android kernel Debugging hibernation and suspend

Debugging hibernation and suspend

    (C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL

1. Testing hibernation (aka suspend to disk or STD)

To check if hibernation works, you can try to hibernate in the "reboot" mode:

# echo reboot > /sys/power/disk

# echo disk > /sys/power/state

and the system should create a hibernation image, reboot, resume and get back to

the command prompt where you have started the transition.  If that happens,

hibernation is most likely to work correctly.  Still, you need to repeat the

test at least a couple of times in a row for confidence.  [This is necessary,

because some problems only show up on a second attempt at suspending and

resuming the system.]  Moreover, hibernating in the "reboot" and "shutdown"

modes causes the PM core to skip some platform-related callbacks which on ACPI

systems might be necessary to make hibernation work.  Thus, if you machine fails

to hibernate or resume in the "reboot" mode, you should try the "platform" mode:

# echo platform > /sys/power/disk

# echo disk > /sys/power/state

which is the default and recommended mode of hibernation.

Unfortunately, the "platform" mode of hibernation does not work on some systems

with broken BIOSes.  In such cases the "shutdown" mode of hibernation might

work:

# echo shutdown > /sys/power/disk

# echo disk > /sys/power/state

(it is similar to the "reboot" mode, but it requires you to press the power

button to make the system resume).

If neither "platform" nor "shutdown" hibernation mode works, you will need to

identify what goes wrong.

a) Test modes of hibernation

To find out why hibernation fails on your system, you can use a special testing

facility available if the kernel is compiled with CONFIG_PM_DEBUG set.  Then,

there is the file /sys/power/pm_test that can be used to make the hibernation

core run in a test mode.  There are 5 test modes available:

freezer

- test the freezing of processes

devices

- test the freezing of processes and suspending of devices

platform

- test the freezing of processes, suspending of devices and platform

  global control methods(*)

processors

- test the freezing of processes, suspending of devices, platform

  global control methods(*) and the disabling of nonboot CPUs

core

- test the freezing of processes, suspending of devices, platform global

  control methods(*), the disabling of nonboot CPUs and suspending of

  platform/system devices

(*) the platform global control methods are only available on ACPI systems

    and are only tested if the hibernation mode is set to "platform"

To use one of them it is necessary to write the corresponding string to

/sys/power/pm_test (eg. "devices" to test the freezing of processes and

suspending devices) and issue the standard hibernation commands.  For example,

to use the "devices" test mode along with the "platform" mode of hibernation,

you should do the following:

# echo devices > /sys/power/pm_test

# echo platform > /sys/power/disk

# echo disk > /sys/power/state

Then, the kernel will try to freeze processes, suspend devices, wait 5 seconds,

resume devices and thaw processes.  If "platform" is written to

/sys/power/pm_test , then after suspending devices the kernel will additionally

invoke the global control methods (eg. ACPI global control methods) used to

prepare the platform firmware for hibernation.  Next, it will wait 5 seconds and

invoke the platform (eg. ACPI) global methods used to cancel hibernation etc.

Writing "none" to /sys/power/pm_test causes the kernel to switch to the normal

hibernation/suspend operations.  Also, when open for reading, /sys/power/pm_test

contains a space-separated list of all available tests (including "none" that

represents the normal functionality) in which the current test level is

indicated by square brackets.

Generally, as you can see, each test level is more "invasive" than the previous

one and the "core" level tests the hardware and drivers as deeply as possible

without creating a hibernation image.  Obviously, if the "devices" test fails,

the "platform" test will fail as well and so on.  Thus, as a rule of thumb, you

should try the test modes starting from "freezer", through "devices", "platform"

and "processors" up to "core" (repeat the test on each level a couple of times

to make sure that any random factors are avoided).

If the "freezer" test fails, there is a task that cannot be frozen (in that case

it usually is possible to identify the offending task by analysing the output of

dmesg obtained after the failing test).  Failure at this level usually means

that there is a problem with the tasks freezer subsystem that should be

reported.

If the "devices" test fails, most likely there is a driver that cannot suspend

or resume its device (in the latter case the system may hang or become unstable

after the test, so please take that into consideration).  To find this driver,

you can carry out a binary search according to the rules:

- if the test fails, unload a half of the drivers currently loaded and repeat

(that would probably involve rebooting the system, so always note what drivers

have been loaded before the test),

- if the test succeeds, load a half of the drivers you have unloaded most

recently and repeat.

Once you have found the failing driver (there can be more than just one of

them), you have to unload it every time before hibernation.  In that case please

make sure to report the problem with the driver.

It is also possible that the "devices" test will still fail after you have

unloaded all modules. In that case, you may want to look in your kernel

configuration for the drivers that can be compiled as modules (and test again

with these drivers compiled as modules).  You may also try to use some special

kernel command line options such as "noapic", "noacpi" or even "acpi=off".

If the "platform" test fails, there is a problem with the handling of the

platform (eg. ACPI) firmware on your system.  In that case the "platform" mode

of hibernation is not likely to work.  You can try the "shutdown" mode, but that

is rather a poor man's workaround.

If the "processors" test fails, the disabling/enabling of nonboot CPUs does not

work (of course, this only may be an issue on SMP systems) and the problem

should be reported.  In that case you can also try to switch the nonboot CPUs

off and on using the /sys/devices/system/cpu/cpu*/online sysfs attributes and

see if that works.

If the "core" test fails, which means that suspending of the system/platform

devices has failed (these devices are suspended on one CPU with interrupts off),

the problem is most probably hardware-related and serious, so it should be

reported.

A failure of any of the "platform", "processors" or "core" tests may cause your

system to hang or become unstable, so please beware.  Such a failure usually

indicates a serious problem that very well may be related to the hardware, but

please report it anyway.

b) Testing minimal configuration

If all of the hibernation test modes work, you can boot the system with the

"init=/bin/bash" command line parameter and attempt to hibernate in the

"reboot", "shutdown" and "platform" modes.  If that does not work, there

probably is a problem with a driver statically compiled into the kernel and you

can try to compile more drivers as modules, so that they can be tested

individually.  Otherwise, there is a problem with a modular driver and you can

find it by loading a half of the modules you normally use and binary searching

in accordance with the algorithm:

- if there are n modules loaded and the attempt to suspend and resume fails,

unload n/2 of the modules and try again (that would probably involve rebooting

the system),

- if there are n modules loaded and the attempt to suspend and resume succeeds,

load n/2 modules more and try again.

Again, if you find the offending module(s), it(they) must be unloaded every time

before hibernation, and please report the problem with it(them).

c) Advanced debugging

In case that hibernation does not work on your system even in the minimal

configuration and compiling more drivers as modules is not practical or some

modules cannot be unloaded, you can use one of the more advanced debugging

techniques to find the problem.  First, if there is a serial port in your box,

you can boot the kernel with the 'no_console_suspend' parameter and try to log

kernel messages using the serial console.  This may provide you with some

information about the reasons of the suspend (resume) failure.  Alternatively,

it may be possible to use a FireWire port for debugging with firescope

(ftp://ftp.firstfloor.org/pub/ak/firescope/).  On x86 it is also possible to

use the PM_TRACE mechanism documented in Documentation/s2ram.txt .

2. Testing suspend to RAM (STR)

To verify that the STR works, it is generally more convenient to use the s2ram

tool available from http://suspend.sf.net and documented at
http://en.opensuse.org/SDB:Suspend_to_RAM.
Namely, after writing "freezer", "devices", "platform", "processors", or "core"

into /sys/power/pm_test (available if the kernel is compiled with

CONFIG_PM_DEBUG set) the suspend code will work in the test mode corresponding

to given string.  The STR test modes are defined in the same way as for

hibernation, so please refer to Section 1 for more information about them.  In

particular, the "core" test allows you to test everything except for the actual

invocation of the platform firmware in order to put the system into the sleep

state.

Among other things, the testing with the help of /sys/power/pm_test may allow

you to identify drivers that fail to suspend or resume their devices.  They

should be unloaded every time before an STR transition.

Next, you can follow the instructions at http://en.opensuse.org/s2ram to test

the system, but if it does not work "out of the box", you may need to boot it

with "init=/bin/bash" and test s2ram in the minimal configuration.  In that

case, you may be able to search for failing drivers by following the procedure

analogous to the one described in section 1.  If you find some failing drivers,

you will have to unload them every time before an STR transition (ie. before

you run s2ram), and please report the problems with them.