Commit 0fd0c48e authored by Andrew Morton's avatar Andrew Morton Committed by Linus Torvalds

[PATCH] swsusp/sleep documentation update

From: Pavel Machek <pavel@ucw.cz>

This kills outdated docs, and adds some helpful docs about video issues
with suspend.
parent 6393cddd
......@@ -17,13 +17,30 @@ From kernel/suspend.c:
You need to append resume=/dev/your_swap_partition to kernel command
line. Then you suspend by echo 4 > /proc/acpi/sleep.
[Notice. Rest docs is pretty outdated (see date!) It should be safe to
use swsusp on ext3/reiserfs these days.]
Pavel's unreliable guide to swsusp mess
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are currently two versions of swap suspend in the kernel, the old
"Pavel's" version in kernel/power/swsusp.c and the new "Patrick's"
version in kernel/power/pmdisk.c. They provide the same functionality;
the old version looks ugly but was tested, while the new version looks
nicer but did not receive so much testing. echo 4 > /proc/acpi/sleep
calls the old version, echo disk > /sys/power/state calls the new one.
[In the future, when the new version is stable enough, two things can
happen:
* the new version is moved into swsusp.c, and swsusp is renamed to swap
suspend (Pavel prefers this)
* pmdisk is kept as is and swsusp.c is removed from the kernel]
Article about goals and implementation of Software Suspend for Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Author: G‚ábor Kuti
Last revised: 2002-04-08
Last revised: 2003-10-20 by Pavel Machek
Idea and goals to achieve
......@@ -36,84 +53,23 @@ 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.
On desk machines the power saving function isn't as important as it is in
laptops but we really may benefit from the second one. Nowadays the number of
desk machines supporting suspend function in their APM is going up but there
are (and there will still be for a long time) machines that don't even support
APM of any kind. On the other hand it is reported that using APM's suspend
some irqs (e.g. ATA disk irq) is lost and it is annoying for the user until
the Linux kernel resets the device.
So I started thinking about implementing Software Suspend which doesn't need
any APM support and - since it uses pretty near only high-level routines - is
supposed to be architecture independent code.
Using the code
The code is experimental right now - testers, extra eyes are welcome. To
compile this support into the kernel, you need CONFIG_EXPERIMENTAL,
and then CONFIG_SOFTWARE_SUSPEND in menu General Setup to be enabled. It
cannot be used as a module and I don't think it will ever be needed.
You have two ways to use this code. The first one is if you've compiled in
sysrq support then you may press Sysrq-D to request suspend. The other way
is with a patched SysVinit (my patch is against 2.76 and available at my
home page). You might call 'swsusp' or 'shutdown -z <time>'. Next way is to
echo 4 > /proc/acpi/sleep.
You have two ways to use this code. The first one is is with a patched
SysVinit (my patch is against 2.76 and available at my home page). You
might call 'swsusp' or 'shutdown -z <time>'. Next way is to echo 4 >
/proc/acpi/sleep.
Either way it saves the state of the machine into active swaps and then
reboots. 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. Warning! Look at section ``Things to implement'' to see what
isn't yet implemented. Also I strongly suggest you to list all active swaps
in /etc/fstab. Firstly because you don't have to specify anything to resume
and secondly if you have more than one swap area you can't decide which one
has the 'root' signature.
the resuming.
In the meantime while the system is suspended you should not touch any of the
hardware!
About the code
Goals reached
The code can be downloaded from
http://falcon.sch.bme.hu/~seasons/linux/. It mainly works but there are still
some of XXXs, TODOs, FIXMEs in the code which seem not to be too important. It
should work all right except for the problems listed in ``Things to
implement''. Notes about the code are really welcome.
How the code works
When suspending is triggered it immediately wakes up process bdflush. Bdflush
checks whether we have anything in our run queue tq_bdflush. Since we queued up
function do_software_suspend, it is called. Here we shrink everything including
dcache, inodes, buffers and memory (here mainly processes are swapped out). We
count how many pages we need to duplicate (we have to be atomical!) then we
create an appropriate sized page directory. It will point to the original and
the new (copied) address of the page. We get the free pages by
__get_free_pages() but since it changes state we have to be able to track it
later so it also flips in a bit in page's flags (a new Nosave flag). We
duplicate pages and then mark them as used (so atomicity is ensured). After
this we write out the image to swaps, do another sync and the machine may
reboot. We also save registers to stack.
By resuming an ``inverse'' method is executed. The image if exists is loaded,
loadling is either triggered by ``resume='' kernel option. We
change our task to bdflush (it is needed because if we don't do this init does
an oops when it is waken up later) and then pages are copied back to their
original location. We restore registers, free previously allocated memory,
activate memory context and task information. Here we should restore hardware
state but even without this the machine is restored and processes are continued
to work. I think hardware state should be restored by some list (using
notify_chain) and probably by some userland program (run-parts?) for users'
pleasure. Check out my patch at the same location for the sysvinit patch.
WARNINGS!
- It does not like pcmcia cards. And this is logical: pcmcia cards need
cardmgr to be initialized. they are not initialized during singleuser boot,
but "resumed" kernel does expect them to be initialized. That leads to
armagedon. You should eject any pcmcia cards before suspending.
Things to implement
- SMP support. I've done an SMP support but since I don't have access to a kind
......@@ -122,34 +78,14 @@ Things to implement
interrupts AFAIK..
- We should only make a copy of data related to kernel segment, since any
process data won't be changed.
- Hardware state restoring. Now there's support for notifying via the notify
chain, event handlers are welcome. Some devices may have microcodes loaded
into them. We should have event handlers for them as well.
- We should support other architectures (There are really only some arch
related functions..)
- We should also restore original state of swaps if the ``noresume'' kernel
option is specified.. Or do we need such a feature to save state for some
other time? Do we need some kind of ``several saved states''? (Linux-HA
people?). There's been some discussion about checkpointing on linux-future.
- Should make more sanity checks. Or are these enough?
Not so important ideas for implementing
- If a real time process is running then don't suspend the machine.
- Support for power.conf file as in Solaris, autoshutdown, special
devicetypes support, maybe in sysctl.
- Introduce timeout for SMP locking. But first locking ought to work :O
- Pre-detect if we don't have enough swap space or free it instead of
calling panic.
- Support for adding/removing hardware while suspended?
- We should not free pages at the beginning so aggressively, most of them
go there anyway..
- If X is active while suspending then by resuming calling svgatextmode
corrupts the virtual console of X.. (Maybe this has been fixed AFAIK).
Drivers we support
- IDE disks are okay
- vesafb
Drivers that need support
- pc_keyb -- perhaps we can wait for vojtech's input patches
......
Video issues with S3 resume
~~~~~~~~~~~~~~~~~~~~~~~~~~~
2003, Pavel Machek
During S3 resume, hardware needs to be reinitialized. For most
devices, this is easy, and kernel driver knows how to do
it. Unfortunately there's one exception: video card. Those are usually
initialized by BIOS, and kernel does not have enough information to
boot video card. (Kernel usually does not even contain video card
driver -- vesafb and vgacon are widely used).
This is not problem for swsusp, because during swsusp resume, BIOS is
run normally so video card is normally initialized.
There are three types of systems where video works after S3 resume:
* systems where video state is preserved over S3. (HP Omnibook xe3)
* systems that initialize video card into vga text mode and where BIOS
works well enough to be able to set video mode. Use
acpi_sleep=s3_mode on these. (Toshiba 4030cdt)
* systems where it is possible to call video bios during S3
resume. Unfortunately, it is not correct to call video BIOS at that
point, but it happens to work on some machines. Use
acpi_sleep=s3_bios (Athlon64 desktop system)
Now, if you pass acpi_sleep=something, and it does not work with your
bios, you'll get hard crash during resume. Be carefull.
You may have system where none of above works. At that point you
either invent another ugly hack that works, or write proper driver for
your video card (good luck getting docs :-(). Maybe suspending from X
(proper X, knowing your hardware, not XF68_FBcon) might have better
chance of working.
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