1Documentation 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 12<http://suspend.sourceforge.net>. You may want to have a look at them if you 13are going to develop your own suspend/resume utilities. 14 15The interface consists of a character device providing the open(), 16release(), read(), and write() operations as well as several ioctl() 17commands defined in kernel/power/power.h. The major and minor 18numbers 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 23assumed 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. 26 27The ioctl() commands recognized by the device are: 28 29SNAPSHOT_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 33SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE 34 35SNAPSHOT_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 44SNAPSHOT_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 50SNAPSHOT_FREE - free memory allocated for the snapshot image 51 52SNAPSHOT_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 57SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last 58 argument should be a pointer to an unsigned int variable that will 59 contain the result if the call is successful). 60 61SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition 62 (the last argument should be a pointer to a loff_t variable that 63 will contain the swap page offset if the call is successful) 64 65SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with 66 SNAPSHOT_GET_SWAP_PAGE 67 68SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument 69 should specify the device's major and minor numbers in the old 70 two-byte format, as returned by the stat() function in the .st_rdev 71 member of the stat structure) 72 73SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE> 74 units) from the beginning of the partition at which the swap header is 75 located (the last ioctl() argument should point to a struct 76 resume_swap_area, as defined in kernel/power/power.h, containing the 77 resume device specification, as for the SNAPSHOT_SET_SWAP_FILE ioctl(), 78 and the offset); for swap partitions the offset is always 0, but it is 79 different to zero for swap files (please see 80 Documentation/swsusp-and-swap-files.txt for details). 81 The SNAPSHOT_SET_SWAP_AREA ioctl() is considered as a replacement for 82 SNAPSHOT_SET_SWAP_FILE which is regarded as obsolete. It is 83 recommended to always use this call, because the code to set the resume 84 partition may be removed from future kernels 85 86SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to 87 immediately enter the suspend-to-RAM state, so this call must always 88 be preceded by the SNAPSHOT_FREEZE call and it is also necessary 89 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call 90 is needed to implement the suspend-to-both mechanism in which the 91 suspend image is first created, as though the system had been suspended 92 to disk, and then the system is suspended to RAM (this makes it possible 93 to resume the system from RAM if there's enough battery power or restore 94 its state on the basis of the saved suspend image otherwise) 95 96SNAPSHOT_PMOPS - enable the usage of the hibernation_ops->prepare, 97 hibernate_ops->enter and hibernation_ops->finish methods (the in-kernel 98 swsusp knows these as the "platform method") which are needed on many 99 machines to (among others) speed up the resume by letting the BIOS skip 100 some steps or to let the system recognise the correct state of the 101 hardware after the resume (in particular on many machines this ensures 102 that unplugged AC adapters get correctly detected and that kacpid does 103 not run wild after the resume). The last ioctl() argument can take one 104 of the three values, defined in kernel/power/power.h: 105 PMOPS_PREPARE - make the kernel carry out the 106 hibernation_ops->prepare() operation 107 PMOPS_ENTER - make the kernel power off the system by calling 108 hibernation_ops->enter() 109 PMOPS_FINISH - make the kernel carry out the 110 hibernation_ops->finish() operation 111 Note that the actual constants are misnamed because they surface 112 internal kernel implementation details that have changed. 113 114The device's read() operation can be used to transfer the snapshot image from 115the kernel. It has the following limitations: 116- you cannot read() more than one virtual memory page at a time 117- read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of 118 a page in the previous call, you will only be able to read() 119 _at_ _most_ 1/2 of the page in the next call) 120 121The device's write() operation is used for uploading the system memory snapshot 122into the kernel. It has the same limitations as the read() operation. 123 124The release() operation frees all memory allocated for the snapshot image 125and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any). 126Thus it is not necessary to use either SNAPSHOT_FREE or 127SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also 128unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are 129still frozen when the device is being closed). 130 131Currently it is assumed that the userland utilities reading/writing the 132snapshot image from/to the kernel will use a swap parition, called the resume 133partition, or a swap file as storage space (if a swap file is used, the resume 134partition is the partition that holds this file). However, this is not really 135required, as they can use, for example, a special (blank) suspend partition or 136a file on a partition that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and 137mounted afterwards. 138 139These utilities SHOULD NOT make any assumptions regarding the ordering of 140data within the snapshot image, except for the image header that MAY be 141assumed to start with an swsusp_info structure, as specified in 142kernel/power/power.h. This structure MAY be used by the userland utilities 143to obtain some information about the snapshot image, such as the size 144of the snapshot image, including the metadata and the header itself, 145contained in the .size member of swsusp_info. 146 147The snapshot image MUST be written to the kernel unaltered (ie. all of the image 148data, metadata and header MUST be written in _exactly_ the same amount, form 149and order in which they have been read). Otherwise, the behavior of the 150resumed system may be totally unpredictable. 151 152While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the 153structure of the snapshot image is consistent with the information stored 154in the image header. If any inconsistencies are detected, 155SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof 156mechanism and the userland utilities using the interface SHOULD use additional 157means, such as checksums, to ensure the integrity of the snapshot image. 158 159The suspending and resuming utilities MUST lock themselves in memory, 160preferrably using mlockall(), before calling SNAPSHOT_FREEZE. 161 162The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT 163in the memory location pointed to by the last argument of ioctl() and proceed 164in accordance with it: 1651. If the value is 1 (ie. the system memory snapshot has just been 166 created and the system is ready for saving it): 167 (a) The suspending utility MUST NOT close the snapshot device 168 _unless_ the whole suspend procedure is to be cancelled, in 169 which case, if the snapshot image has already been saved, the 170 suspending utility SHOULD destroy it, preferrably by zapping 171 its header. If the suspend is not to be cancelled, the 172 system MUST be powered off or rebooted after the snapshot 173 image has been saved. 174 (b) The suspending utility SHOULD NOT attempt to perform any 175 file system operations (including reads) on the file systems 176 that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been 177 called. However, it MAY mount a file system that was not 178 mounted at that time and perform some operations on it (eg. 179 use it for saving the image). 1802. If the value is 0 (ie. the system state has just been restored from 181 the snapshot image), the suspending utility MUST close the snapshot 182 device. Afterwards it will be treated as a regular userland process, 183 so it need not exit. 184 185The resuming utility SHOULD NOT attempt to mount any file systems that could 186be mounted before suspend and SHOULD NOT attempt to perform any operations 187involving such file systems. 188 189For details, please refer to the source code. 190