171 lines
8.4 KiB
Plaintext
Executable File
171 lines
8.4 KiB
Plaintext
Executable File
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://suspend.sourceforge.net>. You may want to have a look at them if you
|
|
are going to develop your own suspend/resume utilities.
|
|
|
|
The interface consists of a character device providing the open(),
|
|
release(), read(), and write() operations as well as several ioctl()
|
|
commands defined in include/linux/suspend_ioctls.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 simultaneous
|
|
reading and writing. It is also impossible to have the device open more than
|
|
once at a time.
|
|
|
|
Even opening the device has side effects. Data structures are
|
|
allocated, and PM_HIBERNATION_PREPARE / PM_RESTORE_PREPARE chains are
|
|
called.
|
|
|
|
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_CREATE_IMAGE
|
|
and SNAPSHOT_ATOMIC_RESTORE to succeed
|
|
|
|
SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
|
|
|
|
SNAPSHOT_CREATE_IMAGE - 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_CREATE_IMAGE 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_PREF_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_GET_IMAGE_SIZE - return the actual size of the hibernation image
|
|
|
|
SNAPSHOT_AVAIL_SWAP_SIZE - 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_ALLOC_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 by
|
|
SNAPSHOT_ALLOC_SWAP_PAGE
|
|
|
|
SNAPSHOT_SET_SWAP_AREA - set the resume partition and the offset (in <PAGE_SIZE>
|
|
units) from the beginning of the partition at which the swap header is
|
|
located (the last ioctl() argument should point to a struct
|
|
resume_swap_area, as defined in kernel/power/suspend_ioctls.h,
|
|
containing the resume device specification and the offset); for swap
|
|
partitions the offset is always 0, but it is different from zero for
|
|
swap files (see Documentation/power/swsusp-and-swap-files.txt for
|
|
details).
|
|
|
|
SNAPSHOT_PLATFORM_SUPPORT - enable/disable the hibernation platform support,
|
|
depending on the argument value (enable, if the argument is nonzero)
|
|
|
|
SNAPSHOT_POWER_OFF - make the kernel transition the system to the hibernation
|
|
state (eg. ACPI S4) using the platform (eg. ACPI) driver
|
|
|
|
SNAPSHOT_S2RAM - suspend to RAM; using this call causes the kernel to
|
|
immediately enter the suspend-to-RAM state, so this call must always
|
|
be preceded by the SNAPSHOT_FREEZE call and it is also necessary
|
|
to use the SNAPSHOT_UNFREEZE call after the system wakes up. This call
|
|
is needed to implement the suspend-to-both mechanism in which the
|
|
suspend image is first created, as though the system had been suspended
|
|
to disk, and then the system is suspended to RAM (this makes it possible
|
|
to resume the system from RAM if there's enough battery power or restore
|
|
its state on the basis of the saved suspend image otherwise)
|
|
|
|
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 across 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_ALLOC_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 partition, called the resume
|
|
partition, or a swap file as storage space (if a swap file is used, the resume
|
|
partition is the partition that holds this file). 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_CREATE_IMAGE and
|
|
mounted afterwards.
|
|
|
|
These utilities MUST NOT make any assumptions regarding the ordering of
|
|
data within the snapshot image. The contents of the image are entirely owned
|
|
by the kernel and its structure may be changed in future kernel releases.
|
|
|
|
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,
|
|
preferably using mlockall(), before calling SNAPSHOT_FREEZE.
|
|
|
|
The suspending utility MUST check the value stored by SNAPSHOT_CREATE_IMAGE
|
|
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, preferably 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_CREATE_IMAGE 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.
|