
+	has been created the read() operation can be used to transfer
+	it out of the kernel
+
+SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
+	uploaded snapshot image; before calling it you should transfer
+	the system memory snapshot back to the kernel using the write()
+	operation; this call will not succeed if the snapshot
+	image is not available to the kernel
+
+SNAPSHOT_FREE - free memory allocated for the snapshot image
+
+SNAPSHOT_PREF_IMAGE_SIZE - set the preferred maximum size of the image
+	(the kernel will do its best to ensure the image size will not exceed
+	this number, but if it turns out to be impossible, the kernel will
+	create the smallest image possible)
+
+SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image
+
+SNAPSHOT_AVAIL_SWAP_SIZE - return the amount of available swap in bytes (the
+	last argument should be a pointer to an unsigned int variable that will
+	contain the result if the call is successful).
+
+SNAPSHOT_ALLOC_SWAP_PAGE - allocate a swap page from the resume partition
+	(the last argument should be a pointer to a loff_t variable that
+	will contain the swap page offset if the call is successful)
+
+SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by
+	SNAPSHOT_ALLOC_SWAP_PAGE
+
+SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE>
+	units) from the beginning of the partition at which the swap header is
+	located (the last ioctl() argument should point to a struct
+	resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
+	containing the resume device specification and the offset); for swap
+	partitions the offset is always 0, but it is different from zero for
+	swap files (see Documentation/power/swsusp-and-swap-files.txt for
+	details).
+
+SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
+	depending on the argument value (enable, if the argument is nonzero)
+
+SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
+	state (eg. ACPI S4) using the platform (eg. ACPI) driver
+
+SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
+	immediately enter the suspend-to-RAM state, so this call must always
+	be preceded by the SNAPSHOT_FREEZE call and it is also necessary
+	to use the SNAPSHOT_UNFREEZE call after the system wakes up.  This call
+	is needed to implement the suspend-to-both mechanism in which the
+	suspend image is first created, as though the system had been suspended
+	to disk, and then the system is suspended to RAM (this makes it possible
+	to resume the system from RAM if there's enough battery power or restore
+	its state on the basis of the saved suspend image otherwise)
+
+The device's read() operation can be used to transfer the snapshot image from
+the kernel.  It has the following limitations:
+- you cannot read() more than one virtual memory page at a time
+- read()s across page boundaries are impossible (ie. if you read() 1/2 of
+	a page in the previous call, you will only be able to read()
+	_at_ _most_ 1/2 of the page in the next call)
+
+The device's write() operation is used for uploading the system memory snapshot
+into the kernel.  It has the same limitations as the read() operation.
+
+The release() operation frees all memory allocated for the snapshot image
+and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
+Thus it is not necessary to use either SNAPSHOT_FREE or
+SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
+unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
+still frozen when the device is being closed).
+
+Currently it is assumed that the userland utilities reading/writing the
+snapshot image from/to the kernel will use a swap partition, called the resume
+partition, or a swap file as storage space (if a swap file is used, the resume
+partition is the partition that holds this file).  However, this is not really
+required, as they can use, for example, a special (blank) suspend partition or
+a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
+mounted afterwards.
+
+These utilities MUST NOT make any assumptions regarding the ordering of
+data within the snapshot image.  The contents of the image are entirely owned
+by the kernel and its structure may be changed in future kernel releases.
+
+The snapshot image MUST be written to the kernel unaltered (ie. all of the image
+data, metadata and header MUST be written in _exactly_ the same amount, form
+and order in which they have been read).  Otherwise, the behavior of the
+resumed system may be totally unpredictable.
+
+While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
+structure of the snapshot image is consistent with the information stored
+in the image header.  If any inconsistencies are detected,
+SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
+mechanism and the userland utilities using the interface SHOULD use additional
+means, such as checksums, to ensure the integrity of the snapshot image.
+
+The suspending and resuming utilities MUST lock themselves in memory,
+preferably using mlockall(), before calling SNAPSHOT_FREEZE.
+
+The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
+in the memory location pointed to by the last argument of ioctl() and proceed
+in accordance with it:
+1. 	If the value is 1 (ie. the system memory snapshot has just been
+	created and the system is ready for saving it):
+	(a)	The suspending utility MUST NOT close the snapshot device
+		_unless_ the whole suspend procedure is to be cancelled, in
+		which case, if the snapshot image has already been saved, the
+		suspending utility SHOULD destroy it, preferably by zapping
+		its header.  If the suspend is not to be cancelled, the
+		system MUST be powered off or rebooted after the snapshot
+		image has been saved.
+	(b)	The suspending utility SHOULD NOT attempt to perform any
+		file system operations (including reads) on the file systems
+		that were mounted before SNAPSHOT_CREATE_IMAGE has been
+		called.  However, it MAY mount a file system that was not
+		mounted at that time and perform some operations on it (eg.
+		use it for saving the image).
+2.	If the value is 0 (ie. the system state has just been restored from
+	the snapshot image), the suspending utility MUST close the snapshot
+	device.  Afterwards it will be treated as a regular userland process,
+	so it need not exit.
+
+The resuming utility SHOULD NOT attempt to mount any file systems that could
+be mounted before suspend and SHOULD NOT attempt to perform any operations
+involving such file systems.
+
+For details, please refer to the source code.