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 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
  (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.