Documentation/power/swsusp.txt - pub/scm/linux/kernel/git/joro/linux - Git at Google

 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.

 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
 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
 - 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..

 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


 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: 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.
	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.

	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
	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
	- 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..

	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


	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: 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.