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-Some warnings, first.
-
- * BIG FAT WARNING *********************************************************
- *
- * If you touch anything on disk between suspend and resume...
- * ...kiss your data goodbye.
- *
- * If you do resume from initrd after your filesystems are mounted...
- * ...bye bye root partition.
- * [this is actually same case as above]
- *
- * 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.
- *
- * If you have any filesystems on USB devices mounted before software suspend,
- * they won't be accessible after resume and you may lose data, as though
- * you have unplugged the USB devices with mounted filesystems on them;
- * see the FAQ below for details. (This is not true for more traditional
- * power states like "standby", which normally don't turn USB off.)
-
-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
-
-. If you have SATA disks, you'll need recent kernels with SATA suspend
-support. For suspend and resume to work, make sure your disk drivers
-are built into kernel -- not modules. [There's way to make
-suspend/resume with modular disk drivers, see FAQ, but you probably
-should not do that.]
-
-If you want to limit the suspend image size to N bytes, do
-
-echo N > /sys/power/image_size
-
-before suspend (it is limited to 500 MB by default).
-
-
-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
-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. If the option ``hibernate=nocompress'' is specified as a boot
-parameter, it saves hibernation image without compression.
-
-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.
-
-
-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: 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: What is the freezing of tasks and why are we using it?
-
-A: The freezing of tasks is a mechanism by which user space processes and some
-kernel threads are controlled during hibernation or system-wide suspend (on some
-architectures). See freezing-of-tasks.txt for details.
-
-Q: What is the difference between "platform" and "shutdown"?
-
-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"
-
-"platform" is actually right thing to do where supported, but
-"shutdown" is most reliable (except on ACPI systems).
-
-Q: I do not understand why you have such strong objections to idea of
-selective suspend.
-
-A: Do selective suspend during runtime power management, that's okay. But
-it's 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 unnecessarily slow. If you want your driver 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 heavily, 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 useful.
-
-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, no one 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.
-
-Q: Can I suspend to a swap file?
-
-A: Generally, yes, you can. However, it requires you to use the "resume=" and
-"resume_offset=" kernel command line parameters, so the resume from a swap file
-cannot be initiated from an initrd or initramfs image. See
-swsusp-and-swap-files.txt for details.
-
-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 useful 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.
-
-Q: How can distributions ship a swsusp-supporting kernel with modular
-disk drivers (especially SATA)?
-
-A: Well, it can be done, load the drivers, then do echo into
-/sys/power/disk/resume file from initrd. Be sure not to mount
-anything, not even read-only mount, or you are going to lose your
-data.
-
-Q: How do I make suspend more verbose?
-
-A: If you want to see any non-error kernel messages on the virtual
-terminal the kernel switches to during suspend, you have to set the
-kernel console loglevel to at least 4 (KERN_WARNING), for example by
-doing
-
- # save the old loglevel
- read LOGLEVEL DUMMY < /proc/sys/kernel/printk
- # set the loglevel so we see the progress bar.
- # if the level is higher than needed, we leave it alone.
- if [ $LOGLEVEL -lt 5 ]; then
- echo 5 > /proc/sys/kernel/printk
- fi
-
- IMG_SZ=0
- read IMG_SZ < /sys/power/image_size
- echo -n disk > /sys/power/state
- RET=$?
- #
- # the logic here is:
- # if image_size > 0 (without kernel support, IMG_SZ will be zero),
- # then try again with image_size set to zero.
- if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
- echo 0 > /sys/power/image_size
- echo -n disk > /sys/power/state
- RET=$?
- fi
-
- # restore previous loglevel
- echo $LOGLEVEL > /proc/sys/kernel/printk
- exit $RET
-
-Q: Is this true that if I have a mounted filesystem on a USB device and
-I suspend to disk, I can lose data unless the filesystem has been mounted
-with "sync"?
-
-A: That's right ... if you disconnect that device, you may lose data.
-In fact, even with "-o sync" you can lose data if your programs have
-information in buffers they haven't written out to a disk you disconnect,
-or if you disconnect before the device finished saving data you wrote.
-
-Software suspend normally powers down USB controllers, which is equivalent
-to disconnecting all USB devices attached to your system.
-
-Your system might well support low-power modes for its USB controllers
-while the system is asleep, maintaining the connection, using true sleep
-modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the
-/sys/power/state file; write "standby" or "mem".) We've not seen any
-hardware that can use these modes through software suspend, although in
-theory some systems might support "platform" modes that won't break the
-USB connections.
-
-Remember that it's always a bad idea to unplug a disk drive containing a
-mounted filesystem. That's true even when your system is asleep! The
-safest thing is to unmount all filesystems on removable media (such USB,
-Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
-before suspending; then remount them after resuming.
-
-There is a work-around for this problem. For more information, see
-Documentation/usb/persist.txt.
-
-Q: Can I suspend-to-disk using a swap partition under LVM?
-
-A: No. You can suspend successfully, but you'll not be able to
-resume. uswsusp should be able to work with LVM. See suspend.sf.net.
-
-Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
-compiled with the similar configuration files. Anyway I found that
-suspend to disk (and resume) is much slower on 2.6.16 compared to
-2.6.15. Any idea for why that might happen or how can I speed it up?
-
-A: This is because the size of the suspend image is now greater than
-for 2.6.15 (by saving more data we can get more responsive system
-after resume).
-
-There's the /sys/power/image_size knob that controls the size of the
-image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
-root), the 2.6.15 behavior should be restored. If it is still too
-slow, take a look at suspend.sf.net -- userland suspend is faster and
-supports LZF compression to speed it up further.