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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	Documentation for userland software suspend interface
2		(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
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
7	done it already.
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://suspend.sourceforge.net>.  You may want to have a look at them if you
13	are going to develop your own suspend/resume utilities.
15	The interface consists of a character device providing the open(),
16	release(), read(), and write() operations as well as several ioctl()
17	commands defined in include/linux/suspend_ioctls.h .  The major and minor
18	numbers of the device are, respectively, 10 and 231, and they can
19	be read from /sys/class/misc/snapshot/dev.
21	The device can be open either for reading or for writing.  If open for
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
25	once at a time.
27	Even opening the device has side effects. Data structures are
28	allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
29	called.
31	The ioctl() commands recognized by the device are:
33	SNAPSHOT_FREEZE - freeze user space processes (the current process is
34		not frozen); this is required for SNAPSHOT_CREATE_IMAGE
35		and SNAPSHOT_ATOMIC_RESTORE to succeed
37	SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
39	SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the
40		last argument of ioctl() should be a pointer to an int variable,
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
49		uploaded snapshot image; before calling it you should transfer
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
64		last argument should be a pointer to an unsigned int variable that will
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)
71	SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by
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
77		resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
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
80		swap files (see Documentation/power/swsusp-and-swap-files.txt for
81		details).
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:
101	- you cannot read() more than one virtual memory page at a time
102	- read()s across page boundaries are impossible (ie. if you read() 1/2 of
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
110	and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
111	Thus it is not necessary to use either SNAPSHOT_FREE or
112	SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
113	unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
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
120	required, as they can use, for example, a special (blank) suspend partition or
121	a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
122	mounted afterwards.
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
131	resumed system may be totally unpredictable.
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,
136	SNAPSHOT_ATOMIC_RESTORE will not succeed.  Still, this is not a fool-proof
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.
140	The suspending and resuming utilities MUST lock themselves in memory,
141	preferably using mlockall(), before calling SNAPSHOT_FREEZE.
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
145	in accordance with it:
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
151			suspending utility SHOULD destroy it, preferably by zapping
152			its header.  If the suspend is not to be cancelled, the
153			system MUST be powered off or rebooted after the snapshot
154			image has been saved.
155		(b)	The suspending utility SHOULD NOT attempt to perform any
156			file system operations (including reads) on the file systems
157			that were mounted before SNAPSHOT_CREATE_IMAGE has been
158			called.  However, it MAY mount a file system that was not
159			mounted at that time and perform some operations on it (eg.
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
163		device.  Afterwards it will be treated as a regular userland process,
164		so it need not exit.
166	The resuming utility SHOULD NOT attempt to mount any file systems that could
167	be mounted before suspend and SHOULD NOT attempt to perform any operations
168	involving such file systems.
170	For details, please refer to the source code.
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