Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 EST.
1 Documentation for userland software suspend interface 2 (C) 2006 Rafael J. Wysocki <firstname.lastname@example.org> 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 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. 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 Even opening the device has side effects. Data structures are 28 allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are 29 called. 30 31 The ioctl() commands recognized by the device are: 32 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 36 37 SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE 38 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 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 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) 60 61 SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image 62 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). 66 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) 70 71 SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by 72 SNAPSHOT_ALLOC_SWAP_PAGE 73 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). 82 83 SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support, 84 depending on the argument value (enable, if the argument is nonzero) 85 86 SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation 87 state (eg. ACPI S4) using the platform (eg. ACPI) driver 88 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) 98 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) 105 106 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. 108 109 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). 115 116 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. 123 124 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. 127 128 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. 132 133 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. 139 140 The suspending and resuming utilities MUST lock themselves in memory, 141 preferably using mlockall(), before calling SNAPSHOT_FREEZE. 142 143 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. 165 166 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. 169 170 For details, please refer to the source code.