Commit 947d2c2c authored by Rafael J. Wysocki's avatar Rafael J. Wysocki

PM / sleep: Update some system sleep documentation

Update some documentation related to system sleep to document new
features and remove outdated information from it.
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: default avatarPavel Machek <pavel@ucw.cz>
Reviewed-by: default avatarChen Yu <yu.c.chen@intel.com>
parent 62822e2e
......@@ -164,7 +164,32 @@ 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
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
......
Power Management Interface
The power management subsystem provides a unified sysfs interface to
userspace, regardless of what architecture or platform one is
running. The interface exists in /sys/power/ directory (assuming sysfs
is mounted at /sys).
/sys/power/state controls system power state. Reading from this file
returns what states are supported, which is hard-coded to 'freeze',
'standby' (Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk'
(Suspend-to-Disk).
Writing to this file one of those strings causes the system to
transition into that state. Please see the file
Documentation/power/states.txt for a description of each of those
states.
/sys/power/disk controls the operating mode of the suspend-to-disk
mechanism. Suspend-to-disk can be handled in several ways. We have a
few options for putting the system to sleep - using the platform driver
(e.g. ACPI or other suspend_ops), powering off the system or rebooting the
system (for testing).
Additionally, /sys/power/disk can be used to turn on one of the two testing
modes of the suspend-to-disk mechanism: 'testproc' or 'test'. If the
suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to
/sys/power/state will cause the kernel to disable nonboot CPUs and freeze
tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs. If it is
in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel
to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait
for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs. Then,
we are able to look in the log messages and work out, for example, which code
is being slow and which device drivers are misbehaving.
Reading from this file will display all supported modes and the currently
selected one in brackets, for example
[shutdown] reboot test testproc
Writing to this file will accept one of
'platform' (only if the platform supports it)
'shutdown'
'reboot'
'testproc'
'test'
/sys/power/image_size controls the size of the image created by
the suspend-to-disk mechanism. It can be written a string
representing a non-negative integer that will be used as an upper
limit of the image size, in bytes. The suspend-to-disk mechanism will
do its best to ensure the image size will not exceed that number. However,
if this turns out to be impossible, it will try to suspend anyway using the
smallest image possible. In particular, if "0" is written to this file, the
suspend image will be as small as possible.
Reading from this file will display the current image size limit, which
is set to 2/5 of available RAM by default.
/sys/power/pm_trace controls the code which saves the last PM event point in
the RTC across reboots, so that you can debug a machine that just hangs
during suspend (or more commonly, during resume). Namely, the RTC is only
used to save the last PM event point if this file contains '1'. Initially it
contains '0' which may be changed to '1' by writing a string representing a
nonzero integer into it.
To use this debugging feature you should attempt to suspend the machine, then
reboot it and run
dmesg -s 1000000 | grep 'hash matches'
CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
set to a random invalid time after a resume.
Power Management Interface for System Sleep
Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
The power management subsystem provides userspace with a unified sysfs interface
for system sleep regardless of the underlying system architecture or platform.
The interface is located in the /sys/power/ directory (assuming that sysfs is
mounted at /sys).
/sys/power/state is the system sleep state control file.
Reading from it returns a list of supported sleep states, encoded as:
'freeze' (Suspend-to-Idle)
'standby' (Power-On Suspend)
'mem' (Suspend-to-RAM)
'disk' (Suspend-to-Disk)
Suspend-to-Idle is always supported. Suspend-to-Disk is always supported
too as long the kernel has been configured to support hibernation at all
(ie. CONFIG_HIBERNATION is set in the kernel configuration file). Support
for Suspend-to-RAM and Power-On Suspend depends on the capabilities of the
platform.
If one of the strings listed in /sys/power/state is written to it, the system
will attempt to transition into the corresponding sleep state. Refer to
Documentation/power/states.txt for a description of each of those states.
/sys/power/disk controls the operating mode of hibernation (Suspend-to-Disk).
Specifically, it tells the kernel what to do after creating a hibernation image.
Reading from it returns a list of supported options encoded as:
'platform' (put the system into sleep using a platform-provided method)
'shutdown' (shut the system down)
'reboot' (reboot the system)
'suspend' (trigger a Suspend-to-RAM transition)
'test_resume' (resume-after-hibernation test mode)
The currently selected option is printed in square brackets.
The 'platform' option is only available if the platform provides a special
mechanism to put the system to sleep after creating a hibernation image (ACPI
does that, for example). The 'suspend' option is available if Suspend-to-RAM
is supported. Refer to Documentation/power/basic_pm_debugging.txt for the
description of the 'test_resume' option.
To select an option, write the string representing it to /sys/power/disk.
/sys/power/image_size controls the size of hibernation images.
It can be written a string representing a non-negative integer that will be
used as a best-effort upper limit of the image size, in bytes. The hibernation
core will do its best to ensure that the image size will not exceed that number.
However, if that turns out to be impossible to achieve, a hibernation image will
still be created and its size will be as small as possible. In particular,
writing '0' to this file will enforce hibernation images to be as small as
possible.
Reading from this file returns the current image size limit, which is set to
around 2/5 of available RAM by default.
/sys/power/pm_trace controls the PM trace mechanism saving the last suspend
or resume event point in the RTC across reboots.
It helps to debug hard lockups or reboots due to device driver failures that
occur during system suspend or resume (which is more common) more effectively.
If /sys/power/pm_trace contains '1', the fingerprint of each suspend/resume
event point in turn will be stored in the RTC memory (overwriting the actual
RTC information), so it will survive a system crash if one occurs right after
storing it and it can be used later to identify the driver that caused the crash
to happen (see Documentation/power/s2ram.txt for more information).
Initially it contains '0' which may be changed to '1' by writing a string
representing a nonzero integer into it.
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