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4 First, the warnings at the beginning of swsusp.txt still apply.
6 Second, you should read the FAQ in swsusp.txt _now_ if you have not
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
15 The interface consists of a character device providing the open(),
18 numbers of the device are, respectively, 10 and 231, and they can
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
27 Even opening the device has side effects. Data structures are
31 The ioctl() commands recognized by the device are:
33 SNAPSHOT_FREEZE - freeze user space processes (the current process is
39 SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the
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
48 SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
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
54 SNAPSHOT_FREE - free memory allocated for the snapshot image
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)
61 SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image
63 SNAPSHOT_AVAIL_SWAP_SIZE - return the amount of available swap in bytes (the
65 contain the result if the call is successful).
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)
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
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
83 SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
84 depending on the argument value (enable, if the argument is nonzero)
86 SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
87 state (eg. ACPI S4) using the platform (eg. ACPI) driver
89 SNAPSHOT_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)
99 The device's read() operation can be used to transfer the snapshot image from
100 the kernel. It has the following limitations:
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)
106 The device's write() operation is used for uploading the system memory snapshot
107 into the kernel. It has the same limitations as the read() operation.
109 The release() operation frees all memory allocated for the snapshot image
112 SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
114 still frozen when the device is being closed).
116 Currently it is assumed that the userland utilities reading/writing the
117 snapshot image from/to the kernel will use a swap partition, called the resume
118 partition, or a swap file as storage space (if a swap file is used, the resume
119 partition is the partition that holds this file). However, this is not really
124 These utilities MUST NOT make any assumptions regarding the ordering of
125 data within the snapshot image. The contents of the image are entirely owned
126 by the kernel and its structure may be changed in future kernel releases.
128 The snapshot image MUST be written to the kernel unaltered (ie. all of the image
129 data, metadata and header MUST be written in _exactly_ the same amount, form
130 and order in which they have been read). Otherwise, the behavior of the
133 While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
134 structure of the snapshot image is consistent with the information stored
135 in the image header. If any inconsistencies are detected,
137 mechanism and the userland utilities using the interface SHOULD use additional
138 means, such as checksums, to ensure the integrity of the snapshot image.
143 The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
144 in the memory location pointed to by the last argument of ioctl() and proceed
146 1. 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
152 its header. If the suspend is not to be cancelled, the
153 system MUST be powered off or rebooted after the snapshot
156 file system operations (including reads) on the file systems
160 use it for saving the image).
161 2. 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
170 For details, please refer to the source code.