1Documentation for userland software suspend interface
   2        (C) 2006 Rafael J. Wysocki <>
   4First, the warnings at the beginning of swsusp.txt still apply.
   6Second, you should read the FAQ in swsusp.txt _now_ if you have not
   7done it already.
   9Now, to use the userland interface for software suspend you need special
  10utilities that will read/write the system memory snapshot from/to the
  11kernel.  Such utilities are available, for example, from
  12<>.  You may want to have a look at them if you
  13are going to develop your own suspend/resume utilities.
  15The interface consists of a character device providing the open(),
  16release(), read(), and write() operations as well as several ioctl()
  17commands defined in include/linux/suspend_ioctls.h .  The major and minor
  18numbers of the device are, respectively, 10 and 231, and they can
  19be read from /sys/class/misc/snapshot/dev.
  21The device can be open either for reading or for writing.  If open for
  22reading, it is considered to be in the suspend mode.  Otherwise it is
  23assumed to be in the resume mode.  The device cannot be open for simultaneous
  24reading and writing.  It is also impossible to have the device open more than
  25once at a time.
  27Even opening the device has side effects. Data structures are
  28allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
  31The ioctl() commands recognized by the device are:
  33SNAPSHOT_FREEZE - freeze user space processes (the current process is
  34        not frozen); this is required for SNAPSHOT_CREATE_IMAGE
  35        and SNAPSHOT_ATOMIC_RESTORE to succeed
  37SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
  39SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the
  40        last argument of ioctl() should be a pointer to an int variable,
  41        the value of which will indicate whether the call returned after
  42        creating the snapshot (1) or after restoring the system memory state
  43        from it (0) (after resume the system finds itself finishing the
  44        SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot
  45        has been created the read() operation can be used to transfer
  46        it out of the kernel
  48SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
  49        uploaded snapshot image; before calling it you should transfer
  50        the system memory snapshot back to the kernel using the write()
  51        operation; this call will not succeed if the snapshot
  52        image is not available to the kernel
  54SNAPSHOT_FREE - free memory allocated for the snapshot image
  56SNAPSHOT_PREF_IMAGE_SIZE - set the preferred maximum size of the image
  57        (the kernel will do its best to ensure the image size will not exceed
  58        this number, but if it turns out to be impossible, the kernel will
  59        create the smallest image possible)
  61SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image
  63SNAPSHOT_AVAIL_SWAP_SIZE - return the amount of available swap in bytes (the
  64        last argument should be a pointer to an unsigned int variable that will
  65        contain the result if the call is successful).
  67SNAPSHOT_ALLOC_SWAP_PAGE - allocate a swap page from the resume partition
  68        (the last argument should be a pointer to a loff_t variable that
  69        will contain the swap page offset if the call is successful)
  71SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by
  74SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE>
  75        units) from the beginning of the partition at which the swap header is
  76        located (the last ioctl() argument should point to a struct
  77        resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
  78        containing the resume device specification and the offset); for swap
  79        partitions the offset is always 0, but it is different from zero for
  80        swap files (see Documentation/power/swsusp-and-swap-files.txt for
  81        details).
  83SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
  84        depending on the argument value (enable, if the argument is nonzero)
  86SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
  87        state (eg. ACPI S4) using the platform (eg. ACPI) driver
  89SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
  90        immediately enter the suspend-to-RAM state, so this call must always
  91        be preceded by the SNAPSHOT_FREEZE call and it is also necessary
  92        to use the SNAPSHOT_UNFREEZE call after the system wakes up.  This call
  93        is needed to implement the suspend-to-both mechanism in which the
  94        suspend image is first created, as though the system had been suspended
  95        to disk, and then the system is suspended to RAM (this makes it possible
  96        to resume the system from RAM if there's enough battery power or restore
  97        its state on the basis of the saved suspend image otherwise)
  99The device's read() operation can be used to transfer the snapshot image from
 100the kernel.  It has the following limitations:
 101- you cannot read() more than one virtual memory page at a time
 102- read()s across page boundaries are impossible (ie. if ypu read() 1/2 of
 103        a page in the previous call, you will only be able to read()
 104        _at_ _most_ 1/2 of the page in the next call)
 106The device's write() operation is used for uploading the system memory snapshot
 107into the kernel.  It has the same limitations as the read() operation.
 109The release() operation frees all memory allocated for the snapshot image
 110and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
 111Thus it is not necessary to use either SNAPSHOT_FREE or
 112SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
 113unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
 114still frozen when the device is being closed).
 116Currently it is assumed that the userland utilities reading/writing the
 117snapshot image from/to the kernel will use a swap partition, called the resume
 118partition, or a swap file as storage space (if a swap file is used, the resume
 119partition is the partition that holds this file).  However, this is not really
 120required, as they can use, for example, a special (blank) suspend partition or
 121a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
 122mounted afterwards.
 124These utilities MUST NOT make any assumptions regarding the ordering of
 125data within the snapshot image.  The contents of the image are entirely owned
 126by the kernel and its structure may be changed in future kernel releases.
 128The snapshot image MUST be written to the kernel unaltered (ie. all of the image
 129data, metadata and header MUST be written in _exactly_ the same amount, form
 130and order in which they have been read).  Otherwise, the behavior of the
 131resumed system may be totally unpredictable.
 133While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
 134structure of the snapshot image is consistent with the information stored
 135in the image header.  If any inconsistencies are detected,
 136SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
 137mechanism and the userland utilities using the interface SHOULD use additional
 138means, such as checksums, to ensure the integrity of the snapshot image.
 140The suspending and resuming utilities MUST lock themselves in memory,
 141preferably using mlockall(), before calling SNAPSHOT_FREEZE.
 143The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
 144in the memory location pointed to by the last argument of ioctl() and proceed
 145in accordance with it:
 1461.      If the value is 1 (ie. the system memory snapshot has just been
 147        created and the system is ready for saving it):
 148        (a)     The suspending utility MUST NOT close the snapshot device
 149                _unless_ the whole suspend procedure is to be cancelled, in
 150                which case, if the snapshot image has already been saved, the
 151                suspending utility SHOULD destroy it, preferably by zapping
 152                its header.  If the suspend is not to be cancelled, the
 153                system MUST be powered off or rebooted after the snapshot
 154                image has been saved.
 155        (b)     The suspending utility SHOULD NOT attempt to perform any
 156                file system operations (including reads) on the file systems
 157                that were mounted before SNAPSHOT_CREATE_IMAGE has been
 158                called.  However, it MAY mount a file system that was not
 159                mounted at that time and perform some operations on it (eg.
 160                use it for saving the image).
 1612.      If the value is 0 (ie. the system state has just been restored from
 162        the snapshot image), the suspending utility MUST close the snapshot
 163        device.  Afterwards it will be treated as a regular userland process,
 164        so it need not exit.
 166The resuming utility SHOULD NOT attempt to mount any file systems that could
 167be mounted before suspend and SHOULD NOT attempt to perform any operations
 168involving such file systems.
 170For details, please refer to the source code.