Loading...
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 (the last argument should be a pointer to a loff_t variable that
73 will contain the result if the call is successful)
74
75SNAPSHOT_AVAIL_SWAP_SIZE
76 return the amount of available swap in bytes
77 (the last argument should be a pointer to a loff_t variable that
78 will contain the result if the call is successful)
79
80SNAPSHOT_ALLOC_SWAP_PAGE
81 allocate a swap page from the resume partition
82 (the last argument should be a pointer to a loff_t variable that
83 will contain the swap page offset if the call is successful)
84
85SNAPSHOT_FREE_SWAP_PAGES
86 free all swap pages allocated by
87 SNAPSHOT_ALLOC_SWAP_PAGE
88
89SNAPSHOT_SET_SWAP_AREA
90 set the resume partition and the offset (in <PAGE_SIZE>
91 units) from the beginning of the partition at which the swap header is
92 located (the last ioctl() argument should point to a struct
93 resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
94 containing the resume device specification and the offset); for swap
95 partitions the offset is always 0, but it is different from zero for
96 swap files (see Documentation/power/swsusp-and-swap-files.rst for
97 details).
98
99SNAPSHOT_PLATFORM_SUPPORT
100 enable/disable the hibernation platform support,
101 depending on the argument value (enable, if the argument is nonzero)
102
103SNAPSHOT_POWER_OFF
104 make the kernel transition the system to the hibernation
105 state (eg. ACPI S4) using the platform (eg. ACPI) driver
106
107SNAPSHOT_S2RAM
108 suspend to RAM; using this call causes the kernel to
109 immediately enter the suspend-to-RAM state, so this call must always
110 be preceded by the SNAPSHOT_FREEZE call and it is also necessary
111 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
112 is needed to implement the suspend-to-both mechanism in which the
113 suspend image is first created, as though the system had been suspended
114 to disk, and then the system is suspended to RAM (this makes it possible
115 to resume the system from RAM if there's enough battery power or restore
116 its state on the basis of the saved suspend image otherwise)
117
118The device's read() operation can be used to transfer the snapshot image from
119the kernel. It has the following limitations:
120
121- you cannot read() more than one virtual memory page at a time
122- read()s across page boundaries are impossible (ie. if you read() 1/2 of
123 a page in the previous call, you will only be able to read()
124 **at most** 1/2 of the page in the next call)
125
126The device's write() operation is used for uploading the system memory snapshot
127into the kernel. It has the same limitations as the read() operation.
128
129The release() operation frees all memory allocated for the snapshot image
130and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
131Thus it is not necessary to use either SNAPSHOT_FREE or
132SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
133unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
134still frozen when the device is being closed).
135
136Currently it is assumed that the userland utilities reading/writing the
137snapshot image from/to the kernel will use a swap partition, called the resume
138partition, or a swap file as storage space (if a swap file is used, the resume
139partition is the partition that holds this file). However, this is not really
140required, as they can use, for example, a special (blank) suspend partition or
141a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
142mounted afterwards.
143
144These utilities MUST NOT make any assumptions regarding the ordering of
145data within the snapshot image. The contents of the image are entirely owned
146by the kernel and its structure may be changed in future kernel releases.
147
148The snapshot image MUST be written to the kernel unaltered (ie. all of the image
149data, metadata and header MUST be written in _exactly_ the same amount, form
150and order in which they have been read). Otherwise, the behavior of the
151resumed system may be totally unpredictable.
152
153While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
154structure of the snapshot image is consistent with the information stored
155in the image header. If any inconsistencies are detected,
156SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
157mechanism and the userland utilities using the interface SHOULD use additional
158means, such as checksums, to ensure the integrity of the snapshot image.
159
160The suspending and resuming utilities MUST lock themselves in memory,
161preferably using mlockall(), before calling SNAPSHOT_FREEZE.
162
163The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
164in the memory location pointed to by the last argument of ioctl() and proceed
165in accordance with it:
166
1671. If the value is 1 (ie. the system memory snapshot has just been
168 created and the system is ready for saving it):
169
170 (a) The suspending utility MUST NOT close the snapshot device
171 _unless_ the whole suspend procedure is to be cancelled, in
172 which case, if the snapshot image has already been saved, the
173 suspending utility SHOULD destroy it, preferably by zapping
174 its header. If the suspend is not to be cancelled, the
175 system MUST be powered off or rebooted after the snapshot
176 image has been saved.
177 (b) The suspending utility SHOULD NOT attempt to perform any
178 file system operations (including reads) on the file systems
179 that were mounted before SNAPSHOT_CREATE_IMAGE has been
180 called. However, it MAY mount a file system that was not
181 mounted at that time and perform some operations on it (eg.
182 use it for saving the image).
183
1842. If the value is 0 (ie. the system state has just been restored from
185 the snapshot image), the suspending utility MUST close the snapshot
186 device. Afterwards it will be treated as a regular userland process,
187 so it need not exit.
188
189The resuming utility SHOULD NOT attempt to mount any file systems that could
190be mounted before suspend and SHOULD NOT attempt to perform any operations
191involving such file systems.
192
193For details, please refer to the source code.
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 (the last argument should be a pointer to a loff_t variable that
73 will contain the result if the call is successful)
74
75SNAPSHOT_AVAIL_SWAP_SIZE
76 return the amount of available swap in bytes
77 (the last argument should be a pointer to a loff_t variable that
78 will contain the result if the call is successful)
79
80SNAPSHOT_ALLOC_SWAP_PAGE
81 allocate a swap page from the resume partition
82 (the last argument should be a pointer to a loff_t variable that
83 will contain the swap page offset if the call is successful)
84
85SNAPSHOT_FREE_SWAP_PAGES
86 free all swap pages allocated by
87 SNAPSHOT_ALLOC_SWAP_PAGE
88
89SNAPSHOT_SET_SWAP_AREA
90 set the resume partition and the offset (in <PAGE_SIZE>
91 units) from the beginning of the partition at which the swap header is
92 located (the last ioctl() argument should point to a struct
93 resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
94 containing the resume device specification and the offset); for swap
95 partitions the offset is always 0, but it is different from zero for
96 swap files (see Documentation/power/swsusp-and-swap-files.rst for
97 details).
98
99SNAPSHOT_PLATFORM_SUPPORT
100 enable/disable the hibernation platform support,
101 depending on the argument value (enable, if the argument is nonzero)
102
103SNAPSHOT_POWER_OFF
104 make the kernel transition the system to the hibernation
105 state (eg. ACPI S4) using the platform (eg. ACPI) driver
106
107SNAPSHOT_S2RAM
108 suspend to RAM; using this call causes the kernel to
109 immediately enter the suspend-to-RAM state, so this call must always
110 be preceded by the SNAPSHOT_FREEZE call and it is also necessary
111 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
112 is needed to implement the suspend-to-both mechanism in which the
113 suspend image is first created, as though the system had been suspended
114 to disk, and then the system is suspended to RAM (this makes it possible
115 to resume the system from RAM if there's enough battery power or restore
116 its state on the basis of the saved suspend image otherwise)
117
118The device's read() operation can be used to transfer the snapshot image from
119the kernel. It has the following limitations:
120
121- you cannot read() more than one virtual memory page at a time
122- read()s across page boundaries are impossible (ie. if you read() 1/2 of
123 a page in the previous call, you will only be able to read()
124 **at most** 1/2 of the page in the next call)
125
126The device's write() operation is used for uploading the system memory snapshot
127into the kernel. It has the same limitations as the read() operation.
128
129The release() operation frees all memory allocated for the snapshot image
130and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
131Thus it is not necessary to use either SNAPSHOT_FREE or
132SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
133unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
134still frozen when the device is being closed).
135
136Currently it is assumed that the userland utilities reading/writing the
137snapshot image from/to the kernel will use a swap partition, called the resume
138partition, or a swap file as storage space (if a swap file is used, the resume
139partition is the partition that holds this file). However, this is not really
140required, as they can use, for example, a special (blank) suspend partition or
141a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
142mounted afterwards.
143
144These utilities MUST NOT make any assumptions regarding the ordering of
145data within the snapshot image. The contents of the image are entirely owned
146by the kernel and its structure may be changed in future kernel releases.
147
148The snapshot image MUST be written to the kernel unaltered (ie. all of the image
149data, metadata and header MUST be written in _exactly_ the same amount, form
150and order in which they have been read). Otherwise, the behavior of the
151resumed system may be totally unpredictable.
152
153While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
154structure of the snapshot image is consistent with the information stored
155in the image header. If any inconsistencies are detected,
156SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
157mechanism and the userland utilities using the interface SHOULD use additional
158means, such as checksums, to ensure the integrity of the snapshot image.
159
160The suspending and resuming utilities MUST lock themselves in memory,
161preferably using mlockall(), before calling SNAPSHOT_FREEZE.
162
163The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
164in the memory location pointed to by the last argument of ioctl() and proceed
165in accordance with it:
166
1671. If the value is 1 (ie. the system memory snapshot has just been
168 created and the system is ready for saving it):
169
170 (a) The suspending utility MUST NOT close the snapshot device
171 _unless_ the whole suspend procedure is to be cancelled, in
172 which case, if the snapshot image has already been saved, the
173 suspending utility SHOULD destroy it, preferably by zapping
174 its header. If the suspend is not to be cancelled, the
175 system MUST be powered off or rebooted after the snapshot
176 image has been saved.
177 (b) The suspending utility SHOULD NOT attempt to perform any
178 file system operations (including reads) on the file systems
179 that were mounted before SNAPSHOT_CREATE_IMAGE has been
180 called. However, it MAY mount a file system that was not
181 mounted at that time and perform some operations on it (eg.
182 use it for saving the image).
183
1842. If the value is 0 (ie. the system state has just been restored from
185 the snapshot image), the suspending utility MUST close the snapshot
186 device. Afterwards it will be treated as a regular userland process,
187 so it need not exit.
188
189The resuming utility SHOULD NOT attempt to mount any file systems that could
190be mounted before suspend and SHOULD NOT attempt to perform any operations
191involving such file systems.
192
193For details, please refer to the source code.