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| Original file line number | Diff line number | Diff line change |
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| Documentation for userland software suspend interface | ||
| (C) 2006 Rafael J. Wysocki <rjw@sisk.pl> | ||
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| First, the warnings at the beginning of swsusp.txt still apply. | ||
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| Second, you should read the FAQ in swsusp.txt _now_ if you have not | ||
| done it already. | ||
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| Now, to use the userland interface for software suspend you need special | ||
| utilities that will read/write the system memory snapshot from/to the | ||
| kernel. Such utilities are available, for example, from | ||
| <http://www.sisk.pl/kernel/utilities/suspend>. You may want to have | ||
| a look at them if you are going to develop your own suspend/resume | ||
| utilities. | ||
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| The interface consists of a character device providing the open(), | ||
| release(), read(), and write() operations as well as several ioctl() | ||
| commands defined in kernel/power/power.h. The major and minor | ||
| numbers of the device are, respectively, 10 and 231, and they can | ||
| be read from /sys/class/misc/snapshot/dev. | ||
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| The device can be open either for reading or for writing. If open for | ||
| reading, it is considered to be in the suspend mode. Otherwise it is | ||
| assumed to be in the resume mode. The device cannot be open for reading | ||
| and writing. It is also impossible to have the device open more than once | ||
| at a time. | ||
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| The ioctl() commands recognized by the device are: | ||
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| SNAPSHOT_FREEZE - freeze user space processes (the current process is | ||
| not frozen); this is required for SNAPSHOT_ATOMIC_SNAPSHOT | ||
| and SNAPSHOT_ATOMIC_RESTORE to succeed | ||
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| SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE | ||
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| SNAPSHOT_ATOMIC_SNAPSHOT - create a snapshot of the system memory; the | ||
| last argument of ioctl() should be a pointer to an int variable, | ||
| the value of which will indicate whether the call returned after | ||
| creating the snapshot (1) or after restoring the system memory state | ||
| from it (0) (after resume the system finds itself finishing the | ||
| SNAPSHOT_ATOMIC_SNAPSHOT ioctl() again); after the snapshot | ||
| has been created the read() operation can be used to transfer | ||
| it out of the kernel | ||
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| SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the | ||
| uploaded snapshot image; before calling it you should transfer | ||
| the system memory snapshot back to the kernel using the write() | ||
| operation; this call will not succeed if the snapshot | ||
| image is not available to the kernel | ||
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| SNAPSHOT_FREE - free memory allocated for the snapshot image | ||
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| SNAPSHOT_SET_IMAGE_SIZE - set the preferred maximum size of the image | ||
| (the kernel will do its best to ensure the image size will not exceed | ||
| this number, but if it turns out to be impossible, the kernel will | ||
| create the smallest image possible) | ||
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| SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last | ||
| argument should be a pointer to an unsigned int variable that will | ||
| contain the result if the call is successful). | ||
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| SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition | ||
| (the last argument should be a pointer to a loff_t variable that | ||
| will contain the swap page offset if the call is successful) | ||
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| SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with | ||
| SNAPSHOT_GET_SWAP_PAGE | ||
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| SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument | ||
| should specify the device's major and minor numbers in the old | ||
| two-byte format, as returned by the stat() function in the .st_rdev | ||
| member of the stat structure); it is recommended to always use this | ||
| call, because the code to set the resume partition could be removed from | ||
| future kernels | ||
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| The device's read() operation can be used to transfer the snapshot image from | ||
| the kernel. It has the following limitations: | ||
| - you cannot read() more than one virtual memory page at a time | ||
| - read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of | ||
| a page in the previous call, you will only be able to read() | ||
| _at_ _most_ 1/2 of the page in the next call) | ||
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| The device's write() operation is used for uploading the system memory snapshot | ||
| into the kernel. It has the same limitations as the read() operation. | ||
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| The release() operation frees all memory allocated for the snapshot image | ||
| and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any). | ||
| Thus it is not necessary to use either SNAPSHOT_FREE or | ||
| SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also | ||
| unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are | ||
| still frozen when the device is being closed). | ||
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| Currently it is assumed that the userland utilities reading/writing the | ||
| snapshot image from/to the kernel will use a swap parition, called the resume | ||
| partition, as storage space. However, this is not really required, as they | ||
| can use, for example, a special (blank) suspend partition or a file on a partition | ||
| that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and mounted afterwards. | ||
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| These utilities SHOULD NOT make any assumptions regarding the ordering of | ||
| data within the snapshot image, except for the image header that MAY be | ||
| assumed to start with an swsusp_info structure, as specified in | ||
| kernel/power/power.h. This structure MAY be used by the userland utilities | ||
| to obtain some information about the snapshot image, such as the size | ||
| of the snapshot image, including the metadata and the header itself, | ||
| contained in the .size member of swsusp_info. | ||
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| The snapshot image MUST be written to the kernel unaltered (ie. all of the image | ||
| data, metadata and header MUST be written in _exactly_ the same amount, form | ||
| and order in which they have been read). Otherwise, the behavior of the | ||
| resumed system may be totally unpredictable. | ||
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| While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the | ||
| structure of the snapshot image is consistent with the information stored | ||
| in the image header. If any inconsistencies are detected, | ||
| SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof | ||
| mechanism and the userland utilities using the interface SHOULD use additional | ||
| means, such as checksums, to ensure the integrity of the snapshot image. | ||
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| The suspending and resuming utilities MUST lock themselves in memory, | ||
| preferrably using mlockall(), before calling SNAPSHOT_FREEZE. | ||
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| The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT | ||
| in the memory location pointed to by the last argument of ioctl() and proceed | ||
| in accordance with it: | ||
| 1. If the value is 1 (ie. the system memory snapshot has just been | ||
| created and the system is ready for saving it): | ||
| (a) The suspending utility MUST NOT close the snapshot device | ||
| _unless_ the whole suspend procedure is to be cancelled, in | ||
| which case, if the snapshot image has already been saved, the | ||
| suspending utility SHOULD destroy it, preferrably by zapping | ||
| its header. If the suspend is not to be cancelled, the | ||
| system MUST be powered off or rebooted after the snapshot | ||
| image has been saved. | ||
| (b) The suspending utility SHOULD NOT attempt to perform any | ||
| file system operations (including reads) on the file systems | ||
| that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been | ||
| called. However, it MAY mount a file system that was not | ||
| mounted at that time and perform some operations on it (eg. | ||
| use it for saving the image). | ||
| 2. If the value is 0 (ie. the system state has just been restored from | ||
| the snapshot image), the suspending utility MUST close the snapshot | ||
| device. Afterwards it will be treated as a regular userland process, | ||
| so it need not exit. | ||
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| The resuming utility SHOULD NOT attempt to mount any file systems that could | ||
| be mounted before suspend and SHOULD NOT attempt to perform any operations | ||
| involving such file systems. | ||
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| For details, please refer to the source code. |
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