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+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) 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.