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.
 *
 * (*) pm interface 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 4 > /proc/acpi/sleep.

Pavel's unreliable guide to swsusp mess
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are currently two versions of swap suspend in the kernel, the old
"Pavel's" version in kernel/power/swsusp.c and the new "Patrick's"
version in kernel/power/pmdisk.c. They provide the same functionality;
the old version looks ugly but was tested, while the new version looks
nicer but did not receive so much testing. echo 4 > /proc/acpi/sleep
calls the old version, echo disk > /sys/power/state calls the new one.

[In the future, when the new version is stable enough, two things can
happen:

* the new version is moved into swsusp.c, and swsusp is renamed to swap
  suspend (Pavel prefers this)

* pmdisk is kept as is and swsusp.c is removed from the kernel]



Article about goals and implementation of Software Suspend for Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Author: G‚ábor 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
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.

Using the code

You have two ways to use this code. The first one is is with a patched
SysVinit (my patch is against 2.76 and available at my home page). You
might call 'swsusp' or 'shutdown -z <time>'. Next way is to echo 4 >
/proc/acpi/sleep.

Either way it saves the state of the machine into active swaps and then
reboots.  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 touch any of the
hardware!

About the code

Things to implement
- SMP support. I've done an SMP support but since I don't have access to a kind
  of this one I cannot test it. Please SMP people test it.  .. Tested it,
  doesn't work. Had no time to figure out why. There is some mess with
  interrupts AFAIK..
- We should only make a copy of data related to kernel segment, since any
  process data won't be changed.
- Should make more sanity checks. Or are these enough?

Not so important ideas for implementing

- If a real time process is running then don't suspend the machine.
- Support for adding/removing hardware while suspended?
- We should not free pages at the beginning so aggressively, most of them
  go there anyway..

Drivers that need support
- pc_keyb -- perhaps we can wait for vojtech's input patches
- do IDE cdroms need some kind of support?
- IDE CD-RW -- how to deal with that?

Sleep states summary (thanx, Ducrot)
====================================

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


FAQ:

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: No.

When swsusp was created, ACPI was not too widespread, so we tried to
avoid using ACPI-specific stuff. ACPI also is/was notoriously
buggy. These days swsusp works on APM-only i386 machines and even
without any power managment at all. Some versions also work on PPC.

That means that machine does not enter S4 on suspend-to-disk, but
simply enters S5. That has few advantages, you can for example boot
windows on next boot, and return to your Linux session later. You
could even have few different Linuxes on your box (not sharing any
partitions), and switch between them.

It also has disadvantages. On HP nx5000, if you unplug power cord
while machine is suspended-to-disk, Linux will fail to notice that.

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;
}

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

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

            if (current->flags & PF_FREEZE)
                    refrigerator(PF_FREEZE);

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.