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.

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/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 accross page boundaries are impossible (ie. if ypu 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 parition, 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,
preferrably 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, preferrably 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.