Some warnings, first.
  
   * BIG FAT WARNING *********************************************************
   *
   * If you touch anything on disk between suspend and resume...
   *				...kiss your data goodbye.
   *
   * If you do resume from initrd after your filesystems are mounted...
   *				...bye bye root partition.
   *			[this is actually same case as above]
   *
   * If you have unsupported (*) devices using DMA, you may have some
   * problems. If your disk driver does not support suspend... (IDE does),
   * it may cause some problems, too. If you change kernel command line
   * between suspend and resume, it may do something wrong. If you change
   * your hardware while system is suspended... well, it was not good idea;
   * but it will probably only crash.
   *
   * (*) suspend/resume support is needed to make it safe.
   *
   * If you have any filesystems on USB devices mounted before software suspend,
   * they won't be accessible after resume and you may lose data, as though
   * you have unplugged the USB devices with mounted filesystems on them;
   * see the FAQ below for details.  (This is not true for more traditional
   * power states like "standby", which normally don't turn USB off.)
  
  You need to append resume=/dev/your_swap_partition to kernel command
  line. Then you suspend by
  
  echo shutdown > /sys/power/disk; echo disk > /sys/power/state
  
  . If you feel ACPI works pretty well on your system, you might try
  
  echo platform > /sys/power/disk; echo disk > /sys/power/state
  
  . If you would like to write hibernation image to swap and then suspend
  to RAM (provided your platform supports it), you can try
  
  echo suspend > /sys/power/disk; echo disk > /sys/power/state
  
  . If you have SATA disks, you'll need recent kernels with SATA suspend
  support. For suspend and resume to work, make sure your disk drivers
  are built into kernel -- not modules. [There's way to make
  suspend/resume with modular disk drivers, see FAQ, but you probably
  should not do that.]
  
  If you want to limit the suspend image size to N bytes, do
  
  echo N > /sys/power/image_size
  
  before suspend (it is limited to 500 MB by default).
  
  . The resume process checks for the presence of the resume device,
  if found, it then checks the contents for the hibernation image signature.
  If both are found, it resumes the hibernation image.
  
  . The resume process may be triggered in two ways:
    1) During lateinit:  If resume=/dev/your_swap_partition is specified on
       the kernel command line, lateinit runs the resume process.  If the
       resume device has not been probed yet, the resume process fails and
       bootup continues.
    2) Manually from an initrd or initramfs:  May be run from
       the init script by using the /sys/power/resume file.  It is vital
       that this be done prior to remounting any filesystems (even as
       read-only) otherwise data may be corrupted.
  
  Article about goals and implementation of Software Suspend for Linux
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Author: Gábor Kuti
  Last revised: 2003-10-20 by Pavel Machek
  
  Idea and goals to achieve
  
  Nowadays it is common in several laptops that they have a suspend button. It
  saves the state of the machine to a filesystem or to a partition and switches
  to standby mode. Later resuming the machine the saved state is loaded back to
  ram and the machine can continue its work. It has two real benefits. First we
  save ourselves the time machine goes down and later boots up, energy costs
  are real high when running from batteries. The other gain is that we don't have to
  interrupt our programs so processes that are calculating something for a long
  time shouldn't need to be written interruptible.
  
  swsusp saves the state of the machine into active swaps and then reboots or
  powerdowns.  You must explicitly specify the swap partition to resume from with
  ``resume='' kernel option. If signature is found it loads and restores saved
  state. If the option ``noresume'' is specified as a boot parameter, it skips
  the resuming.  If the option ``hibernate=nocompress'' is specified as a boot
  parameter, it saves hibernation image without compression.
  
  In the meantime while the system is suspended you should not add/remove any
  of the hardware, write to the filesystems, etc.
  
  Sleep states summary
  ====================
  
  There are three different interfaces you can use, /proc/acpi should
  work like this:
  
  In a really perfect world:
  echo 1 > /proc/acpi/sleep       # for standby
  echo 2 > /proc/acpi/sleep       # for suspend to ram
  echo 3 > /proc/acpi/sleep       # for suspend to ram, but with more power conservative
  echo 4 > /proc/acpi/sleep       # for suspend to disk
  echo 5 > /proc/acpi/sleep       # for shutdown unfriendly the system
  
  and perhaps
  echo 4b > /proc/acpi/sleep      # for suspend to disk via s4bios
  
  Frequently Asked Questions
  ==========================
  
  Q: well, suspending a server is IMHO a really stupid thing,
  but... (Diego Zuccato):
  
  A: You bought new UPS for your server. How do you install it without
  bringing machine down? Suspend to disk, rearrange power cables,
  resume.
  
  You have your server on UPS. Power died, and UPS is indicating 30
  seconds to failure. What do you do? Suspend to disk.
  
  
  Q: Maybe I'm missing something, but why don't the regular I/O paths work?
  
  A: We do use the regular I/O paths. However we cannot restore the data
  to its original location as we load it. That would create an
  inconsistent kernel state which would certainly result in an oops.
  Instead, we load the image into unused memory and then atomically copy
  it back to it original location. This implies, of course, a maximum
  image size of half the amount of memory.
  
  There are two solutions to this:
  
  * require half of memory to be free during suspend. That way you can
  read "new" data onto free spots, then cli and copy
  
  * assume we had special "polling" ide driver that only uses memory
  between 0-640KB. That way, I'd have to make sure that 0-640KB is free
  during suspending, but otherwise it would work...
  
  suspend2 shares this fundamental limitation, but does not include user
  data and disk caches into "used memory" by saving them in
  advance. That means that the limitation goes away in practice.
  
  Q: Does linux support ACPI S4?
  
  A: Yes. That's what echo platform > /sys/power/disk does.
  
  Q: What is 'suspend2'?
  
  A: suspend2 is 'Software Suspend 2', a forked implementation of
  suspend-to-disk which is available as separate patches for 2.4 and 2.6
  kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
  highmem and preemption. It also has a extensible architecture that
  allows for arbitrary transformations on the image (compression,
  encryption) and arbitrary backends for writing the image (eg to swap
  or an NFS share[Work In Progress]). Questions regarding suspend2
  should be sent to the mailing list available through the suspend2
  website, and not to the Linux Kernel Mailing List. We are working
  toward merging suspend2 into the mainline kernel.
  
  Q: What is the freezing of tasks and why are we using it?
  
  A: The freezing of tasks is a mechanism by which user space processes and some
  kernel threads are controlled during hibernation or system-wide suspend (on some
  architectures).  See freezing-of-tasks.txt for details.
  
  Q: What is the difference between "platform" and "shutdown"?
  
  A:
  
  shutdown: save state in linux, then tell bios to powerdown
  
  platform: save state in linux, then tell bios to powerdown and blink
            "suspended led"
  
  "platform" is actually right thing to do where supported, but
  "shutdown" is most reliable (except on ACPI systems).
  
  Q: I do not understand why you have such strong objections to idea of
  selective suspend.
  
  A: Do selective suspend during runtime power management, that's okay. But
  it's useless for suspend-to-disk. (And I do not see how you could use
  it for suspend-to-ram, I hope you do not want that).
  
  Lets see, so you suggest to
  
  * SUSPEND all but swap device and parents
  * Snapshot
  * Write image to disk
  * SUSPEND swap device and parents
  * Powerdown
  
  Oh no, that does not work, if swap device or its parents uses DMA,
  you've corrupted data. You'd have to do
  
  * SUSPEND all but swap device and parents
  * FREEZE swap device and parents
  * Snapshot
  * UNFREEZE swap device and parents
  * Write
  * SUSPEND swap device and parents
  
  Which means that you still need that FREEZE state, and you get more
  complicated code. (And I have not yet introduce details like system
  devices).
  
  Q: There don't seem to be any generally useful behavioral
  distinctions between SUSPEND and FREEZE.
  
  A: Doing SUSPEND when you are asked to do FREEZE is always correct,
  but it may be unnecessarily slow. If you want your driver to stay simple,
  slowness may not matter to you. It can always be fixed later.
  
  For devices like disk it does matter, you do not want to spindown for
  FREEZE.
  
  Q: After resuming, system is paging heavily, leading to very bad interactivity.
  
  A: Try running
  
  cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null
  
  after resume. swapoff -a; swapon -a may also be useful.
  
  Q: What happens to devices during swsusp? They seem to be resumed
  during system suspend?
  
  A: That's correct. We need to resume them if we want to write image to
  disk. Whole sequence goes like
  
        Suspend part
        ~~~~~~~~~~~~
        running system, user asks for suspend-to-disk
  
        user processes are stopped
  
        suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
        		      with state snapshot
  
        state snapshot: copy of whole used memory is taken with interrupts disabled
  
        resume(): devices are woken up so that we can write image to swap
  
        write image to swap
  
        suspend(PMSG_SUSPEND): suspend devices so that we can power off
  
        turn the power off
  
        Resume part
        ~~~~~~~~~~~
        (is actually pretty similar)
  
        running system, user asks for suspend-to-disk
  
        user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows)
  
        read image from disk
  
        suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
        		      with image restoration
  
        image restoration: rewrite memory with image
  
        resume(): devices are woken up so that system can continue
  
        thaw all user processes
  
  Q: What is this 'Encrypt suspend image' for?
  
  A: First of all: it is not a replacement for dm-crypt encrypted swap.
  It cannot protect your computer while it is suspended. Instead it does
  protect from leaking sensitive data after resume from suspend.
  
  Think of the following: you suspend while an application is running
  that keeps sensitive data in memory. The application itself prevents
  the data from being swapped out. Suspend, however, must write these
  data to swap to be able to resume later on. Without suspend encryption
  your sensitive data are then stored in plaintext on disk.  This means
  that after resume your sensitive data are accessible to all
  applications having direct access to the swap device which was used
  for suspend. If you don't need swap after resume these data can remain
  on disk virtually forever. Thus it can happen that your system gets
  broken in weeks later and sensitive data which you thought were
  encrypted and protected are retrieved and stolen from the swap device.
  To prevent this situation you should use 'Encrypt suspend image'.
  
  During suspend a temporary key is created and this key is used to
  encrypt the data written to disk. When, during resume, the data was
  read back into memory the temporary key is destroyed which simply
  means that all data written to disk during suspend are then
  inaccessible so they can't be stolen later on.  The only thing that
  you must then take care of is that you call 'mkswap' for the swap
  partition used for suspend as early as possible during regular
  boot. This asserts that any temporary key from an oopsed suspend or
  from a failed or aborted resume is erased from the swap device.
  
  As a rule of thumb use encrypted swap to protect your data while your
  system is shut down or suspended. Additionally use the encrypted
  suspend image to prevent sensitive data from being stolen after
  resume.
  
  Q: Can I suspend to a swap file?
  
  A: Generally, yes, you can.  However, it requires you to use the "resume=" and
  "resume_offset=" kernel command line parameters, so the resume from a swap file
  cannot be initiated from an initrd or initramfs image.  See
  swsusp-and-swap-files.txt for details.
  
  Q: Is there a maximum system RAM size that is supported by swsusp?
  
  A: It should work okay with highmem.
  
  Q: Does swsusp (to disk) use only one swap partition or can it use
  multiple swap partitions (aggregate them into one logical space)?
  
  A: Only one swap partition, sorry.
  
  Q: If my application(s) causes lots of memory & swap space to be used
  (over half of the total system RAM), is it correct that it is likely
  to be useless to try to suspend to disk while that app is running?
  
  A: No, it should work okay, as long as your app does not mlock()
  it. Just prepare big enough swap partition.
  
  Q: What information is useful for debugging suspend-to-disk problems?
  
  A: Well, last messages on the screen are always useful. If something
  is broken, it is usually some kernel driver, therefore trying with as
  little as possible modules loaded helps a lot. I also prefer people to
  suspend from console, preferably without X running. Booting with
  init=/bin/bash, then swapon and starting suspend sequence manually
  usually does the trick. Then it is good idea to try with latest
  vanilla kernel.
  
  Q: How can distributions ship a swsusp-supporting kernel with modular
  disk drivers (especially SATA)?
  
  A: Well, it can be done, load the drivers, then do echo into
  /sys/power/resume file from initrd. Be sure not to mount
  anything, not even read-only mount, or you are going to lose your
  data.
  
  Q: How do I make suspend more verbose?
  
  A: If you want to see any non-error kernel messages on the virtual
  terminal the kernel switches to during suspend, you have to set the
  kernel console loglevel to at least 4 (KERN_WARNING), for example by
  doing
  
  	# save the old loglevel
  	read LOGLEVEL DUMMY < /proc/sys/kernel/printk
  	# set the loglevel so we see the progress bar.
  	# if the level is higher than needed, we leave it alone.
  	if [ $LOGLEVEL -lt 5 ]; then
  	        echo 5 > /proc/sys/kernel/printk
  		fi
  
          IMG_SZ=0
          read IMG_SZ < /sys/power/image_size
          echo -n disk > /sys/power/state
          RET=$?
          #
          # the logic here is:
          # if image_size > 0 (without kernel support, IMG_SZ will be zero),
          # then try again with image_size set to zero.
  	if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
                  echo 0 > /sys/power/image_size
                  echo -n disk > /sys/power/state
                  RET=$?
          fi
  
  	# restore previous loglevel
  	echo $LOGLEVEL > /proc/sys/kernel/printk
  	exit $RET
  
  Q: Is this true that if I have a mounted filesystem on a USB device and
  I suspend to disk, I can lose data unless the filesystem has been mounted
  with "sync"?
  
  A: That's right ... if you disconnect that device, you may lose data.
  In fact, even with "-o sync" you can lose data if your programs have
  information in buffers they haven't written out to a disk you disconnect,
  or if you disconnect before the device finished saving data you wrote.
  
  Software suspend normally powers down USB controllers, which is equivalent
  to disconnecting all USB devices attached to your system.
  
  Your system might well support low-power modes for its USB controllers
  while the system is asleep, maintaining the connection, using true sleep
  modes like "suspend-to-RAM" or "standby".  (Don't write "disk" to the
  /sys/power/state file; write "standby" or "mem".)  We've not seen any
  hardware that can use these modes through software suspend, although in
  theory some systems might support "platform" modes that won't break the
  USB connections.
  
  Remember that it's always a bad idea to unplug a disk drive containing a
  mounted filesystem.  That's true even when your system is asleep!  The
  safest thing is to unmount all filesystems on removable media (such USB,
  Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
  before suspending; then remount them after resuming.
  
  There is a work-around for this problem.  For more information, see
  Documentation/usb/persist.txt.
  
  Q: Can I suspend-to-disk using a swap partition under LVM?
  
  A: No. You can suspend successfully, but you'll not be able to
  resume. uswsusp should be able to work with LVM. See suspend.sf.net.
  
  Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
  compiled with the similar configuration files. Anyway I found that
  suspend to disk (and resume) is much slower on 2.6.16 compared to
  2.6.15. Any idea for why that might happen or how can I speed it up?
  
  A: This is because the size of the suspend image is now greater than
  for 2.6.15 (by saving more data we can get more responsive system
  after resume).
  
  There's the /sys/power/image_size knob that controls the size of the
  image.  If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
  root), the 2.6.15 behavior should be restored.  If it is still too
  slow, take a look at suspend.sf.net -- userland suspend is faster and
  supports LZF compression to speed it up further.