[PATCH] swsusp: update documentation

From kernel/suspend.c:

Some warnings, first.

 * 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 do resume from initrd after your filesystems are mounted...

 *				...bye bye root partition.

 *			[this is actually same case as above]

 *

 * If you change your hardware while system is suspended...

 *			        ...well, it was not good idea.

 * 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.

echo platform > /sys/power/disk; echo disk > /sys/power/state

Encrypted suspend image:

------------------------

If you want to store your suspend image encrypted with a temporary

key to prevent data gathering after resume you must compile

crypto and the aes algorithm into the kernel - modules won't work

as they cannot be loaded at resume time.

Article about goals and implementation of Software Suspend for Linux

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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: 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 image to prevent sensitive data from being stolen after

resume.

Q: Why we cannot suspend to a swap file?

Q: Why can't we suspend to a swap file?

A: Because accessing swap file needs the filesystem mounted, and

filesystem might do something wrong (like replaying the journal)

during mount. [Probably could be solved by modifying every filesystem

to support some kind of "really read-only!" option. Patches welcome.]

during mount.

There are few ways to get that fixed:

1) Probably could be solved by modifying every filesystem to support

some kind of "really read-only!" option. Patches welcome.

2) suspend2 gets around that by storing absolute positions in on-disk

image (and blocksize), with resume parameter pointing directly to

suspend header.

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 usefull 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.

IBM TP X20			??? (*)

IBM TP X30			s3_bios (2)

IBM TP X31 / Type 2672-XXH      none (1), use radeontool (http://fdd.com/software/radeon/) to turn off backlight.

IBM TP X32			none (1), but backlight is on and video is trashed after long suspend

IBM Thinkpad X40 Type 2371-7JG  s3_bios,s3_mode (4)

Medion MD4220			??? (*)

Samsung P35			vbetool needed (6)

config SOFTWARE_SUSPEND

	bool "Software Suspend"

	depends on EXPERIMENTAL && PM && SWAP && ((X86 && SMP) || ((FVR || PPC32 || X86) && !SMP))

	depends on PM && SWAP && (X86 || ((FVR || PPC32) && !SMP))

	---help---

	  Enable the possibility of suspending the machine.

	  It doesn't need APM.