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Documentation for userland software suspend interface

        (C) 2006 Rafael J. Wysocki 

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

.  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 kernel/power/power.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_ATOMIC_SNAPSHOT

        and SNAPSHOT_ATOMIC_RESTORE to succeed

SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE

SNAPSHOT_ATOMIC_SNAPSHOT - 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_ATOMIC_SNAPSHOT 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_SET_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_AVAIL_SWAP - 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_GET_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 with

        SNAPSHOT_GET_SWAP_PAGE

SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument

        should specify the device's major and minor numbers in the old

        two-byte format, as returned by the stat() function in the .st_rdev

        member of the stat structure)

SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in 

        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/power.h, containing the

        resume device specification, as for the SNAPSHOT_SET_SWAP_FILE ioctl(),

        and the offset); for swap partitions the offset is always 0, but it is

        different to zero for swap files (please see

        Documentation/swsusp-and-swap-files.txt for details).

        The SNAPSHOT_SET_SWAP_AREA ioctl() is considered as a replacement for

        SNAPSHOT_SET_SWAP_FILE which is regarded as obsolete.   It is

        recommended to always use this call, because the code to set the resume

        partition may be removed from future kernels

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)

SNAPSHOT_PMOPS - enable the usage of the hibernation_ops->prepare,

        hibernate_ops->enter and hibernation_ops->finish methods (the in-kernel

        swsusp knows these as the "platform method") which are needed on many

        machines to (among others) speed up the resume by letting the BIOS skip

        some steps or to let the system recognise the correct state of the

        hardware after the resume (in particular on many machines this ensures

        that unplugged AC adapters get correctly detected and that kacpid does

        not run wild after the resume).  The last ioctl() argument can take one

        of the three values, defined in kernel/power/power.h:

        PMOPS_PREPARE - make the kernel carry out the

                hibernation_ops->prepare() operation

        PMOPS_ENTER - make the kernel power off the system by calling

                hibernation_ops->enter()

        PMOPS_FINISH - make the kernel carry out the

                hibernation_ops->finish() operation

        Note that the actual constants are misnamed because they surface

        internal kernel implementation details that have changed.

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_GET_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_ATOMIC_SNAPSHOT and

mounted afterwards.

These utilities SHOULD NOT make any assumptions regarding the ordering of

data within the snapshot image, except for the image header that MAY be

assumed to start with an swsusp_info structure, as specified in

kernel/power/power.h.  This structure MAY be used by the userland utilities

to obtain some information about the snapshot image, such as the size

of the snapshot image, including the metadata and the header itself,

contained in the .size member of swsusp_info.

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_ATOMIC_SNAPSHOT

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_ATOMIC_SNAPSHOT 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.