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 73SNAPSHOT_AVAIL_SWAP_SIZE 74 return the amount of available swap in bytes (the 75 last argument should be a pointer to an unsigned int variable that will 76 contain the result if the call is successful). 77 78SNAPSHOT_ALLOC_SWAP_PAGE 79 allocate a swap page from the resume partition 80 (the last argument should be a pointer to a loff_t variable that 81 will contain the swap page offset if the call is successful) 82 83SNAPSHOT_FREE_SWAP_PAGES 84 free all swap pages allocated by 85 SNAPSHOT_ALLOC_SWAP_PAGE 86 87SNAPSHOT_SET_SWAP_AREA 88 set the resume partition and the offset (in <PAGE_SIZE> 89 units) from the beginning of the partition at which the swap header is 90 located (the last ioctl() argument should point to a struct 91 resume_swap_area, as defined in kernel/power/suspend_ioctls.h, 92 containing the resume device specification and the offset); for swap 93 partitions the offset is always 0, but it is different from zero for 94 swap files (see Documentation/power/swsusp-and-swap-files.rst for 95 details). 96 97SNAPSHOT_PLATFORM_SUPPORT 98 enable/disable the hibernation platform support, 99 depending on the argument value (enable, if the argument is nonzero) 100 101SNAPSHOT_POWER_OFF 102 make the kernel transition the system to the hibernation 103 state (eg. ACPI S4) using the platform (eg. ACPI) driver 104 105SNAPSHOT_S2RAM 106 suspend to RAM; using this call causes the kernel to 107 immediately enter the suspend-to-RAM state, so this call must always 108 be preceded by the SNAPSHOT_FREEZE call and it is also necessary 109 to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call 110 is needed to implement the suspend-to-both mechanism in which the 111 suspend image is first created, as though the system had been suspended 112 to disk, and then the system is suspended to RAM (this makes it possible 113 to resume the system from RAM if there's enough battery power or restore 114 its state on the basis of the saved suspend image otherwise) 115 116The device's read() operation can be used to transfer the snapshot image from 117the kernel. It has the following limitations: 118 119- you cannot read() more than one virtual memory page at a time 120- read()s across page boundaries are impossible (ie. if you read() 1/2 of 121 a page in the previous call, you will only be able to read() 122 **at most** 1/2 of the page in the next call) 123 124The device's write() operation is used for uploading the system memory snapshot 125into the kernel. It has the same limitations as the read() operation. 126 127The release() operation frees all memory allocated for the snapshot image 128and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any). 129Thus it is not necessary to use either SNAPSHOT_FREE or 130SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also 131unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are 132still frozen when the device is being closed). 133 134Currently it is assumed that the userland utilities reading/writing the 135snapshot image from/to the kernel will use a swap partition, called the resume 136partition, or a swap file as storage space (if a swap file is used, the resume 137partition is the partition that holds this file). However, this is not really 138required, as they can use, for example, a special (blank) suspend partition or 139a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and 140mounted afterwards. 141 142These utilities MUST NOT make any assumptions regarding the ordering of 143data within the snapshot image. The contents of the image are entirely owned 144by the kernel and its structure may be changed in future kernel releases. 145 146The snapshot image MUST be written to the kernel unaltered (ie. all of the image 147data, metadata and header MUST be written in _exactly_ the same amount, form 148and order in which they have been read). Otherwise, the behavior of the 149resumed system may be totally unpredictable. 150 151While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the 152structure of the snapshot image is consistent with the information stored 153in the image header. If any inconsistencies are detected, 154SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof 155mechanism and the userland utilities using the interface SHOULD use additional 156means, such as checksums, to ensure the integrity of the snapshot image. 157 158The suspending and resuming utilities MUST lock themselves in memory, 159preferably using mlockall(), before calling SNAPSHOT_FREEZE. 160 161The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE 162in the memory location pointed to by the last argument of ioctl() and proceed 163in accordance with it: 164 1651. If the value is 1 (ie. the system memory snapshot has just been 166 created and the system is ready for saving it): 167 168 (a) The suspending utility MUST NOT close the snapshot device 169 _unless_ the whole suspend procedure is to be cancelled, in 170 which case, if the snapshot image has already been saved, the 171 suspending utility SHOULD destroy it, preferably by zapping 172 its header. If the suspend is not to be cancelled, the 173 system MUST be powered off or rebooted after the snapshot 174 image has been saved. 175 (b) The suspending utility SHOULD NOT attempt to perform any 176 file system operations (including reads) on the file systems 177 that were mounted before SNAPSHOT_CREATE_IMAGE has been 178 called. However, it MAY mount a file system that was not 179 mounted at that time and perform some operations on it (eg. 180 use it for saving the image). 181 1822. If the value is 0 (ie. the system state has just been restored from 183 the snapshot image), the suspending utility MUST close the snapshot 184 device. Afterwards it will be treated as a regular userland process, 185 so it need not exit. 186 187The resuming utility SHOULD NOT attempt to mount any file systems that could 188be mounted before suspend and SHOULD NOT attempt to perform any operations 189involving such file systems. 190 191For details, please refer to the source code. 192