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diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt new file mode 100644 index 000000000000..c7c3459fde43 --- /dev/null +++ b/Documentation/power/swsusp.txt @@ -0,0 +1,235 @@ +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: Gbor 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. |