Documentation/power/swsusp.txt - kernel/msm-4.9 - Gitiles

 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: 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.

 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:

             if (current->flags & PF_FREEZE)
                     refrigerator(PF_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.


 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.
	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: 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.

	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:

	if (current->flags & PF_FREEZE)
	refrigerator(PF_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.


	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.