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Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -08001Documentation for userland software suspend interface
2 (C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
3
4First, the warnings at the beginning of swsusp.txt still apply.
5
6Second, you should read the FAQ in swsusp.txt _now_ if you have not
7done it already.
8
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
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080012<http://suspend.sourceforge.net>. You may want to have a look at them if you
13are going to develop your own suspend/resume utilities.
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080014
15The interface consists of a character device providing the open(),
16release(), read(), and write() operations as well as several ioctl()
17commands defined in kernel/power/power.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.
20
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
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080023assumed 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.
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080026
27The ioctl() commands recognized by the device are:
28
29SNAPSHOT_FREEZE - freeze user space processes (the current process is
30 not frozen); this is required for SNAPSHOT_ATOMIC_SNAPSHOT
31 and SNAPSHOT_ATOMIC_RESTORE to succeed
32
33SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
34
35SNAPSHOT_ATOMIC_SNAPSHOT - create a snapshot of the system memory; the
36 last argument of ioctl() should be a pointer to an int variable,
37 the value of which will indicate whether the call returned after
38 creating the snapshot (1) or after restoring the system memory state
39 from it (0) (after resume the system finds itself finishing the
40 SNAPSHOT_ATOMIC_SNAPSHOT ioctl() again); after the snapshot
41 has been created the read() operation can be used to transfer
42 it out of the kernel
43
44SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
45 uploaded snapshot image; before calling it you should transfer
46 the system memory snapshot back to the kernel using the write()
47 operation; this call will not succeed if the snapshot
48 image is not available to the kernel
49
50SNAPSHOT_FREE - free memory allocated for the snapshot image
51
52SNAPSHOT_SET_IMAGE_SIZE - set the preferred maximum size of the image
53 (the kernel will do its best to ensure the image size will not exceed
54 this number, but if it turns out to be impossible, the kernel will
55 create the smallest image possible)
56
57SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last
58 argument should be a pointer to an unsigned int variable that will
59 contain the result if the call is successful).
60
61SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition
62 (the last argument should be a pointer to a loff_t variable that
63 will contain the swap page offset if the call is successful)
64
65SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with
66 SNAPSHOT_GET_SWAP_PAGE
67
68SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument
69 should specify the device's major and minor numbers in the old
70 two-byte format, as returned by the stat() function in the .st_rdev
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080071 member of the stat structure)
72
73SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE>
74 units) from the beginning of the partition at which the swap header is
75 located (the last ioctl() argument should point to a struct
76 resume_swap_area, as defined in kernel/power/power.h, containing the
77 resume device specification, as for the SNAPSHOT_SET_SWAP_FILE ioctl(),
78 and the offset); for swap partitions the offset is always 0, but it is
79 different to zero for swap files (please see
80 Documentation/swsusp-and-swap-files.txt for details).
81 The SNAPSHOT_SET_SWAP_AREA ioctl() is considered as a replacement for
82 SNAPSHOT_SET_SWAP_FILE which is regarded as obsolete. It is
83 recommended to always use this call, because the code to set the resume
84 partition may be removed from future kernels
85
86SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
87 immediately enter the suspend-to-RAM state, so this call must always
88 be preceded by the SNAPSHOT_FREEZE call and it is also necessary
89 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
90 is needed to implement the suspend-to-both mechanism in which the
91 suspend image is first created, as though the system had been suspended
92 to disk, and then the system is suspended to RAM (this makes it possible
93 to resume the system from RAM if there's enough battery power or restore
94 its state on the basis of the saved suspend image otherwise)
95
96SNAPSHOT_PMOPS - enable the usage of the pmops->prepare, pmops->enter and
97 pmops->finish methods (the in-kernel swsusp knows these as the "platform
98 method") which are needed on many machines to (among others) speed up
99 the resume by letting the BIOS skip some steps or to let the system
100 recognise the correct state of the hardware after the resume (in
101 particular on many machines this ensures that unplugged AC
102 adapters get correctly detected and that kacpid does not run wild after
103 the resume). The last ioctl() argument can take one of the three
104 values, defined in kernel/power/power.h:
105 PMOPS_PREPARE - make the kernel carry out the
106 pm_ops->prepare(PM_SUSPEND_DISK) operation
107 PMOPS_ENTER - make the kernel power off the system by calling
108 pm_ops->enter(PM_SUSPEND_DISK)
109 PMOPS_FINISH - make the kernel carry out the
110 pm_ops->finish(PM_SUSPEND_DISK) operation
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800111
112The device's read() operation can be used to transfer the snapshot image from
113the kernel. It has the following limitations:
114- you cannot read() more than one virtual memory page at a time
115- read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of
116 a page in the previous call, you will only be able to read()
117 _at_ _most_ 1/2 of the page in the next call)
118
119The device's write() operation is used for uploading the system memory snapshot
120into the kernel. It has the same limitations as the read() operation.
121
122The release() operation frees all memory allocated for the snapshot image
123and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any).
124Thus it is not necessary to use either SNAPSHOT_FREE or
125SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
126unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
127still frozen when the device is being closed).
128
129Currently it is assumed that the userland utilities reading/writing the
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -0800130snapshot image from/to the kernel will use a swap parition, called the resume
131partition, or a swap file as storage space (if a swap file is used, the resume
132partition is the partition that holds this file). However, this is not really
133required, as they can use, for example, a special (blank) suspend partition or
134a file on a partition that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and
135mounted afterwards.
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800136
137These utilities SHOULD NOT make any assumptions regarding the ordering of
138data within the snapshot image, except for the image header that MAY be
139assumed to start with an swsusp_info structure, as specified in
140kernel/power/power.h. This structure MAY be used by the userland utilities
141to obtain some information about the snapshot image, such as the size
142of the snapshot image, including the metadata and the header itself,
143contained in the .size member of swsusp_info.
144
145The snapshot image MUST be written to the kernel unaltered (ie. all of the image
146data, metadata and header MUST be written in _exactly_ the same amount, form
147and order in which they have been read). Otherwise, the behavior of the
148resumed system may be totally unpredictable.
149
150While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
151structure of the snapshot image is consistent with the information stored
152in the image header. If any inconsistencies are detected,
153SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
154mechanism and the userland utilities using the interface SHOULD use additional
155means, such as checksums, to ensure the integrity of the snapshot image.
156
157The suspending and resuming utilities MUST lock themselves in memory,
158preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
159
160The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT
161in the memory location pointed to by the last argument of ioctl() and proceed
162in accordance with it:
1631. If the value is 1 (ie. the system memory snapshot has just been
164 created and the system is ready for saving it):
165 (a) The suspending utility MUST NOT close the snapshot device
166 _unless_ the whole suspend procedure is to be cancelled, in
167 which case, if the snapshot image has already been saved, the
168 suspending utility SHOULD destroy it, preferrably by zapping
169 its header. If the suspend is not to be cancelled, the
170 system MUST be powered off or rebooted after the snapshot
171 image has been saved.
172 (b) The suspending utility SHOULD NOT attempt to perform any
173 file system operations (including reads) on the file systems
174 that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been
175 called. However, it MAY mount a file system that was not
176 mounted at that time and perform some operations on it (eg.
177 use it for saving the image).
1782. If the value is 0 (ie. the system state has just been restored from
179 the snapshot image), the suspending utility MUST close the snapshot
180 device. Afterwards it will be treated as a regular userland process,
181 so it need not exit.
182
183The resuming utility SHOULD NOT attempt to mount any file systems that could
184be mounted before suspend and SHOULD NOT attempt to perform any operations
185involving such file systems.
186
187For details, please refer to the source code.