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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 | |
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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. | |
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14 | |
15 | The interface consists of a character device providing the open(), | |
16 | release(), read(), and write() operations as well as several ioctl() | |
3010f8ca | 17 | commands defined in include/linux/suspend_ioctls.h . The major and minor |
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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 | |
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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. | |
6e1819d6 | 26 | |
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27 | Even opening the device has side effects. Data structures are |
28 | allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are | |
29 | called. | |
30 | ||
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31 | The ioctl() commands recognized by the device are: |
32 | ||
33 | SNAPSHOT_FREEZE - freeze user space processes (the current process is | |
cc5d207c | 34 | not frozen); this is required for SNAPSHOT_CREATE_IMAGE |
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35 | and SNAPSHOT_ATOMIC_RESTORE to succeed |
36 | ||
37 | SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE | |
38 | ||
cc5d207c | 39 | SNAPSHOT_CREATE_IMAGE - create a snapshot of the system memory; the |
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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 | |
cc5d207c | 44 | SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot |
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45 | has been created the read() operation can be used to transfer |
46 | it out of the kernel | |
47 | ||
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 | |
53 | ||
54 | SNAPSHOT_FREE - free memory allocated for the snapshot image | |
55 | ||
cc5d207c | 56 | SNAPSHOT_PREF_IMAGE_SIZE - set the preferred maximum size of the image |
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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) | |
60 | ||
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61 | SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image |
62 | ||
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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 | |
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65 | contain the result if the call is successful). |
66 | ||
cc5d207c | 67 | SNAPSHOT_ALLOC_SWAP_PAGE - allocate a swap page from the resume partition |
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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) | |
70 | ||
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71 | SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by |
72 | SNAPSHOT_ALLOC_SWAP_PAGE | |
6e1819d6 | 73 | |
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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 | |
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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/swsusp-and-swap-files.txt for details). | |
bf73bae6 | 81 | |
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82 | SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support, |
83 | depending on the argument value (enable, if the argument is nonzero) | |
84 | ||
85 | SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation | |
86 | state (eg. ACPI S4) using the platform (eg. ACPI) driver | |
87 | ||
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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 | ||
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98 | The device's read() operation can be used to transfer the snapshot image from |
99 | the 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 | ||
105 | The device's write() operation is used for uploading the system memory snapshot | |
106 | into the kernel. It has the same limitations as the read() operation. | |
107 | ||
108 | The release() operation frees all memory allocated for the snapshot image | |
cc5d207c | 109 | and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any). |
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110 | Thus it is not necessary to use either SNAPSHOT_FREE or |
111 | SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also | |
112 | unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are | |
113 | still frozen when the device is being closed). | |
114 | ||
115 | Currently it is assumed that the userland utilities reading/writing the | |
19f59460 | 116 | snapshot image from/to the kernel will use a swap partition, called the resume |
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117 | partition, or a swap file as storage space (if a swap file is used, the resume |
118 | partition is the partition that holds this file). However, this is not really | |
119 | required, as they can use, for example, a special (blank) suspend partition or | |
cc5d207c | 120 | a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and |
bf73bae6 | 121 | mounted afterwards. |
6e1819d6 | 122 | |
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123 | These utilities MUST NOT make any assumptions regarding the ordering of |
124 | data within the snapshot image. The contents of the image are entirely owned | |
125 | by the kernel and its structure may be changed in future kernel releases. | |
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126 | |
127 | The snapshot image MUST be written to the kernel unaltered (ie. all of the image | |
128 | data, metadata and header MUST be written in _exactly_ the same amount, form | |
129 | and order in which they have been read). Otherwise, the behavior of the | |
130 | resumed system may be totally unpredictable. | |
131 | ||
132 | While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the | |
133 | structure of the snapshot image is consistent with the information stored | |
134 | in the image header. If any inconsistencies are detected, | |
135 | SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof | |
136 | mechanism and the userland utilities using the interface SHOULD use additional | |
137 | means, such as checksums, to ensure the integrity of the snapshot image. | |
138 | ||
139 | The suspending and resuming utilities MUST lock themselves in memory, | |
140 | preferrably using mlockall(), before calling SNAPSHOT_FREEZE. | |
141 | ||
cc5d207c | 142 | The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE |
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143 | in the memory location pointed to by the last argument of ioctl() and proceed |
144 | in accordance with it: | |
145 | 1. 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 | |
cc5d207c | 156 | that were mounted before SNAPSHOT_CREATE_IMAGE has been |
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157 | 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). | |
160 | 2. 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 | ||
165 | The resuming utility SHOULD NOT attempt to mount any file systems that could | |
166 | be mounted before suspend and SHOULD NOT attempt to perform any operations | |
167 | involving such file systems. | |
168 | ||
169 | For details, please refer to the source code. |