Suspend to RAM

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From Wikipedia:Sleep mode:

Sleep mode can go by many different names, including Stand By, Sleep, and Suspend. When placed in this sleep mode, aside from the RAM which is required to restore the machine's state, the computer attempts to cut power to all unneeded parts of the machine. Because of the large power savings, most laptops automatically enter this mode when the computer is running on batteries and the lid is closed.

Suspend to RAM in Arch Linux using one of the following methods:

Suspend Framework

There are three canonical suspend backends - kernel, uswsusp and tuxonice - which instead of being configured or called directly, can be plugged into the suspend framework provided by pm-utils.

Backends/Methods

kernel

The first approach is to directly inform the kernel to enter a suspended state; the exact method and state depends on the level of hardware support.

/usr/lib/pm-utils/pm-functions::294 (comments added): 

if grep -q mem /sys/power/state; then
    SUSPEND_MODULE="kernel"
    # Suspend-to-RAM
    # ACPI State S3
    # Device State D3
    # Greatest power savings, slower resume
    do_suspend() { echo -n "mem" >/sys/power/state; }
elif [ -c /dev/pmu ] && pm-pmu --check; then
    SUSPEND_MODULE="kernel"
    # Suspend using Macintosh-style PMU
    # Fallback for older kernels in which the sysfs interface does not support this hardware
    do_suspend() { pm-pmu --suspend; }
elif grep -q standby /sys/power/state; then
    SUSPEND_MODULE="kernel"
    # Power-On-Suspend
    # ACPI State S1
    # Device State D1 (supported devices)
    # Minimal power savings, fast resume
    do_suspend() { echo -n "standby" >/sys/power/state; }
fi

uswsusp

While Suspend-to-RAM may generally work, often-times 'hacks' are needed to reinitialize the video card or turn on the backlight. The uswsusp ('Userspace Software Suspend') package provides s2ram, a wrapper around the kernel's suspend-to-RAM mechanism which perform some graphics adapter manipulations from userspace before suspending and after resuming. This includes:

  • passing acpi_sleep=s3_bios to the kernel
  • passing acpi_sleep=s3_mode to the kernel
  • passing both of the above (acpi_sleep=s3_bios,s3_mode) to the kernel
  • POSTing the video card from userspace after resume using vbetool
  • saving the VBE state before suspend and restoring it after resume using vbetool

This is accomplished by a hardware whitelist maintained by HAL - s2ram translates the HAL database options into s2ram parameters.

Since HAL is deprecated and KMS drivers can save the state of the grahic card directly without userspace quirks, s2ram development is discontinued and no further whitelist entries are accepted. If a KMS driver is in use, s2ram will directly suspend the machine.

tuxonice

...

Configuration

pm-utils is suspend and powerstate settings framework implementing extensible hooks run in alphanumerical order as root from {/usr/lib/pm-utils/,/etc/pm/}{sleep.d/,power.d/}. The suspend backend is specified by the SLEEP_MODULE configuration variable in /etc/pm/config.d and defaults to the kernel backend. The Arch Linux package ships with support for the following backends: 

pacman -Ql pm-utils | grep module.d
pm-utils /usr/lib/pm-utils/module.d/
pm-utils /usr/lib/pm-utils/module.d/kernel
pm-utils /usr/lib/pm-utils/module.d/tuxonice
pm-utils /usr/lib/pm-utils/module.d/uswsusp

Furthermore, pm-utils ships with its own video quirks database in /usr/lib/pm-utils/video-quirks/.

However to be able to use the alternative suspend backends - uswsusp or tuxonice - the respective packages need to be installed

Internals

This outlines the internal actions when pm-suspend is run, describing how pm-utils gracefully falls back onto the kernel method if the requirements of other methods are not met. 

$ pm-suspend

The first step is set-up preliminary variables and source parent scripts: 

export STASHNAME=pm-suspend
export METHOD="$(echo ${0##*pm-} |tr - _)"
. "/usr/lib/pm-utils/pm-functions"

The variable METHOD is extracted from the executable name, suspend from pm-suspend and hibernate from pm-hibernate.

The location of runtime configuration parameters is defined in /usr/lib/pm-utils/pm-functions as PM_UTILS_RUNDIR="/var/run/pm-utils" and STORAGEDIR="${PM_UTILS_RUNDIR}/${STASHNAME}/storage". Therefore STORAGEDIR="/var/run/pm-utils/pm-suspend/storage"; this is where pm-suspend will cache its configuration. Disabled hooks are stored as plain text files with the hook name prefixed by "disable_hook:". Configuration parameters are appended to the parameters file: 

$  ls -lah /var/run/pm-utils/pm-suspend/storage/
-rw-r--r-- 1 root root  20 May 19 09:57 disable_hook:99video
-rw-r--r-- 1 root root   0 May 19 02:59 parameters
-rw-r--r-- 1 root root 247 May 19 02:59 parameters.rm
-rw-r--r-- 1 root root   9 May 19 02:59 state:cpu0_governor
-rw-r--r-- 1 root root   9 May 19 02:59 state:cpu1_governor

Then pm-functions will source the files located in /etc/pm/config.d/ in addition to /usr/lib/pm-utils/defaults. Upon returning, pm-functions will proceed to source the files specified by $SLEEP_METHOD as /usr/lib/pm-utils/module.d/$SLEEP_METHOD[...] if they exist: 

for mod in $SLEEP_MODULE; do
    mod="${PM_UTILS_LIBDIR}/module.d/${mod}"
    [ -f "$mod" ] || continue
    . "$mod"
done

Otherwise, if $SLEEP_MODULE is empty, do_suspend() will be set to the kernel backend as described above: 

if [ -z "$SUSPEND_MODULE" ]; then
    if grep -q mem /sys/power/state; then
       SUSPEND_MODULE="kernel"
       do_suspend() { echo -n "mem" >/sys/power/state; }
    elif [ -c /dev/pmu ] && pm-pmu --check; then
       SUSPEND_MODULE="kernel"
       do_suspend() { pm-pmu --suspend; }
    elif grep -q standby /sys/power/state; then
       SUSPEND_MODULE="kernel"
       do_suspend() { echo -n "standby" >/sys/power/state; }
    fi
fi

Assuming $SLEEP_MODULE is not empty and uswsusp is specified, /usr/lib/pm-utils/module.d/uswsusp is executed. This script checks several requirements (these are the requirements for being able to use uswsusp):

  • [ -z $SUSPEND_MODULE ]
  • command_exists s2ram
  • grep -q mem /sys/power/state || ( [ -c /dev/pmu ] && pm-pmu --check; );

If these requirements are met, do_suspend() is defined as: 

do_suspend()
{
   uswsusp_get_quirks
   s2ram --force $OPTS
}

Most importantly, the uswsusp module runs: 

add_before_hooks uswsusp_hooks
add_module_help uswsusp_help

The first function, add_before_hook disables the pm-utils hooks 99video since this functionality is subsumed by s2ram. The second function, add_module_help, adds uswsusp-module-specific help, which in essence replaces the help function provided by 99video.


Back to pm-suspend: 

command_exists "check_$METHOD" && command_exists "do_$METHOD"
"check_$METHOD"

This verifies that the check_suspend and do_suspend methods have been defined. The check_suspend method simply verifies that $SUSPEND_MODULE is not empty. 

check_suspend() { [ -n "$SUSPEND_MODULE" ]; }

Lastly, pm-suspend must run all hooks that have not been disabled, sync file-system buffers, and run do_suspend: 

if run_hooks sleep "$ACTION $METHOD"; then
    # Sleep only if we know how and if a hook did not inhibit us.
    log "$(date): performing $METHOD"
    sync
    "do_$METHOD" || r=128
    log "$(date): Awake."

run_hooks -> _run_hooks -> foreach hook, check if inhibited and update parameters, then run_hook run_hook "suspend" -> hook_ok: check that the hook is not disabled run the hook with the suspend argument, log the exit status sync the file system buffers run do_suspend()

Which of What to Use

Framework or Not?

Directly calling the kernel backend method is significantly faster than calling pm-suspend, since running all the hooks provided by the pm-utils framework invariable takes time. Even uswsusp is faster than pm-suspend. However, the recommended approach is to use pm-utils as it can properly:

  • set hardware clock
  • restore wireless
  • etc...

In fact, only the pm-utils approach can be called without special privileges, see pm-utils#Suspend.2FHibernate as regular user

Which Backend/Method

  • kernel - hooks provided by pm-utils (including video quirks) with kernel method. Recommended.
  • uswsusp - hooks provided by pm-utils except video99 with s2ram assuming responsibility for video quirks. Not necessary unless the kernel method explicitly fails to work.
  • tuxonice - ...


Other Resources

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