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1 | Documentation for userland software suspend interface | |
2 | (C) 2006 Rafael J. Wysocki <rjw@sisk.pl> | |
3 | ||
4 | First, the warnings at the beginning of swsusp.txt still apply. | |
5 | ||
6 | Second, you should read the FAQ in swsusp.txt _now_ if you have not | |
7 | done it already. | |
8 | ||
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. | |
14 | ||
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 kernel/power/power.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. | |
20 | ||
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. | |
26 | ||
27 | The ioctl() commands recognized by the device are: | |
28 | ||
29 | SNAPSHOT_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 | ||
33 | SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE | |
34 | ||
35 | SNAPSHOT_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 | ||
44 | SNAPSHOT_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 | ||
50 | SNAPSHOT_FREE - free memory allocated for the snapshot image | |
51 | ||
52 | SNAPSHOT_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 | ||
57 | SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image | |
58 | ||
59 | SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last | |
60 | argument should be a pointer to an unsigned int variable that will | |
61 | contain the result if the call is successful). | |
62 | ||
63 | SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition | |
64 | (the last argument should be a pointer to a loff_t variable that | |
65 | will contain the swap page offset if the call is successful) | |
66 | ||
67 | SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with | |
68 | SNAPSHOT_GET_SWAP_PAGE | |
69 | ||
70 | SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument | |
71 | should specify the device's major and minor numbers in the old | |
72 | two-byte format, as returned by the stat() function in the .st_rdev | |
73 | member of the stat structure) | |
74 | ||
75 | SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE> | |
76 | units) from the beginning of the partition at which the swap header is | |
77 | located (the last ioctl() argument should point to a struct | |
78 | resume_swap_area, as defined in kernel/power/power.h, containing the | |
79 | resume device specification, as for the SNAPSHOT_SET_SWAP_FILE ioctl(), | |
80 | and the offset); for swap partitions the offset is always 0, but it is | |
81 | different to zero for swap files (please see | |
82 | Documentation/swsusp-and-swap-files.txt for details). | |
83 | The SNAPSHOT_SET_SWAP_AREA ioctl() is considered as a replacement for | |
84 | SNAPSHOT_SET_SWAP_FILE which is regarded as obsolete. It is | |
85 | recommended to always use this call, because the code to set the resume | |
86 | partition may be removed from future kernels | |
87 | ||
88 | SNAPSHOT_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 | ||
98 | SNAPSHOT_PMOPS - enable the usage of the hibernation_ops->prepare, | |
99 | hibernate_ops->enter and hibernation_ops->finish methods (the in-kernel | |
100 | swsusp knows these as the "platform method") which are needed on many | |
101 | machines to (among others) speed up the resume by letting the BIOS skip | |
102 | some steps or to let the system recognise the correct state of the | |
103 | hardware after the resume (in particular on many machines this ensures | |
104 | that unplugged AC adapters get correctly detected and that kacpid does | |
105 | not run wild after the resume). The last ioctl() argument can take one | |
106 | of the three values, defined in kernel/power/power.h: | |
107 | PMOPS_PREPARE - make the kernel carry out the | |
108 | hibernation_ops->prepare() operation | |
109 | PMOPS_ENTER - make the kernel power off the system by calling | |
110 | hibernation_ops->enter() | |
111 | PMOPS_FINISH - make the kernel carry out the | |
112 | hibernation_ops->finish() operation | |
113 | Note that the actual constants are misnamed because they surface | |
114 | internal kernel implementation details that have changed. | |
115 | ||
116 | The device's read() operation can be used to transfer the snapshot image from | |
117 | the kernel. It has the following limitations: | |
118 | - you cannot read() more than one virtual memory page at a time | |
119 | - read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of | |
120 | a page in the previous call, you will only be able to read() | |
121 | _at_ _most_ 1/2 of the page in the next call) | |
122 | ||
123 | The device's write() operation is used for uploading the system memory snapshot | |
124 | into the kernel. It has the same limitations as the read() operation. | |
125 | ||
126 | The release() operation frees all memory allocated for the snapshot image | |
127 | and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any). | |
128 | Thus it is not necessary to use either SNAPSHOT_FREE or | |
129 | SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also | |
130 | unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are | |
131 | still frozen when the device is being closed). | |
132 | ||
133 | Currently it is assumed that the userland utilities reading/writing the | |
134 | snapshot image from/to the kernel will use a swap parition, called the resume | |
135 | partition, or a swap file as storage space (if a swap file is used, the resume | |
136 | partition is the partition that holds this file). However, this is not really | |
137 | required, as they can use, for example, a special (blank) suspend partition or | |
138 | a file on a partition that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and | |
139 | mounted afterwards. | |
140 | ||
141 | These utilities MUST NOT make any assumptions regarding the ordering of | |
142 | data within the snapshot image. The contents of the image are entirely owned | |
143 | by the kernel and its structure may be changed in future kernel releases. | |
144 | ||
145 | The snapshot image MUST be written to the kernel unaltered (ie. all of the image | |
146 | data, metadata and header MUST be written in _exactly_ the same amount, form | |
147 | and order in which they have been read). Otherwise, the behavior of the | |
148 | resumed system may be totally unpredictable. | |
149 | ||
150 | While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the | |
151 | structure of the snapshot image is consistent with the information stored | |
152 | in the image header. If any inconsistencies are detected, | |
153 | SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof | |
154 | mechanism and the userland utilities using the interface SHOULD use additional | |
155 | means, such as checksums, to ensure the integrity of the snapshot image. | |
156 | ||
157 | The suspending and resuming utilities MUST lock themselves in memory, | |
158 | preferrably using mlockall(), before calling SNAPSHOT_FREEZE. | |
159 | ||
160 | The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT | |
161 | in the memory location pointed to by the last argument of ioctl() and proceed | |
162 | in accordance with it: | |
163 | 1. If the value is 1 (ie. the system memory snapshot has just been | |
164 | created and the system is ready for saving it): | |
165 | (a) The suspending utility MUST NOT close the snapshot device | |
166 | _unless_ the whole suspend procedure is to be cancelled, in | |
167 | which case, if the snapshot image has already been saved, the | |
168 | suspending utility SHOULD destroy it, preferrably by zapping | |
169 | its header. If the suspend is not to be cancelled, the | |
170 | system MUST be powered off or rebooted after the snapshot | |
171 | image has been saved. | |
172 | (b) The suspending utility SHOULD NOT attempt to perform any | |
173 | file system operations (including reads) on the file systems | |
174 | that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been | |
175 | called. However, it MAY mount a file system that was not | |
176 | mounted at that time and perform some operations on it (eg. | |
177 | use it for saving the image). | |
178 | 2. If the value is 0 (ie. the system state has just been restored from | |
179 | the snapshot image), the suspending utility MUST close the snapshot | |
180 | device. Afterwards it will be treated as a regular userland process, | |
181 | so it need not exit. | |
182 | ||
183 | The resuming utility SHOULD NOT attempt to mount any file systems that could | |
184 | be mounted before suspend and SHOULD NOT attempt to perform any operations | |
185 | involving such file systems. | |
186 | ||
187 | For details, please refer to the source code. |