Documentation/power/userland-swsusp.txt

   1 Documentation for userland software suspend interface
   2         (C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
   3
   4 First, the warnings at the beginning of swsusp.txt still apply.
   5
   6 Second, you should read the FAQ in swsusp.txt _now_ if you have not
   7 done it already.
   8
   9 Now, to use the userland interface for software suspend you need special
  10 utilities that will read/write the system memory snapshot from/to the
  11 kernel.  Such utilities are available, for example, from
  12 <http://suspend.sourceforge.net>.  You may want to have a look at them if you
  13 are going to develop your own suspend/resume utilities.
  14
  15 The interface consists of a character device providing the open(),
  16 release(), read(), and write() operations as well as several ioctl()
  17 commands defined in include/linux/suspend_ioctls.h .  The major and minor
  18 numbers of the device are, respectively, 10 and 231, and they can
  19 be read from /sys/class/misc/snapshot/dev.
  20
  21 The device can be open either for reading or for writing.  If open for
  22 reading, it is considered to be in the suspend mode.  Otherwise it is
  23 assumed to be in the resume mode.  The device cannot be open for simultaneous
  24 reading and writing.  It is also impossible to have the device open more than
  25 once at a time.
  26
  27 Even opening the device has side effects. Data structures are
  28 allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
  29 called.
  30
  31 The ioctl() commands recognized by the device are:
  32
  33 SNAPSHOT_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
  36
  37 SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
  38
  39 SNAPSHOT_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
  47
  48 SNAPSHOT_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
  53
  54 SNAPSHOT_FREE - free memory allocated for the snapshot image
  55
  56 SNAPSHOT_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)
  60
  61 SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image
  62
  63 SNAPSHOT_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).
  66
  67 SNAPSHOT_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)
  70
  71 SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by
  72         SNAPSHOT_ALLOC_SWAP_PAGE
  73
  74 SNAPSHOT_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/swsusp-and-swap-files.txt for details).
  81
  82 SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
  83         depending on the argument value (enable, if the argument is nonzero)
  84
  85 SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
  86         state (eg. ACPI S4) using the platform (eg. ACPI) driver
  87
  88 SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
  89         immediately enter the suspend-to-RAM state, so this call must always
  90         be preceded by the SNAPSHOT_FREEZE call and it is also necessary
  91         to use the SNAPSHOT_UNFREEZE call after the system wakes up.  This call
  92         is needed to implement the suspend-to-both mechanism in which the
  93         suspend image is first created, as though the system had been suspended
  94         to disk, and then the system is suspended to RAM (this makes it possible
  95         to resume the system from RAM if there's enough battery power or restore
  96         its state on the basis of the saved suspend image otherwise)
  97
  98 The device's read() operation can be used to transfer the snapshot image from
  99 the kernel.  It has the following limitations:
 100 - you cannot read() more than one virtual memory page at a time
 101 - read()s across page boundaries are impossible (ie. if ypu read() 1/2 of
 102         a page in the previous call, you will only be able to read()
 103         _at_ _most_ 1/2 of the page in the next call)
 104
 105 The device's write() operation is used for uploading the system memory snapshot
 106 into the kernel.  It has the same limitations as the read() operation.
 107
 108 The release() operation frees all memory allocated for the snapshot image
 109 and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
 110 Thus it is not necessary to use either SNAPSHOT_FREE or
 111 SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
 112 unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
 113 still frozen when the device is being closed).
 114
 115 Currently it is assumed that the userland utilities reading/writing the
 116 snapshot image from/to the kernel will use a swap partition, called the resume
 117 partition, or a swap file as storage space (if a swap file is used, the resume
 118 partition is the partition that holds this file).  However, this is not really
 119 required, as they can use, for example, a special (blank) suspend partition or
 120 a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
 121 mounted afterwards.
 122
 123 These utilities MUST NOT make any assumptions regarding the ordering of
 124 data within the snapshot image.  The contents of the image are entirely owned
 125 by the kernel and its structure may be changed in future kernel releases.
 126
 127 The snapshot image MUST be written to the kernel unaltered (ie. all of the image
 128 data, metadata and header MUST be written in _exactly_ the same amount, form
 129 and order in which they have been read).  Otherwise, the behavior of the
 130 resumed system may be totally unpredictable.
 131
 132 While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
 133 structure of the snapshot image is consistent with the information stored
 134 in the image header.  If any inconsistencies are detected,
 135 SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
 136 mechanism and the userland utilities using the interface SHOULD use additional
 137 means, such as checksums, to ensure the integrity of the snapshot image.
 138
 139 The suspending and resuming utilities MUST lock themselves in memory,
 140 preferably using mlockall(), before calling SNAPSHOT_FREEZE.
 141
 142 The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
 143 in the memory location pointed to by the last argument of ioctl() and proceed
 144 in accordance with it:
 145 1.      If the value is 1 (ie. the system memory snapshot has just been
 146         created and the system is ready for saving it):
 147         (a)     The suspending utility MUST NOT close the snapshot device
 148                 _unless_ the whole suspend procedure is to be cancelled, in
 149                 which case, if the snapshot image has already been saved, the
 150                 suspending utility SHOULD destroy it, preferably by zapping
 151                 its header.  If the suspend is not to be cancelled, the
 152                 system MUST be powered off or rebooted after the snapshot
 153                 image has been saved.
 154         (b)     The suspending utility SHOULD NOT attempt to perform any
 155                 file system operations (including reads) on the file systems
 156                 that were mounted before SNAPSHOT_CREATE_IMAGE has been
 157                 called.  However, it MAY mount a file system that was not
 158                 mounted at that time and perform some operations on it (eg.
 159                 use it for saving the image).
 160 2.      If the value is 0 (ie. the system state has just been restored from
 161         the snapshot image), the suspending utility MUST close the snapshot
 162         device.  Afterwards it will be treated as a regular userland process,
 163         so it need not exit.
 164
 165 The resuming utility SHOULD NOT attempt to mount any file systems that could
 166 be mounted before suspend and SHOULD NOT attempt to perform any operations
 167 involving such file systems.
 168
 169 For details, please refer to the source code.