[PATCH] swsusp: userland interface

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

utilities.

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

	and SNAPSHOT_ATOMIC_RESTORE to succeed

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_GET_SWAP_PAGE

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.

	sys_chroot(".");

	mount_devfs_fs ();

	current->flags |= PF_NOFREEZE;

	pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);

	if (pid > 0) {

		while (pid != sys_wait4(-1, NULL, 0, NULL))

obj-y				:= main.o process.o console.o

obj-$(CONFIG_PM_LEGACY)		+= pm.o

obj-$(CONFIG_SOFTWARE_SUSPEND)	+= swsusp.o disk.o snapshot.o swap.o

obj-$(CONFIG_SOFTWARE_SUSPEND)	+= swsusp.o disk.o snapshot.o swap.o user.o

obj-$(CONFIG_SUSPEND_SMP)	+= smp.o

	int			cpus;

	unsigned long		image_pages;

	unsigned long		pages;

	unsigned long		size;

} __attribute__((aligned(PAGE_SIZE)));

extern int snapshot_write_next(struct snapshot_handle *handle, size_t count);

int snapshot_image_loaded(struct snapshot_handle *handle);

#define SNAPSHOT_IOC_MAGIC	'3'

#define SNAPSHOT_FREEZE			_IO(SNAPSHOT_IOC_MAGIC, 1)

#define SNAPSHOT_UNFREEZE		_IO(SNAPSHOT_IOC_MAGIC, 2)

#define SNAPSHOT_ATOMIC_SNAPSHOT	_IOW(SNAPSHOT_IOC_MAGIC, 3, void *)

#define SNAPSHOT_ATOMIC_RESTORE		_IO(SNAPSHOT_IOC_MAGIC, 4)

#define SNAPSHOT_FREE			_IO(SNAPSHOT_IOC_MAGIC, 5)

#define SNAPSHOT_SET_IMAGE_SIZE		_IOW(SNAPSHOT_IOC_MAGIC, 6, unsigned long)

#define SNAPSHOT_AVAIL_SWAP		_IOR(SNAPSHOT_IOC_MAGIC, 7, void *)

#define SNAPSHOT_GET_SWAP_PAGE		_IOR(SNAPSHOT_IOC_MAGIC, 8, void *)

#define SNAPSHOT_FREE_SWAP_PAGES	_IO(SNAPSHOT_IOC_MAGIC, 9)

#define SNAPSHOT_SET_SWAP_FILE		_IOW(SNAPSHOT_IOC_MAGIC, 10, unsigned int)

#define SNAPSHOT_IOC_MAXNR	10

/**

 *	The bitmap is used for tracing allocated swap pages

 *

struct pbe *pagedir_nosave;

static unsigned int nr_copy_pages;

static unsigned int nr_meta_pages;

static unsigned long *buffer;

#ifdef CONFIG_HIGHMEM

unsigned int count_highmem_pages(void)

	nr_copy_pages = 0;

	nr_meta_pages = 0;

	pagedir_nosave = NULL;

	buffer = NULL;

}

	info->cpus = num_online_cpus();

	info->image_pages = nr_copy_pages;

	info->pages = nr_copy_pages + nr_meta_pages + 1;

	info->size = info->pages;

	info->size <<= PAGE_SHIFT;

}

/**

int snapshot_read_next(struct snapshot_handle *handle, size_t count)

{

	static unsigned long *buffer;

	if (handle->page > nr_meta_pages + nr_copy_pages)

		return 0;

	if (!buffer) {

int snapshot_write_next(struct snapshot_handle *handle, size_t count)

{

	static unsigned long *buffer;

	int error = 0;

	if (handle->prev && handle->page > nr_meta_pages + nr_copy_pages)