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 include/linux/suspend_ioctls.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 27Even opening the device has side effects. Data structures are 28allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are 29called. 30 31The ioctl() commands recognized by the device are: 32 33SNAPSHOT_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 37SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE 38 39SNAPSHOT_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 48SNAPSHOT_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 54SNAPSHOT_FREE - free memory allocated for the snapshot image 55 56SNAPSHOT_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 61SNAPSHOT_GET_IMAGE_SIZE - return the actual size of the hibernation image 62 63SNAPSHOT_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 67SNAPSHOT_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 71SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated by 72 SNAPSHOT_ALLOC_SWAP_PAGE 73 74SNAPSHOT_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/swsusp-and-swap-files.txt for details). 81 82SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support, 83 depending on the argument value (enable, if the argument is nonzero) 84 85SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation 86 state (eg. ACPI S4) using the platform (eg. ACPI) driver 87 88SNAPSHOT_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 98The device's read() operation can be used to transfer the snapshot image from 99the 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 105The device's write() operation is used for uploading the system memory snapshot 106into the kernel. It has the same limitations as the read() operation. 107 108The release() operation frees all memory allocated for the snapshot image 109and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any). 110Thus it is not necessary to use either SNAPSHOT_FREE or 111SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also 112unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are 113still frozen when the device is being closed). 114 115Currently it is assumed that the userland utilities reading/writing the 116snapshot image from/to the kernel will use a swap partition, called the resume 117partition, or a swap file as storage space (if a swap file is used, the resume 118partition is the partition that holds this file). However, this is not really 119required, as they can use, for example, a special (blank) suspend partition or 120a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and 121mounted afterwards. 122 123These utilities MUST NOT make any assumptions regarding the ordering of 124data within the snapshot image. The contents of the image are entirely owned 125by the kernel and its structure may be changed in future kernel releases. 126 127The snapshot image MUST be written to the kernel unaltered (ie. all of the image 128data, metadata and header MUST be written in _exactly_ the same amount, form 129and order in which they have been read). Otherwise, the behavior of the 130resumed system may be totally unpredictable. 131 132While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the 133structure of the snapshot image is consistent with the information stored 134in the image header. If any inconsistencies are detected, 135SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof 136mechanism and the userland utilities using the interface SHOULD use additional 137means, such as checksums, to ensure the integrity of the snapshot image. 138 139The suspending and resuming utilities MUST lock themselves in memory, 140preferrably using mlockall(), before calling SNAPSHOT_FREEZE. 141 142The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE 143in the memory location pointed to by the last argument of ioctl() and proceed 144in accordance with it: 1451. 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 156 that were mounted before SNAPSHOT_CREATE_IMAGE has been 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). 1602. 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 165The resuming utility SHOULD NOT attempt to mount any file systems that could 166be mounted before suspend and SHOULD NOT attempt to perform any operations 167involving such file systems. 168 169For details, please refer to the source code. 170