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From kernel/suspend.c:

 * BIG FAT WARNING *********************************************************
 *
 * If you have unsupported (*) devices using DMA...
 *				...say goodbye to your data.
 *
 * If you touch anything on disk between suspend and resume...
 *				...kiss your data goodbye.
 *
 * If your disk driver does not support suspend... (IDE does)
 *				...you'd better find out how to get along
 *				   without your data.
 *
 * If you change kernel command line between suspend and resume...
 *			        ...prepare for nasty fsck or worse.
 *
 * If you change your hardware while system is suspended...
 *			        ...well, it was not good idea.
 *
 * (*) suspend/resume support is needed to make it safe.

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



Article about goals and implementation of Software Suspend for Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Author: G��bor 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.

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.

Ethernet card in your server died. You want to replace it. Your
server is not hotplug capable. What do you do? Suspend to disk,
replace ethernet card, resume. If you are fast your users will not
even see broken connections.


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: My machine doesn't work with ACPI. How can I use swsusp than ?

A: Do a reboot() syscall with right parameters. Warning: glibc gets in
its way, so check with strace:

reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, 0xd000fce2)

(Thanks to Peter Osterlund:)

#include <unistd.h>
#include <syscall.h>

#define LINUX_REBOOT_MAGIC1     0xfee1dead
#define LINUX_REBOOT_MAGIC2     672274793
#define LINUX_REBOOT_CMD_SW_SUSPEND     0xD000FCE2

int main()
{
    syscall(SYS_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
            LINUX_REBOOT_CMD_SW_SUSPEND, 0);
    return 0;
}

Also /sys/ interface should be still present.

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: A kernel thread must voluntarily freeze itself (call 'refrigerator').
I found some kernel threads that don't do it, and they don't freeze
so the system can't sleep. Is this a known behavior?

A: All such kernel threads need to be fixed, one by one. Select the
place where the thread is safe to be frozen (no kernel semaphores
should be held at that point and it must be safe to sleep there), and
add:

       try_to_freeze();

If the thread is needed for writing the image to storage, you should
instead set the PF_NOFREEZE process flag when creating the thread (and
be very carefull).


Q: What is the difference between between "platform", "shutdown" and
"firmware" in /sys/power/disk?

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"

firmware: tell bios to save state itself [needs BIOS-specific suspend
	  partition, and has very little to do with swsusp]

"platform" is actually right thing to do, but "shutdown" is most
reliable.

Q: I do not understand why you have such strong objections to idea of
selective suspend.

A: Do selective suspend during runtime power managment, that's okay. But
its 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 unneccessarily slow. If you want USB 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 heavilly, 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 usefull.

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, noone 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.