1===================================================== 2Documentation for userland software suspend interface 3===================================================== 4 5 (C) 2006 Rafael J. Wysocki <rjw@sisk.pl> 6 7First, the warnings at the beginning of swsusp.txt still apply. 8 9Second, you should read the FAQ in swsusp.txt _now_ if you have not 10done it already. 11 12Now, to use the userland interface for software suspend you need special 13utilities that will read/write the system memory snapshot from/to the 14kernel. Such utilities are available, for example, from 15<http://suspend.sourceforge.net>. You may want to have a look at them if you 16are going to develop your own suspend/resume utilities. 17 18The interface consists of a character device providing the open(), 19release(), read(), and write() operations as well as several ioctl() 20commands defined in include/linux/suspend_ioctls.h . The major and minor 21numbers of the device are, respectively, 10 and 231, and they can 22be read from /sys/class/misc/snapshot/dev. 23 24The device can be open either for reading or for writing. If open for 25reading, it is considered to be in the suspend mode. Otherwise it is 26assumed to be in the resume mode. The device cannot be open for simultaneous 27reading and writing. It is also impossible to have the device open more than 28once at a time. 29 30Even opening the device has side effects. Data structures are 31allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are 32called. 33 34The ioctl() commands recognized by the device are: 35 36SNAPSHOT_FREEZE 37 freeze user space processes (the current process is 38 not frozen); this is required for SNAPSHOT_CREATE_IMAGE 39 and SNAPSHOT_ATOMIC_RESTORE to succeed 40 41SNAPSHOT_UNFREEZE 42 thaw user space processes frozen by SNAPSHOT_FREEZE 43 44SNAPSHOT_CREATE_IMAGE 45 create a snapshot of the system memory; the 46 last argument of ioctl() should be a pointer to an int variable, 47 the value of which will indicate whether the call returned after 48 creating the snapshot (1) or after restoring the system memory state 49 from it (0) (after resume the system finds itself finishing the 50 SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot 51 has been created the read() operation can be used to transfer 52 it out of the kernel 53 54SNAPSHOT_ATOMIC_RESTORE 55 restore the system memory state from the 56 uploaded snapshot image; before calling it you should transfer 57 the system memory snapshot back to the kernel using the write() 58 operation; this call will not succeed if the snapshot 59 image is not available to the kernel 60 61SNAPSHOT_FREE 62 free memory allocated for the snapshot image 63 64SNAPSHOT_PREF_IMAGE_SIZE 65 set the preferred maximum size of the image 66 (the kernel will do its best to ensure the image size will not exceed 67 this number, but if it turns out to be impossible, the kernel will 68 create the smallest image possible) 69 70SNAPSHOT_GET_IMAGE_SIZE 71 return the actual size of the hibernation image 72 (the last argument should be a pointer to a loff_t variable that 73 will contain the result if the call is successful) 74 75SNAPSHOT_AVAIL_SWAP_SIZE 76 return the amount of available swap in bytes 77 (the last argument should be a pointer to a loff_t variable that 78 will contain the result if the call is successful) 79 80SNAPSHOT_ALLOC_SWAP_PAGE 81 allocate a swap page from the resume partition 82 (the last argument should be a pointer to a loff_t variable that 83 will contain the swap page offset if the call is successful) 84 85SNAPSHOT_FREE_SWAP_PAGES 86 free all swap pages allocated by 87 SNAPSHOT_ALLOC_SWAP_PAGE 88 89SNAPSHOT_SET_SWAP_AREA 90 set the resume partition and the offset (in <PAGE_SIZE> 91 units) from the beginning of the partition at which the swap header is 92 located (the last ioctl() argument should point to a struct 93 resume_swap_area, as defined in kernel/power/suspend_ioctls.h, 94 containing the resume device specification and the offset); for swap 95 partitions the offset is always 0, but it is different from zero for 96 swap files (see Documentation/power/swsusp-and-swap-files.rst for 97 details). 98 99SNAPSHOT_PLATFORM_SUPPORT 100 enable/disable the hibernation platform support, 101 depending on the argument value (enable, if the argument is nonzero) 102 103SNAPSHOT_POWER_OFF 104 make the kernel transition the system to the hibernation 105 state (eg. ACPI S4) using the platform (eg. ACPI) driver 106 107SNAPSHOT_S2RAM 108 suspend to RAM; using this call causes the kernel to 109 immediately enter the suspend-to-RAM state, so this call must always 110 be preceded by the SNAPSHOT_FREEZE call and it is also necessary 111 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call 112 is needed to implement the suspend-to-both mechanism in which the 113 suspend image is first created, as though the system had been suspended 114 to disk, and then the system is suspended to RAM (this makes it possible 115 to resume the system from RAM if there's enough battery power or restore 116 its state on the basis of the saved suspend image otherwise) 117 118The device's read() operation can be used to transfer the snapshot image from 119the kernel. It has the following limitations: 120 121- you cannot read() more than one virtual memory page at a time 122- read()s across page boundaries are impossible (ie. if you read() 1/2 of 123 a page in the previous call, you will only be able to read() 124 **at most** 1/2 of the page in the next call) 125 126The device's write() operation is used for uploading the system memory snapshot 127into the kernel. It has the same limitations as the read() operation. 128 129The release() operation frees all memory allocated for the snapshot image 130and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any). 131Thus it is not necessary to use either SNAPSHOT_FREE or 132SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also 133unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are 134still frozen when the device is being closed). 135 136Currently it is assumed that the userland utilities reading/writing the 137snapshot image from/to the kernel will use a swap partition, called the resume 138partition, or a swap file as storage space (if a swap file is used, the resume 139partition is the partition that holds this file). However, this is not really 140required, as they can use, for example, a special (blank) suspend partition or 141a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and 142mounted afterwards. 143 144These utilities MUST NOT make any assumptions regarding the ordering of 145data within the snapshot image. The contents of the image are entirely owned 146by the kernel and its structure may be changed in future kernel releases. 147 148The snapshot image MUST be written to the kernel unaltered (ie. all of the image 149data, metadata and header MUST be written in _exactly_ the same amount, form 150and order in which they have been read). Otherwise, the behavior of the 151resumed system may be totally unpredictable. 152 153While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the 154structure of the snapshot image is consistent with the information stored 155in the image header. If any inconsistencies are detected, 156SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof 157mechanism and the userland utilities using the interface SHOULD use additional 158means, such as checksums, to ensure the integrity of the snapshot image. 159 160The suspending and resuming utilities MUST lock themselves in memory, 161preferably using mlockall(), before calling SNAPSHOT_FREEZE. 162 163The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE 164in the memory location pointed to by the last argument of ioctl() and proceed 165in accordance with it: 166 1671. If the value is 1 (ie. the system memory snapshot has just been 168 created and the system is ready for saving it): 169 170 (a) The suspending utility MUST NOT close the snapshot device 171 _unless_ the whole suspend procedure is to be cancelled, in 172 which case, if the snapshot image has already been saved, the 173 suspending utility SHOULD destroy it, preferably by zapping 174 its header. If the suspend is not to be cancelled, the 175 system MUST be powered off or rebooted after the snapshot 176 image has been saved. 177 (b) The suspending utility SHOULD NOT attempt to perform any 178 file system operations (including reads) on the file systems 179 that were mounted before SNAPSHOT_CREATE_IMAGE has been 180 called. However, it MAY mount a file system that was not 181 mounted at that time and perform some operations on it (eg. 182 use it for saving the image). 183 1842. If the value is 0 (ie. the system state has just been restored from 185 the snapshot image), the suspending utility MUST close the snapshot 186 device. Afterwards it will be treated as a regular userland process, 187 so it need not exit. 188 189The resuming utility SHOULD NOT attempt to mount any file systems that could 190be mounted before suspend and SHOULD NOT attempt to perform any operations 191involving such file systems. 192 193For details, please refer to the source code. 194