1=====================================================
2Documentation for userland software suspend interface
3=====================================================
4
5	(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
6
7First, the warnings at the beginning of swsusp.txt still apply.
8
9Second, you should read the FAQ in swsusp.txt _now_ if you have not
10done it already.
11
12Now, to use the userland interface for software suspend you need special
13utilities that will read/write the system memory snapshot from/to the
14kernel.  Such utilities are available, for example, from
15<http://suspend.sourceforge.net>.  You may want to have a look at them if you
16are going to develop your own suspend/resume utilities.
17
18The interface consists of a character device providing the open(),
19release(), read(), and write() operations as well as several ioctl()
20commands defined in include/linux/suspend_ioctls.h .  The major and minor
21numbers of the device are, respectively, 10 and 231, and they can
22be read from /sys/class/misc/snapshot/dev.
23
24The device can be open either for reading or for writing.  If open for
25reading, it is considered to be in the suspend mode.  Otherwise it is
26assumed to be in the resume mode.  The device cannot be open for simultaneous
27reading and writing.  It is also impossible to have the device open more than
28once at a time.
29
30Even opening the device has side effects. Data structures are
31allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
32called.
33
34The ioctl() commands recognized by the device are:
35
36SNAPSHOT_FREEZE
37	freeze user space processes (the current process is
38	not frozen); this is required for SNAPSHOT_CREATE_IMAGE
39	and SNAPSHOT_ATOMIC_RESTORE to succeed
40
41SNAPSHOT_UNFREEZE
42	thaw user space processes frozen by SNAPSHOT_FREEZE
43
44SNAPSHOT_CREATE_IMAGE
45	create a snapshot of the system memory; the
46	last argument of ioctl() should be a pointer to an int variable,
47	the value of which will indicate whether the call returned after
48	creating the snapshot (1) or after restoring the system memory state
49	from it (0) (after resume the system finds itself finishing the
50	SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot
51	has been created the read() operation can be used to transfer
52	it out of the kernel
53
54SNAPSHOT_ATOMIC_RESTORE
55	restore the system memory state from the
56	uploaded snapshot image; before calling it you should transfer
57	the system memory snapshot back to the kernel using the write()
58	operation; this call will not succeed if the snapshot
59	image is not available to the kernel
60
61SNAPSHOT_FREE
62	free memory allocated for the snapshot image
63
64SNAPSHOT_PREF_IMAGE_SIZE
65	set the preferred maximum size of the image
66	(the kernel will do its best to ensure the image size will not exceed
67	this number, but if it turns out to be impossible, the kernel will
68	create the smallest image possible)
69
70SNAPSHOT_GET_IMAGE_SIZE
71	return the actual size of the hibernation image
72
73SNAPSHOT_AVAIL_SWAP_SIZE
74	return the amount of available swap in bytes (the
75	last argument should be a pointer to an unsigned int variable that will
76	contain the result if the call is successful).
77
78SNAPSHOT_ALLOC_SWAP_PAGE
79	allocate a swap page from the resume partition
80	(the last argument should be a pointer to a loff_t variable that
81	will contain the swap page offset if the call is successful)
82
83SNAPSHOT_FREE_SWAP_PAGES
84	free all swap pages allocated by
85	SNAPSHOT_ALLOC_SWAP_PAGE
86
87SNAPSHOT_SET_SWAP_AREA
88	set the resume partition and the offset (in <PAGE_SIZE>
89	units) from the beginning of the partition at which the swap header is
90	located (the last ioctl() argument should point to a struct
91	resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
92	containing the resume device specification and the offset); for swap
93	partitions the offset is always 0, but it is different from zero for
94	swap files (see Documentation/power/swsusp-and-swap-files.rst for
95	details).
96
97SNAPSHOT_PLATFORM_SUPPORT
98	enable/disable the hibernation platform support,
99	depending on the argument value (enable, if the argument is nonzero)
100
101SNAPSHOT_POWER_OFF
102	make the kernel transition the system to the hibernation
103	state (eg. ACPI S4) using the platform (eg. ACPI) driver
104
105SNAPSHOT_S2RAM
106	suspend to RAM; using this call causes the kernel to
107	immediately enter the suspend-to-RAM state, so this call must always
108	be preceded by the SNAPSHOT_FREEZE call and it is also necessary
109	to use the SNAPSHOT_UNFREEZE call after the system wakes up.  This call
110	is needed to implement the suspend-to-both mechanism in which the
111	suspend image is first created, as though the system had been suspended
112	to disk, and then the system is suspended to RAM (this makes it possible
113	to resume the system from RAM if there's enough battery power or restore
114	its state on the basis of the saved suspend image otherwise)
115
116The device's read() operation can be used to transfer the snapshot image from
117the kernel.  It has the following limitations:
118
119- you cannot read() more than one virtual memory page at a time
120- read()s across page boundaries are impossible (ie. if you read() 1/2 of
121  a page in the previous call, you will only be able to read()
122  **at most** 1/2 of the page in the next call)
123
124The device's write() operation is used for uploading the system memory snapshot
125into the kernel.  It has the same limitations as the read() operation.
126
127The release() operation frees all memory allocated for the snapshot image
128and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
129Thus it is not necessary to use either SNAPSHOT_FREE or
130SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
131unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
132still frozen when the device is being closed).
133
134Currently it is assumed that the userland utilities reading/writing the
135snapshot image from/to the kernel will use a swap partition, called the resume
136partition, or a swap file as storage space (if a swap file is used, the resume
137partition is the partition that holds this file).  However, this is not really
138required, as they can use, for example, a special (blank) suspend partition or
139a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
140mounted afterwards.
141
142These utilities MUST NOT make any assumptions regarding the ordering of
143data within the snapshot image.  The contents of the image are entirely owned
144by the kernel and its structure may be changed in future kernel releases.
145
146The snapshot image MUST be written to the kernel unaltered (ie. all of the image
147data, metadata and header MUST be written in _exactly_ the same amount, form
148and order in which they have been read).  Otherwise, the behavior of the
149resumed system may be totally unpredictable.
150
151While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
152structure of the snapshot image is consistent with the information stored
153in the image header.  If any inconsistencies are detected,
154SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
155mechanism and the userland utilities using the interface SHOULD use additional
156means, such as checksums, to ensure the integrity of the snapshot image.
157
158The suspending and resuming utilities MUST lock themselves in memory,
159preferably using mlockall(), before calling SNAPSHOT_FREEZE.
160
161The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
162in the memory location pointed to by the last argument of ioctl() and proceed
163in accordance with it:
164
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
168	(a)	The suspending utility MUST NOT close the snapshot device
169		_unless_ the whole suspend procedure is to be cancelled, in
170		which case, if the snapshot image has already been saved, the
171		suspending utility SHOULD destroy it, preferably by zapping
172		its header.  If the suspend is not to be cancelled, the
173		system MUST be powered off or rebooted after the snapshot
174		image has been saved.
175	(b)	The suspending utility SHOULD NOT attempt to perform any
176		file system operations (including reads) on the file systems
177		that were mounted before SNAPSHOT_CREATE_IMAGE has been
178		called.  However, it MAY mount a file system that was not
179		mounted at that time and perform some operations on it (eg.
180		use it for saving the image).
181
1822.	If the value is 0 (ie. the system state has just been restored from
183	the snapshot image), the suspending utility MUST close the snapshot
184	device.  Afterwards it will be treated as a regular userland process,
185	so it need not exit.
186
187The resuming utility SHOULD NOT attempt to mount any file systems that could
188be mounted before suspend and SHOULD NOT attempt to perform any operations
189involving such file systems.
190
191For details, please refer to the source code.
192