blob: aea7e920966753193c6139d84e7839f7de485ebe [file] [log] [blame]
Pavel Machekd7ae79c2005-09-06 15:16:21 -07001Some warnings, first.
Linus Torvalds1da177e2005-04-16 15:20:36 -07002
3 * BIG FAT WARNING *********************************************************
4 *
Linus Torvalds1da177e2005-04-16 15:20:36 -07005 * If you touch anything on disk between suspend and resume...
6 * ...kiss your data goodbye.
7 *
Pavel Machekd7ae79c2005-09-06 15:16:21 -07008 * If you do resume from initrd after your filesystems are mounted...
9 * ...bye bye root partition.
10 * [this is actually same case as above]
Linus Torvalds1da177e2005-04-16 15:20:36 -070011 *
Pavel Machekd7ae79c2005-09-06 15:16:21 -070012 * If you have unsupported (*) devices using DMA, you may have some
13 * problems. If your disk driver does not support suspend... (IDE does),
14 * it may cause some problems, too. If you change kernel command line
15 * between suspend and resume, it may do something wrong. If you change
16 * your hardware while system is suspended... well, it was not good idea;
17 * but it will probably only crash.
Linus Torvalds1da177e2005-04-16 15:20:36 -070018 *
19 * (*) suspend/resume support is needed to make it safe.
Pavel Machek543cc272006-03-23 03:00:02 -080020 *
David Brownellb9827e42006-05-16 17:33:14 -070021 * If you have any filesystems on USB devices mounted before software suspend,
Pavel Machek543cc272006-03-23 03:00:02 -080022 * they won't be accessible after resume and you may lose data, as though
David Brownellb9827e42006-05-16 17:33:14 -070023 * you have unplugged the USB devices with mounted filesystems on them;
24 * see the FAQ below for details. (This is not true for more traditional
25 * power states like "standby", which normally don't turn USB off.)
Linus Torvalds1da177e2005-04-16 15:20:36 -070026
27You need to append resume=/dev/your_swap_partition to kernel command
28line. Then you suspend by
29
30echo shutdown > /sys/power/disk; echo disk > /sys/power/state
31
32. If you feel ACPI works pretty well on your system, you might try
33
34echo platform > /sys/power/disk; echo disk > /sys/power/state
35
Pavel Machek543cc272006-03-23 03:00:02 -080036. If you have SATA disks, you'll need recent kernels with SATA suspend
37support. For suspend and resume to work, make sure your disk drivers
38are built into kernel -- not modules. [There's way to make
39suspend/resume with modular disk drivers, see FAQ, but you probably
40should not do that.]
41
Rafael J. Wysocki853609b2006-02-01 03:05:07 -080042If you want to limit the suspend image size to N bytes, do
Rafael J. Wysockica0aec02006-01-06 00:15:56 -080043
44echo N > /sys/power/image_size
45
46before suspend (it is limited to 500 MB by default).
Linus Torvalds1da177e2005-04-16 15:20:36 -070047
48
49Article about goals and implementation of Software Suspend for Linux
50~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
John Anthony Kazos Jrbe2a6082007-05-09 08:50:42 +020051Author: G‚ábor Kuti
Linus Torvalds1da177e2005-04-16 15:20:36 -070052Last revised: 2003-10-20 by Pavel Machek
53
54Idea and goals to achieve
55
56Nowadays it is common in several laptops that they have a suspend button. It
57saves the state of the machine to a filesystem or to a partition and switches
58to standby mode. Later resuming the machine the saved state is loaded back to
59ram and the machine can continue its work. It has two real benefits. First we
60save ourselves the time machine goes down and later boots up, energy costs
61are real high when running from batteries. The other gain is that we don't have to
62interrupt our programs so processes that are calculating something for a long
63time shouldn't need to be written interruptible.
64
65swsusp saves the state of the machine into active swaps and then reboots or
66powerdowns. You must explicitly specify the swap partition to resume from with
67``resume='' kernel option. If signature is found it loads and restores saved
68state. If the option ``noresume'' is specified as a boot parameter, it skips
69the resuming.
70
71In the meantime while the system is suspended you should not add/remove any
72of the hardware, write to the filesystems, etc.
73
74Sleep states summary
75====================
76
77There are three different interfaces you can use, /proc/acpi should
78work like this:
79
80In a really perfect world:
81echo 1 > /proc/acpi/sleep # for standby
82echo 2 > /proc/acpi/sleep # for suspend to ram
83echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative
84echo 4 > /proc/acpi/sleep # for suspend to disk
85echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system
86
87and perhaps
88echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios
89
90Frequently Asked Questions
91==========================
92
93Q: well, suspending a server is IMHO a really stupid thing,
94but... (Diego Zuccato):
95
96A: You bought new UPS for your server. How do you install it without
97bringing machine down? Suspend to disk, rearrange power cables,
98resume.
99
100You have your server on UPS. Power died, and UPS is indicating 30
101seconds to failure. What do you do? Suspend to disk.
102
Linus Torvalds1da177e2005-04-16 15:20:36 -0700103
104Q: Maybe I'm missing something, but why don't the regular I/O paths work?
105
106A: We do use the regular I/O paths. However we cannot restore the data
107to its original location as we load it. That would create an
108inconsistent kernel state which would certainly result in an oops.
109Instead, we load the image into unused memory and then atomically copy
110it back to it original location. This implies, of course, a maximum
111image size of half the amount of memory.
112
113There are two solutions to this:
114
115* require half of memory to be free during suspend. That way you can
116read "new" data onto free spots, then cli and copy
117
118* assume we had special "polling" ide driver that only uses memory
119between 0-640KB. That way, I'd have to make sure that 0-640KB is free
120during suspending, but otherwise it would work...
121
122suspend2 shares this fundamental limitation, but does not include user
123data and disk caches into "used memory" by saving them in
124advance. That means that the limitation goes away in practice.
125
126Q: Does linux support ACPI S4?
127
128A: Yes. That's what echo platform > /sys/power/disk does.
129
Linus Torvalds1da177e2005-04-16 15:20:36 -0700130Q: What is 'suspend2'?
131
132A: suspend2 is 'Software Suspend 2', a forked implementation of
133suspend-to-disk which is available as separate patches for 2.4 and 2.6
134kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
135highmem and preemption. It also has a extensible architecture that
136allows for arbitrary transformations on the image (compression,
137encryption) and arbitrary backends for writing the image (eg to swap
138or an NFS share[Work In Progress]). Questions regarding suspend2
139should be sent to the mailing list available through the suspend2
140website, and not to the Linux Kernel Mailing List. We are working
141toward merging suspend2 into the mainline kernel.
142
Rafael J. Wysocki83144182007-07-17 04:03:35 -0700143Q: What is the freezing of tasks and why are we using it?
Linus Torvalds1da177e2005-04-16 15:20:36 -0700144
Rafael J. Wysocki83144182007-07-17 04:03:35 -0700145A: The freezing of tasks is a mechanism by which user space processes and some
146kernel threads are controlled during hibernation or system-wide suspend (on some
147architectures). See freezing-of-tasks.txt for details.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700148
Johannes Berg11d77d02007-04-30 15:09:53 -0700149Q: What is the difference between "platform" and "shutdown"?
Linus Torvalds1da177e2005-04-16 15:20:36 -0700150
151A:
152
153shutdown: save state in linux, then tell bios to powerdown
154
155platform: save state in linux, then tell bios to powerdown and blink
156 "suspended led"
157
Johannes Berg11d77d02007-04-30 15:09:53 -0700158"platform" is actually right thing to do where supported, but
159"shutdown" is most reliable (except on ACPI systems).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700160
161Q: I do not understand why you have such strong objections to idea of
162selective suspend.
163
Matt LaPlante2fe0ae72006-10-03 22:50:39 +0200164A: Do selective suspend during runtime power management, that's okay. But
165it's useless for suspend-to-disk. (And I do not see how you could use
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166it for suspend-to-ram, I hope you do not want that).
167
168Lets see, so you suggest to
169
170* SUSPEND all but swap device and parents
171* Snapshot
172* Write image to disk
173* SUSPEND swap device and parents
174* Powerdown
175
176Oh no, that does not work, if swap device or its parents uses DMA,
177you've corrupted data. You'd have to do
178
179* SUSPEND all but swap device and parents
180* FREEZE swap device and parents
181* Snapshot
182* UNFREEZE swap device and parents
183* Write
184* SUSPEND swap device and parents
185
186Which means that you still need that FREEZE state, and you get more
187complicated code. (And I have not yet introduce details like system
188devices).
189
190Q: There don't seem to be any generally useful behavioral
191distinctions between SUSPEND and FREEZE.
192
193A: Doing SUSPEND when you are asked to do FREEZE is always correct,
David Brownellb9827e42006-05-16 17:33:14 -0700194but it may be unneccessarily slow. If you want your driver to stay simple,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700195slowness may not matter to you. It can always be fixed later.
196
197For devices like disk it does matter, you do not want to spindown for
198FREEZE.
199
Matt LaPlante2fe0ae72006-10-03 22:50:39 +0200200Q: After resuming, system is paging heavily, leading to very bad interactivity.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201
202A: Try running
203
204cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null
205
Adrian Bunka58a4142006-01-10 00:08:17 +0100206after resume. swapoff -a; swapon -a may also be useful.
Pavel Machekfc5fb2c2005-06-25 14:55:07 -0700207
208Q: What happens to devices during swsusp? They seem to be resumed
209during system suspend?
210
211A: That's correct. We need to resume them if we want to write image to
212disk. Whole sequence goes like
213
214 Suspend part
215 ~~~~~~~~~~~~
216 running system, user asks for suspend-to-disk
217
218 user processes are stopped
219
220 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
221 with state snapshot
222
223 state snapshot: copy of whole used memory is taken with interrupts disabled
224
225 resume(): devices are woken up so that we can write image to swap
226
227 write image to swap
228
229 suspend(PMSG_SUSPEND): suspend devices so that we can power off
230
231 turn the power off
232
233 Resume part
234 ~~~~~~~~~~~
235 (is actually pretty similar)
236
237 running system, user asks for suspend-to-disk
238
239 user processes are stopped (in common case there are none, but with resume-from-initrd, noone knows)
240
241 read image from disk
242
243 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
244 with image restoration
245
246 image restoration: rewrite memory with image
247
248 resume(): devices are woken up so that system can continue
249
250 thaw all user processes
251
252Q: What is this 'Encrypt suspend image' for?
253
254A: First of all: it is not a replacement for dm-crypt encrypted swap.
255It cannot protect your computer while it is suspended. Instead it does
256protect from leaking sensitive data after resume from suspend.
257
258Think of the following: you suspend while an application is running
259that keeps sensitive data in memory. The application itself prevents
260the data from being swapped out. Suspend, however, must write these
261data to swap to be able to resume later on. Without suspend encryption
262your sensitive data are then stored in plaintext on disk. This means
263that after resume your sensitive data are accessible to all
264applications having direct access to the swap device which was used
265for suspend. If you don't need swap after resume these data can remain
266on disk virtually forever. Thus it can happen that your system gets
267broken in weeks later and sensitive data which you thought were
268encrypted and protected are retrieved and stolen from the swap device.
269To prevent this situation you should use 'Encrypt suspend image'.
270
271During suspend a temporary key is created and this key is used to
272encrypt the data written to disk. When, during resume, the data was
273read back into memory the temporary key is destroyed which simply
274means that all data written to disk during suspend are then
275inaccessible so they can't be stolen later on. The only thing that
276you must then take care of is that you call 'mkswap' for the swap
277partition used for suspend as early as possible during regular
278boot. This asserts that any temporary key from an oopsed suspend or
279from a failed or aborted resume is erased from the swap device.
280
281As a rule of thumb use encrypted swap to protect your data while your
282system is shut down or suspended. Additionally use the encrypted
283suspend image to prevent sensitive data from being stolen after
284resume.
Pavel Machek7e958882005-09-03 15:56:56 -0700285
Rafael J. Wysockiecbd0da2006-12-06 20:34:13 -0800286Q: Can I suspend to a swap file?
Pavel Machek7e958882005-09-03 15:56:56 -0700287
Rafael J. Wysockiecbd0da2006-12-06 20:34:13 -0800288A: Generally, yes, you can. However, it requires you to use the "resume=" and
289"resume_offset=" kernel command line parameters, so the resume from a swap file
290cannot be initiated from an initrd or initramfs image. See
291swsusp-and-swap-files.txt for details.
Pavel Machekd7ae79c2005-09-06 15:16:21 -0700292
293Q: Is there a maximum system RAM size that is supported by swsusp?
294
295A: It should work okay with highmem.
296
297Q: Does swsusp (to disk) use only one swap partition or can it use
298multiple swap partitions (aggregate them into one logical space)?
299
300A: Only one swap partition, sorry.
301
302Q: If my application(s) causes lots of memory & swap space to be used
303(over half of the total system RAM), is it correct that it is likely
304to be useless to try to suspend to disk while that app is running?
305
306A: No, it should work okay, as long as your app does not mlock()
307it. Just prepare big enough swap partition.
308
Adrian Bunka58a4142006-01-10 00:08:17 +0100309Q: What information is useful for debugging suspend-to-disk problems?
Pavel Machekd7ae79c2005-09-06 15:16:21 -0700310
311A: Well, last messages on the screen are always useful. If something
312is broken, it is usually some kernel driver, therefore trying with as
313little as possible modules loaded helps a lot. I also prefer people to
314suspend from console, preferably without X running. Booting with
315init=/bin/bash, then swapon and starting suspend sequence manually
316usually does the trick. Then it is good idea to try with latest
317vanilla kernel.
318
Pavel Machek543cc272006-03-23 03:00:02 -0800319Q: How can distributions ship a swsusp-supporting kernel with modular
320disk drivers (especially SATA)?
321
322A: Well, it can be done, load the drivers, then do echo into
323/sys/power/disk/resume file from initrd. Be sure not to mount
324anything, not even read-only mount, or you are going to lose your
325data.
326
327Q: How do I make suspend more verbose?
328
329A: If you want to see any non-error kernel messages on the virtual
330terminal the kernel switches to during suspend, you have to set the
Pavel Macheke084dbd2006-06-23 02:04:50 -0700331kernel console loglevel to at least 4 (KERN_WARNING), for example by
332doing
Pavel Machek543cc272006-03-23 03:00:02 -0800333
Pavel Macheke084dbd2006-06-23 02:04:50 -0700334 # save the old loglevel
335 read LOGLEVEL DUMMY < /proc/sys/kernel/printk
336 # set the loglevel so we see the progress bar.
337 # if the level is higher than needed, we leave it alone.
338 if [ $LOGLEVEL -lt 5 ]; then
339 echo 5 > /proc/sys/kernel/printk
340 fi
341
342 IMG_SZ=0
343 read IMG_SZ < /sys/power/image_size
344 echo -n disk > /sys/power/state
345 RET=$?
346 #
347 # the logic here is:
348 # if image_size > 0 (without kernel support, IMG_SZ will be zero),
349 # then try again with image_size set to zero.
350 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
351 echo 0 > /sys/power/image_size
352 echo -n disk > /sys/power/state
353 RET=$?
354 fi
355
356 # restore previous loglevel
357 echo $LOGLEVEL > /proc/sys/kernel/printk
358 exit $RET
Pavel Machek543cc272006-03-23 03:00:02 -0800359
360Q: Is this true that if I have a mounted filesystem on a USB device and
361I suspend to disk, I can lose data unless the filesystem has been mounted
362with "sync"?
363
David Brownellb9827e42006-05-16 17:33:14 -0700364A: That's right ... if you disconnect that device, you may lose data.
365In fact, even with "-o sync" you can lose data if your programs have
366information in buffers they haven't written out to a disk you disconnect,
367or if you disconnect before the device finished saving data you wrote.
Pavel Machek543cc272006-03-23 03:00:02 -0800368
David Brownellb9827e42006-05-16 17:33:14 -0700369Software suspend normally powers down USB controllers, which is equivalent
370to disconnecting all USB devices attached to your system.
371
372Your system might well support low-power modes for its USB controllers
373while the system is asleep, maintaining the connection, using true sleep
374modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the
375/sys/power/state file; write "standby" or "mem".) We've not seen any
376hardware that can use these modes through software suspend, although in
Johannes Berg11d77d02007-04-30 15:09:53 -0700377theory some systems might support "platform" modes that won't break the
378USB connections.
Pavel Machek543cc272006-03-23 03:00:02 -0800379
380Remember that it's always a bad idea to unplug a disk drive containing a
David Brownellb9827e42006-05-16 17:33:14 -0700381mounted filesystem. That's true even when your system is asleep! The
382safest thing is to unmount all filesystems on removable media (such USB,
383Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
384before suspending; then remount them after resuming.
Pavel Machekd7ae79c2005-09-06 15:16:21 -0700385
Alan Stern0458d5b2007-05-04 11:52:20 -0400386There is a work-around for this problem. For more information, see
387Documentation/usb/persist.txt.
388
Pavel Macheke084dbd2006-06-23 02:04:50 -0700389Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
390compiled with the similar configuration files. Anyway I found that
391suspend to disk (and resume) is much slower on 2.6.16 compared to
3922.6.15. Any idea for why that might happen or how can I speed it up?
393
394A: This is because the size of the suspend image is now greater than
395for 2.6.15 (by saving more data we can get more responsive system
396after resume).
397
398There's the /sys/power/image_size knob that controls the size of the
399image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
400root), the 2.6.15 behavior should be restored. If it is still too
401slow, take a look at suspend.sf.net -- userland suspend is faster and
402supports LZF compression to speed it up further.