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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()
Rafael J. Wysocki3010f8c2007-10-26 01:05:05 +020017commands defined in include/linux/suspend_ioctls.h . The major and minor
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080018numbers 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
Pavel Machekbc6a0cb2010-04-23 20:32:29 +020027Even opening the device has side effects. Data structures are
28allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
29called.
30
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080031The ioctl() commands recognized by the device are:
32
33SNAPSHOT_FREEZE - freeze user space processes (the current process is
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020034 not frozen); this is required for SNAPSHOT_CREATE_IMAGE
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080035 and SNAPSHOT_ATOMIC_RESTORE to succeed
36
37SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
38
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020039SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080040 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
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020044 SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080045 has been created the read() operation can be used to transfer
46 it out of the kernel
47
48SNAPSHOT_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
53
54SNAPSHOT_FREE - free memory allocated for the snapshot image
55
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020056SNAPSHOT_PREF_IMAGE_SIZE - set the preferred maximum size of the image
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080057 (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)
60
Rafael J. Wysockiaf508b32007-10-26 00:59:31 +020061SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image
62
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020063SNAPSHOT_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
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080065 contain the result if the call is successful).
66
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020067SNAPSHOT_ALLOC_SWAP_PAGE - allocate a swap page from the resume partition
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080068 (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)
70
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +020071SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by
72 SNAPSHOT_ALLOC_SWAP_PAGE
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080073
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080074SNAPSHOT_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
Rafael J. Wysocki3010f8c2007-10-26 01:05:05 +020077 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/swsusp-and-swap-files.txt for details).
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080081
Rafael J. Wysockieb57c1c2007-10-26 01:01:10 +020082SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
83 depending on the argument value (enable, if the argument is nonzero)
84
85SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
86 state (eg. ACPI S4) using the platform (eg. ACPI) driver
87
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -080088SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
89 immediately enter the suspend-to-RAM state, so this call must always
90 be preceded by the SNAPSHOT_FREEZE call and it is also necessary
91 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
92 is needed to implement the suspend-to-both mechanism in which the
93 suspend image is first created, as though the system had been suspended
94 to disk, and then the system is suspended to RAM (this makes it possible
95 to resume the system from RAM if there's enough battery power or restore
96 its state on the basis of the saved suspend image otherwise)
97
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -080098The device's read() operation can be used to transfer the snapshot image from
99the kernel. It has the following limitations:
100- you cannot read() more than one virtual memory page at a time
101- read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of
102 a page in the previous call, you will only be able to read()
103 _at_ _most_ 1/2 of the page in the next call)
104
105The device's write() operation is used for uploading the system memory snapshot
106into the kernel. It has the same limitations as the read() operation.
107
108The release() operation frees all memory allocated for the snapshot image
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +0200109and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800110Thus it is not necessary to use either SNAPSHOT_FREE or
111SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
112unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
113still frozen when the device is being closed).
114
115Currently it is assumed that the userland utilities reading/writing the
Matt LaPlante19f59462009-04-27 15:06:31 +0200116snapshot image from/to the kernel will use a swap partition, called the resume
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -0800117partition, or a swap file as storage space (if a swap file is used, the resume
118partition is the partition that holds this file). However, this is not really
119required, as they can use, for example, a special (blank) suspend partition or
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +0200120a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
Rafael J. Wysockibf73bae2006-12-06 20:34:16 -0800121mounted afterwards.
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800122
Rafael J. Wysockiaf508b32007-10-26 00:59:31 +0200123These utilities MUST NOT make any assumptions regarding the ordering of
124data within the snapshot image. The contents of the image are entirely owned
125by the kernel and its structure may be changed in future kernel releases.
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800126
127The snapshot image MUST be written to the kernel unaltered (ie. all of the image
128data, metadata and header MUST be written in _exactly_ the same amount, form
129and order in which they have been read). Otherwise, the behavior of the
130resumed system may be totally unpredictable.
131
132While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
133structure of the snapshot image is consistent with the information stored
134in the image header. If any inconsistencies are detected,
135SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
136mechanism and the userland utilities using the interface SHOULD use additional
137means, such as checksums, to ensure the integrity of the snapshot image.
138
139The suspending and resuming utilities MUST lock themselves in memory,
140preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
141
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +0200142The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800143in the memory location pointed to by the last argument of ioctl() and proceed
144in accordance with it:
1451. If the value is 1 (ie. the system memory snapshot has just been
146 created and the system is ready for saving it):
147 (a) The suspending utility MUST NOT close the snapshot device
148 _unless_ the whole suspend procedure is to be cancelled, in
149 which case, if the snapshot image has already been saved, the
150 suspending utility SHOULD destroy it, preferrably by zapping
151 its header. If the suspend is not to be cancelled, the
152 system MUST be powered off or rebooted after the snapshot
153 image has been saved.
154 (b) The suspending utility SHOULD NOT attempt to perform any
155 file system operations (including reads) on the file systems
Rafael J. Wysockicc5d2072007-10-26 01:03:33 +0200156 that were mounted before SNAPSHOT_CREATE_IMAGE has been
Rafael J. Wysocki6e1819d2006-03-23 03:00:03 -0800157 called. However, it MAY mount a file system that was not
158 mounted at that time and perform some operations on it (eg.
159 use it for saving the image).
1602. If the value is 0 (ie. the system state has just been restored from
161 the snapshot image), the suspending utility MUST close the snapshot
162 device. Afterwards it will be treated as a regular userland process,
163 so it need not exit.
164
165The resuming utility SHOULD NOT attempt to mount any file systems that could
166be mounted before suspend and SHOULD NOT attempt to perform any operations
167involving such file systems.
168
169For details, please refer to the source code.