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8.7 KiB
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194 lines
8.7 KiB
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=====================================================
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Documentation for userland software suspend interface
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=====================================================
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(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
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done it already.
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Now, to use the userland interface for software suspend you need special
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utilities that will read/write the system memory snapshot from/to the
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kernel. Such utilities are available, for example, from
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<http://suspend.sourceforge.net>. You may want to have a look at them if you
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are going to develop your own suspend/resume utilities.
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The interface consists of a character device providing the open(),
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release(), read(), and write() operations as well as several ioctl()
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commands defined in include/linux/suspend_ioctls.h . The major and minor
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numbers of the device are, respectively, 10 and 231, and they can
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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
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reading, it is considered to be in the suspend mode. Otherwise it is
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assumed to be in the resume mode. The device cannot be open for simultaneous
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reading and writing. It is also impossible to have the device open more than
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once at a time.
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Even opening the device has side effects. Data structures are
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allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
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called.
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The ioctl() commands recognized by the device are:
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SNAPSHOT_FREEZE
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freeze user space processes (the current process is
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not frozen); this is required for SNAPSHOT_CREATE_IMAGE
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and SNAPSHOT_ATOMIC_RESTORE to succeed
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SNAPSHOT_UNFREEZE
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thaw user space processes frozen by SNAPSHOT_FREEZE
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SNAPSHOT_CREATE_IMAGE
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create a snapshot of the system memory; the
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last argument of ioctl() should be a pointer to an int variable,
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the value of which will indicate whether the call returned after
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creating the snapshot (1) or after restoring the system memory state
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from it (0) (after resume the system finds itself finishing the
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SNAPSHOT_CREATE_IMAGE ioctl() again); after the snapshot
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has been created the read() operation can be used to transfer
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it out of the kernel
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SNAPSHOT_ATOMIC_RESTORE
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restore the system memory state from the
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uploaded snapshot image; before calling it you should transfer
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the system memory snapshot back to the kernel using the write()
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operation; this call will not succeed if the snapshot
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image is not available to the kernel
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SNAPSHOT_FREE
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free memory allocated for the snapshot image
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SNAPSHOT_PREF_IMAGE_SIZE
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set the preferred maximum size of the image
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(the kernel will do its best to ensure the image size will not exceed
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this number, but if it turns out to be impossible, the kernel will
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create the smallest image possible)
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SNAPSHOT_GET_IMAGE_SIZE
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return the actual size of the hibernation image
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(the last argument should be a pointer to a loff_t variable that
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will contain the result if the call is successful)
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SNAPSHOT_AVAIL_SWAP_SIZE
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return the amount of available swap in bytes
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(the last argument should be a pointer to a loff_t variable that
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will contain the result if the call is successful)
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SNAPSHOT_ALLOC_SWAP_PAGE
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allocate a swap page from the resume partition
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(the last argument should be a pointer to a loff_t variable that
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will contain the swap page offset if the call is successful)
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SNAPSHOT_FREE_SWAP_PAGES
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free all swap pages allocated by
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SNAPSHOT_ALLOC_SWAP_PAGE
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SNAPSHOT_SET_SWAP_AREA
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set the resume partition and the offset (in <PAGE_SIZE>
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units) from the beginning of the partition at which the swap header is
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located (the last ioctl() argument should point to a struct
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resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
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containing the resume device specification and the offset); for swap
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partitions the offset is always 0, but it is different from zero for
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swap files (see Documentation/power/swsusp-and-swap-files.rst for
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details).
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SNAPSHOT_PLATFORM_SUPPORT
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enable/disable the hibernation platform support,
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depending on the argument value (enable, if the argument is nonzero)
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SNAPSHOT_POWER_OFF
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make the kernel transition the system to the hibernation
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state (eg. ACPI S4) using the platform (eg. ACPI) driver
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SNAPSHOT_S2RAM
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suspend to RAM; using this call causes the kernel to
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immediately enter the suspend-to-RAM state, so this call must always
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be preceded by the SNAPSHOT_FREEZE call and it is also necessary
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to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
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is needed to implement the suspend-to-both mechanism in which the
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suspend image is first created, as though the system had been suspended
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to disk, and then the system is suspended to RAM (this makes it possible
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to resume the system from RAM if there's enough battery power or restore
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its state on the basis of the saved suspend image otherwise)
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The device's read() operation can be used to transfer the snapshot image from
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the kernel. It has the following limitations:
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- you cannot read() more than one virtual memory page at a time
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- read()s across page boundaries are impossible (ie. if you read() 1/2 of
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a page in the previous call, you will only be able to read()
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**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
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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
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and all swap pages allocated with SNAPSHOT_ALLOC_SWAP_PAGE (if any).
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Thus it is not necessary to use either SNAPSHOT_FREE or
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SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
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unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
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still frozen when the device is being closed).
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Currently it is assumed that the userland utilities reading/writing the
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snapshot image from/to the kernel will use a swap partition, called the resume
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partition, or a swap file as storage space (if a swap file is used, the resume
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partition is the partition that holds this file). However, this is not really
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required, as they can use, for example, a special (blank) suspend partition or
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a file on a partition that is unmounted before SNAPSHOT_CREATE_IMAGE and
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mounted afterwards.
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These utilities MUST NOT make any assumptions regarding the ordering of
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data within the snapshot image. The contents of the image are entirely owned
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by the kernel and its structure may be changed in future kernel releases.
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The snapshot image MUST be written to the kernel unaltered (ie. all of the image
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data, metadata and header MUST be written in _exactly_ the same amount, form
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and order in which they have been read). Otherwise, the behavior of the
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resumed system may be totally unpredictable.
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While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
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structure of the snapshot image is consistent with the information stored
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in the image header. If any inconsistencies are detected,
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SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
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mechanism and the userland utilities using the interface SHOULD use additional
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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,
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preferably using mlockall(), before calling SNAPSHOT_FREEZE.
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The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
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in the memory location pointed to by the last argument of ioctl() and proceed
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in accordance with it:
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1. If the value is 1 (ie. the system memory snapshot has just been
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created and the system is ready for saving it):
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(a) The suspending utility MUST NOT close the snapshot device
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_unless_ the whole suspend procedure is to be cancelled, in
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which case, if the snapshot image has already been saved, the
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suspending utility SHOULD destroy it, preferably by zapping
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its header. If the suspend is not to be cancelled, the
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system MUST be powered off or rebooted after the snapshot
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image has been saved.
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(b) The suspending utility SHOULD NOT attempt to perform any
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file system operations (including reads) on the file systems
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that were mounted before SNAPSHOT_CREATE_IMAGE has been
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called. However, it MAY mount a file system that was not
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mounted at that time and perform some operations on it (eg.
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use it for saving the image).
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2. If the value is 0 (ie. the system state has just been restored from
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the snapshot image), the suspending utility MUST close the snapshot
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device. Afterwards it will be treated as a regular userland process,
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so it need not exit.
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The resuming utility SHOULD NOT attempt to mount any file systems that could
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be mounted before suspend and SHOULD NOT attempt to perform any operations
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involving such file systems.
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For details, please refer to the source code.
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