src/kernel/linux/v4.19/Documentation/power/basic-pm-debugging.txt

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 your 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 a few
seconds (5 by default, but configurable by the suspend.pm_test_delay module
parameter), 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 a
configurable number of 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) Using the "test_resume" hibernation option

/sys/power/disk generally tells the kernel what to do after creating a
hibernation image.  One of the available options is "test_resume" which
causes the just created image to be used for immediate restoration.  Namely,
after doing:

# echo test_resume > /sys/power/disk
# echo disk > /sys/power/state

a hibernation image will be created and a resume from it will be triggered
immediately without involving the platform firmware in any way.

That test can be used to check if failures to resume from hibernation are
related to bad interactions with the platform firmware.  That is, if the above
works every time, but resume from actual hibernation does not work or is
unreliable, the platform firmware may be responsible for the failures.

On architectures and platforms that support using different kernels to restore
hibernation images (that is, the kernel used to read the image from storage and
load it into memory is different from the one included in the image) or support
kernel address space randomization, it also can be used to check if failures
to resume may be related to the differences between the restore and image
kernels.

d) 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
(http://v3.sk/~lkundrak/firescope/).  On x86 it is also possible to
use the PM_TRACE mechanism documented in Documentation/power/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 (S2RAM_LINK).

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 S2RAM_LINK 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.

There is a debugfs entry which shows the suspend to RAM statistics. Here is an
example of its output.
	# mount -t debugfs none /sys/kernel/debug
	# cat /sys/kernel/debug/suspend_stats
	success: 20
	fail: 5
	failed_freeze: 0
	failed_prepare: 0
	failed_suspend: 5
	failed_suspend_noirq: 0
	failed_resume: 0
	failed_resume_noirq: 0
	failures:
	  last_failed_dev:	alarm
				adc
	  last_failed_errno:	-16
				-16
	  last_failed_step:	suspend
				suspend
Field success means the success number of suspend to RAM, and field fail means
the failure number. Others are the failure number of different steps of suspend
to RAM. suspend_stats just lists the last 2 failed devices, error number and
failed step of suspend.