linux/Documentation/power/userland-swsusp.rst

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Documentation for userland software suspend interface
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	(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.rst 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.