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+Documentation for userland software suspend interface
+ (C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
+First, the warnings at the beginning of swsusp.txt still apply.
+Second, you should read the FAQ in swsusp.txt _now_ if you have not
+done it already.
+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
+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.
+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.
+The ioctl() commands recognized by the device are:
+SNAPSHOT_FREEZE - freeze user space processes (the current process is
+ not frozen); this is required for SNAPSHOT_ATOMIC_SNAPSHOT
+SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
+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
+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
+SNAPSHOT_FREE - free memory allocated for the snapshot image
+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)
+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).
+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)
+SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with
+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
+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)
+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.
+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).
+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.
+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.
+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.
+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.
+The suspending and resuming utilities MUST lock themselves in memory,
+preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
+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.
+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.
+For details, please refer to the source code.