1Documentation 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
12<http://suspend.sourceforge.net>.  You may want to have a look at them if you
13are going to develop your own suspend/resume utilities.
14
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
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.
26
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
71	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 hibernation_ops->prepare,
97	hibernate_ops->enter and hibernation_ops->finish methods (the in-kernel
98	swsusp knows these as the "platform method") which are needed on many
99	machines to (among others) speed up the resume by letting the BIOS skip
100	some steps or to let the system recognise the correct state of the
101	hardware after the resume (in particular on many machines this ensures
102	that unplugged AC adapters get correctly detected and that kacpid does
103	not run wild after the resume).  The last ioctl() argument can take one
104	of the three values, defined in kernel/power/power.h:
105	PMOPS_PREPARE - make the kernel carry out the
106		hibernation_ops->prepare() operation
107	PMOPS_ENTER - make the kernel power off the system by calling
108		hibernation_ops->enter()
109	PMOPS_FINISH - make the kernel carry out the
110		hibernation_ops->finish() operation
111	Note that the actual constants are misnamed because they surface
112	internal kernel implementation details that have changed.
113
114The device's read() operation can be used to transfer the snapshot image from
115the kernel.  It has the following limitations:
116- you cannot read() more than one virtual memory page at a time
117- read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of
118	a page in the previous call, you will only be able to read()
119	_at_ _most_ 1/2 of the page in the next call)
120
121The device's write() operation is used for uploading the system memory snapshot
122into the kernel.  It has the same limitations as the read() operation.
123
124The release() operation frees all memory allocated for the snapshot image
125and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any).
126Thus it is not necessary to use either SNAPSHOT_FREE or
127SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
128unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
129still frozen when the device is being closed).
130
131Currently it is assumed that the userland utilities reading/writing the
132snapshot image from/to the kernel will use a swap parition, called the resume
133partition, or a swap file as storage space (if a swap file is used, the resume
134partition is the partition that holds this file).  However, this is not really
135required, as they can use, for example, a special (blank) suspend partition or
136a file on a partition that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and
137mounted afterwards.
138
139These utilities SHOULD NOT make any assumptions regarding the ordering of
140data within the snapshot image, except for the image header that MAY be
141assumed to start with an swsusp_info structure, as specified in
142kernel/power/power.h.  This structure MAY be used by the userland utilities
143to obtain some information about the snapshot image, such as the size
144of the snapshot image, including the metadata and the header itself,
145contained in the .size member of swsusp_info.
146
147The snapshot image MUST be written to the kernel unaltered (ie. all of the image
148data, metadata and header MUST be written in _exactly_ the same amount, form
149and order in which they have been read).  Otherwise, the behavior of the
150resumed system may be totally unpredictable.
151
152While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
153structure of the snapshot image is consistent with the information stored
154in the image header.  If any inconsistencies are detected,
155SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
156mechanism and the userland utilities using the interface SHOULD use additional
157means, such as checksums, to ensure the integrity of the snapshot image.
158
159The suspending and resuming utilities MUST lock themselves in memory,
160preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
161
162The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT
163in the memory location pointed to by the last argument of ioctl() and proceed
164in accordance with it:
1651. 	If the value is 1 (ie. the system memory snapshot has just been
166	created and the system is ready for saving it):
167	(a)	The suspending utility MUST NOT close the snapshot device
168		_unless_ the whole suspend procedure is to be cancelled, in
169		which case, if the snapshot image has already been saved, the
170		suspending utility SHOULD destroy it, preferrably by zapping
171		its header.  If the suspend is not to be cancelled, the
172		system MUST be powered off or rebooted after the snapshot
173		image has been saved.
174	(b)	The suspending utility SHOULD NOT attempt to perform any
175		file system operations (including reads) on the file systems
176		that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been
177		called.  However, it MAY mount a file system that was not
178		mounted at that time and perform some operations on it (eg.
179		use it for saving the image).
1802.	If the value is 0 (ie. the system state has just been restored from
181	the snapshot image), the suspending utility MUST close the snapshot
182	device.  Afterwards it will be treated as a regular userland process,
183	so it need not exit.
184
185The resuming utility SHOULD NOT attempt to mount any file systems that could
186be mounted before suspend and SHOULD NOT attempt to perform any operations
187involving such file systems.
188
189For details, please refer to the source code.
190