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://www.sisk.pl/kernel/utilities/suspend>. You may want to have
13 a look at them if you are going to develop your own suspend/resume
14 utilities.
15
16 The interface consists of a character device providing the open(),
17 release(), read(), and write() operations as well as several ioctl()
18 commands defined in kernel/power/power.h. The major and minor
19 numbers of the device are, respectively, 10 and 231, and they can
20 be read from /sys/class/misc/snapshot/dev.
21
22 The device can be open either for reading or for writing. If open for
23 reading, it is considered to be in the suspend mode. Otherwise it is
24 assumed to be in the resume mode. The device cannot be open for reading
25 and writing. It is also impossible to have the device open more than once
26 at a time.
27
28 The ioctl() commands recognized by the device are:
29
30 SNAPSHOT_FREEZE - freeze user space processes (the current process is
31 not frozen); this is required for SNAPSHOT_ATOMIC_SNAPSHOT
32 and SNAPSHOT_ATOMIC_RESTORE to succeed
33
34 SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
35
36 SNAPSHOT_ATOMIC_SNAPSHOT - create a snapshot of the system memory; the
37 last argument of ioctl() should be a pointer to an int variable,
38 the value of which will indicate whether the call returned after
39 creating the snapshot (1) or after restoring the system memory state
40 from it (0) (after resume the system finds itself finishing the
41 SNAPSHOT_ATOMIC_SNAPSHOT ioctl() again); after the snapshot
42 has been created the read() operation can be used to transfer
43 it out of the kernel
44
45 SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
46 uploaded snapshot image; before calling it you should transfer
47 the system memory snapshot back to the kernel using the write()
48 operation; this call will not succeed if the snapshot
49 image is not available to the kernel
50
51 SNAPSHOT_FREE - free memory allocated for the snapshot image
52
53 SNAPSHOT_SET_IMAGE_SIZE - set the preferred maximum size of the image
54 (the kernel will do its best to ensure the image size will not exceed
55 this number, but if it turns out to be impossible, the kernel will
56 create the smallest image possible)
57
58 SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last
59 argument should be a pointer to an unsigned int variable that will
60 contain the result if the call is successful).
61
62 SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition
63 (the last argument should be a pointer to a loff_t variable that
64 will contain the swap page offset if the call is successful)
65
66 SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with
67 SNAPSHOT_GET_SWAP_PAGE
68
69 SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument
70 should specify the device's major and minor numbers in the old
71 two-byte format, as returned by the stat() function in the .st_rdev
72 member of the stat structure); it is recommended to always use this
73 call, because the code to set the resume partition could be removed from
74 future kernels
75
76 The device's read() operation can be used to transfer the snapshot image from
77 the kernel. It has the following limitations:
78 - you cannot read() more than one virtual memory page at a time
79 - read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of
80 a page in the previous call, you will only be able to read()
81 _at_ _most_ 1/2 of the page in the next call)
82
83 The device's write() operation is used for uploading the system memory snapshot
84 into the kernel. It has the same limitations as the read() operation.
85
86 The release() operation frees all memory allocated for the snapshot image
87 and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any).
88 Thus it is not necessary to use either SNAPSHOT_FREE or
89 SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
90 unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
91 still frozen when the device is being closed).
92
93 Currently it is assumed that the userland utilities reading/writing the
94 snapshot image from/to the kernel will use a swap parition, called the resume
95 partition, as storage space. However, this is not really required, as they
96 can use, for example, a special (blank) suspend partition or a file on a partition
97 that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and mounted afterwards.
98
99 These utilities SHOULD NOT make any assumptions regarding the ordering of
100 data within the snapshot image, except for the image header that MAY be
101 assumed to start with an swsusp_info structure, as specified in
102 kernel/power/power.h. This structure MAY be used by the userland utilities
103 to obtain some information about the snapshot image, such as the size
104 of the snapshot image, including the metadata and the header itself,
105 contained in the .size member of swsusp_info.
106
107 The snapshot image MUST be written to the kernel unaltered (ie. all of the image
108 data, metadata and header MUST be written in _exactly_ the same amount, form
109 and order in which they have been read). Otherwise, the behavior of the
110 resumed system may be totally unpredictable.
111
112 While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
113 structure of the snapshot image is consistent with the information stored
114 in the image header. If any inconsistencies are detected,
115 SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
116 mechanism and the userland utilities using the interface SHOULD use additional
117 means, such as checksums, to ensure the integrity of the snapshot image.
118
119 The suspending and resuming utilities MUST lock themselves in memory,
120 preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
121
122 The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT
123 in the memory location pointed to by the last argument of ioctl() and proceed
124 in accordance with it:
125 1. If the value is 1 (ie. the system memory snapshot has just been
126 created and the system is ready for saving it):
127 (a) The suspending utility MUST NOT close the snapshot device
128 _unless_ the whole suspend procedure is to be cancelled, in
129 which case, if the snapshot image has already been saved, the
130 suspending utility SHOULD destroy it, preferrably by zapping
131 its header. If the suspend is not to be cancelled, the
132 system MUST be powered off or rebooted after the snapshot
133 image has been saved.
134 (b) The suspending utility SHOULD NOT attempt to perform any
135 file system operations (including reads) on the file systems
136 that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been
137 called. However, it MAY mount a file system that was not
138 mounted at that time and perform some operations on it (eg.
139 use it for saving the image).
140 2. If the value is 0 (ie. the system state has just been restored from
141 the snapshot image), the suspending utility MUST close the snapshot
142 device. Afterwards it will be treated as a regular userland process,
143 so it need not exit.
144
145 The resuming utility SHOULD NOT attempt to mount any file systems that could
146 be mounted before suspend and SHOULD NOT attempt to perform any operations
147 involving such file systems.
148
149 For details, please refer to the source code.
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