Power Management from Linux Kernel to Android

Power Management
from Linux Kernel to Android
Matt Hsu & Jim Huang (jserv)
from 0xlab

June 19, 2009

1
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2

Agenda

Introduction to Linux Power Management
Concepts behind Android Power Management
Design and Implementation
Room for Improvements

3

Goal of Power Management
APM vs. ACPI
APM emulation
No silver bullet
Manage power in different power state
Lighthouse in the sea

4

Power Management Basics

Prolong Life

5

Advanced Power Management

APM-Aware APM-Aware APM-Aware APM-Aware
Applications
Application Application Device Driver Device Driver

Operating System APM Driver OS dependent

APM Interface OS independent

BIOS APM BIOS

APM BIOS Add-In Add-In
Controlled Device Device
Hardware

6

APM power state

Full On
Device Off Switch
Responsiveness • APM Enable • APM Disable
Decreases • Enable Call • Disable Call
• Off Switch
POWER • Off Call
MANAGED APM Enabled

• Short Inactivity Resume
• Standby Call Event

APM Standby
Off Switch

Wakeup
Event

APM Suspend
Off Switch
• Long Inactivity
Hibernation • Suspend Interrupt
• Suspend Call

On Switch Power
Off Usage
Increases

7

Advanced Configuration and Power Interface

8

ACPI Power State Transition

9

APM vs. ACPI
APM
Control resides in BIOS
Uses activity timeouts to determine when to power down a device
BIOS rarely used in embedded systems
Makes power-management decisions without informing OS or individual
applications
No knowledge of add-in cards or new devices (e.g. USB, IEEE 1394)

ACPI
Control divided between BIOS and OS
Decisions managed through the OS
Enables sophisticated power policies for general-purpose computers with
standard usage patterns and hardware
No knowledge of device-specific scenarios (e.g. need to provide predictable
response times or to respond to critical events over extended period)
10

APM emulation

/dev/apm_bois user_pmd

pm_prepare
pm_enter mach_pm_ops
kapmd pm_finish

battery_drv pmu_drv
get_pm_status_drv

11

No silver bullet
for building most power-saving
mobile system.
12

Power State Transition

13

Runtime / Standby

Application-driven power management
Micro manage your device
WiFi -enable PS poll mode
Switch on/off device by demand (Android's concept)
Gating off unused device clock
Keep the flexibility of CPU
Gating CPU freq dynamically
Tickless idle
Using DMA instead PIO

14

Suspend/Sleep Mode

STR vs STD
Suspend to RAM
Suspend to Disk/Hibernate

STR: A technique by which systems preserve state in RAM
during suspend and restore system state from RAM upon resume

STR is suitable for mobile device if hardware supports this
function

15

Suspend/Resume Control Path

# echo mem >/sys/power/state System running

Stop tasks
Restarting tasks
pm_registered_dev
pm_registered_dev
|-> bus_suspend
|-> bus_suspend pm_registered_dev
| | → dev_suspend
→ dev_suspend |-> bus_resume
| → → bus_suspend
| bus_suspend | → dev_resume
............
............ | → bus_resume
............

- keep GPIO state
- Set SDRAM as self-refresh mode - restore GPIO state
- Request CPU to sleep

bootloader
Suspend
16

Practical Example -S3C244x (1)

17

Practical Example -S3C244x (2)

18

A ship on the beach is a lighthouse to the sea

Provide a kernel module to dump and check GPIOs
Provide power source as independent as possible
Don't ignore the poweroff state
To most difficult part of debugging STR is resuming

19

Basically, it is the slim wrapper for Linux
Power Management

20

Where is Power Manager?

PM?
PM?

21

In fact, power ghost exists everywhere

Let “grep -ri power” tell you about the details!
It is layered into several components, but implementation
involves in some hacks.
Check: CONFIG_PM, sysfs, device drivers, libhardware_legacy,
libril, init.rc, powerd, alarm, vold, JNI, PowerManager,
BatteryManager, ...

22

Base: Linux Kernel
Directory: arch/arm/mach-omap2/
Directory: arch/arm/mach-omap2/
board-omap3beagle.c
board-omap3beagle.c
gpmc.c
gpmc.c
pm.[ch]
pm.[ch]

Android does rely on Linux Kernel 2.6 for core system services
Memory/Process Management
Device Driver Model
sysfs, kobject/uevent, netlink
Android Kernel extensions
Key Idea: Android attempts to provide
Key Idea: Android attempts to provide
Binder an abstraction layer between
an abstraction layer between
android_power hardware and the related software stack.
hardware and the related software stack.
/sys/android_power/, /sys/power/ 23

Android's PM Concepts

Android PM is built on top of standard Linux Power Management.
It can support more aggressive PM, but looks fairly simple now.
Components make requests to keep the power on through
“Wake Locks”.
PM does support several types of “Wake Locks”.

If there are no active wake locks, CPU will
be turned off.
If there is are partial wake locks, screen
and keyboard will be turned off.
24

25

Types of Wake Locks
ACQUIRE_CAUSES_WAKEUP
Normally wake locks don't actually wake the device, they just cause it to
remain on once it's already on. Think of the video player app as the normal
behavior.

FULL_WAKE_LOCK
Wake lock that ensures that the screen and keyboard are on at full
brightness.

ON_AFTER_RELEASE
When this wake lock is released, poke the user activity timer so the screen
stays on for a little longer.

PARTIAL_WAKE_LOCK
Wake lock that ensures that the CPU is running. The screen might not be on.

SCREEN_BRIGHT_WAKE_LOCK
Wake lock that ensures that the screen is on at full brightness; the keyboard
backlight will be allowed to go off.

SCREEN_DIM_WAKE_LOCK
Wake lock that ensures that the screen is on, but the keyboard backlight will
be allowed to go off, and the screen backlight will be allowed to go dim. 26

PM State Machine
Touchscreen or keyboard user activity
Touchscreen or keyboard user activity
event or full wake locks acquired.
event or full wake locks acquired.

All partial wake locks
All partial wake locks
released
released

Partial wake locks acquired
Partial wake locks acquired

27

Basically, it is the slim wrapper for Linux Power Management


28


29

android_os_Power
frameworks/base/core/jni/android_os_Power.cpp
frameworks/base/core/jni/android_os_Power.cpp
...
...
static JNINativeMethod method_table[] = {
static JNINativeMethod method_table[] = {
{ "acquireWakeLock", "(ILjava/lang/String;)V", (void*)acquireWakeLock },
{ "acquireWakeLock", "(ILjava/lang/String;)V", (void*)acquireWakeLock },
{ "releaseWakeLock", "(Ljava/lang/String;)V", (void*)releaseWakeLock },
{ "releaseWakeLock", "(Ljava/lang/String;)V", (void*)releaseWakeLock },
{ "setLastUserActivityTimeout", "(J)I", (void*)setLastUserActivityTimeout },
{ "setLastUserActivityTimeout", "(J)I", (void*)setLastUserActivityTimeout },
{ "setLightBrightness", "(II)I", (void*)setLightBrightness },
{ "setLightBrightness", "(II)I", (void*)setLightBrightness },
{ "setScreenState", "(Z)I", (void*)setScreenState },
{ "setScreenState", "(Z)I", (void*)setScreenState },
{ "shutdown", "()V", (void*)android_os_Power_shutdown },
{ "shutdown", "()V", (void*)android_os_Power_shutdown },
{ "reboot", "(Ljava/lang/String;)V", (void*)android_os_Power_reboot },
{ "reboot", "(Ljava/lang/String;)V", (void*)android_os_Power_reboot },
};
};

int register_android_os_Power(JNIEnv *env)
int register_android_os_Power(JNIEnv *env)
{{
return AndroidRuntime::registerNativeMethods(
return AndroidRuntime::registerNativeMethods(
env, "android/os/Power",
env, "android/os/Power",
method_table, NELEM(method_table));
method_table, NELEM(method_table));
}}

static void
static void
acquireWakeLock(JNIEnv *env, jobject clazz,
acquireWakeLock(JNIEnv *env, jobject clazz,
jint lock, jstring idObj)
jint lock, jstring idObj)
{
{
if (idObj == NULL) {
if (idObj == NULL) {
throw_NullPointerException(env, "id is null");
throw_NullPointerException(env, "id is null");
return ;
return ;
}
}

const char *id = env->GetStringUTFChars(idObj, NULL);
const char *id = env->GetStringUTFChars(idObj, NULL);

acquire_wake_lock(lock, id);
acquire_wake_lock(lock, id);

env->ReleaseStringUTFChars(idObj, id);
env->ReleaseStringUTFChars(idObj, id); 30
}
0xlab – connect your device to application – http://0xlab.org/ }

Power
hardware/libhardware_legacy/power/power.c
hardware/libhardware_legacy/power/power.c
...
...
int
int
acquire_wake_lock(int lock, const char* id)
acquire_wake_lock(int lock, const char* id)
{
{
initialize_fds();
initialize_fds();
const char * const OLD_PATHS[] = {
const char * const OLD_PATHS[] = { if (g_error) return g_error;
if (g_error) return g_error;
"/sys/android_power/acquire_partial_wake_lock",
"/sys/android_power/acquire_partial_wake_lock",
"/sys/android_power/release_wake_lock",
"/sys/android_power/release_wake_lock", int fd;
int fd;
"/sys/android_power/request_state"
"/sys/android_power/request_state" if (lock == PARTIAL_WAKE_LOCK) {
if (lock == PARTIAL_WAKE_LOCK) {
};
}; fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK];
fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK];
}
}
const char * const NEW_PATHS[] = {
const char * const NEW_PATHS[] = { else {
else {
"/sys/power/wake_lock",
"/sys/power/wake_lock", return EINVAL;
return EINVAL;
"/sys/power/wake_unlock",
"/sys/power/wake_unlock", }
}
"/sys/power/state"
"/sys/power/state" return write(fd, id, strlen(id));
return write(fd, id, strlen(id));
};
}; }
}
(Kernel interface changes in Android Cupcake)
(Kernel interface changes in Android Cupcake)

static inline void
static inline void
initialize_fds(void)
initialize_fds(void)
{
{
if (g_initialized == 0) {
if (g_initialized == 0) {
if(open_file_descriptors(NEW_PATHS) < 0) {
if(open_file_descriptors(NEW_PATHS) < 0) {
open_file_descriptors(OLD_PATHS);
open_file_descriptors(OLD_PATHS);
on_state = "wake";
on_state = "wake";
off_state = "standby";
off_state = "standby";
}
}
g_initialized = 1;
g_initialized = 1;
}
}
}
} 31

Android PM Kernel APIs
Source code (Cupcake, linux-2.6.27)
kernel/kernel/power/userwake.c
kernel/kernel/power/wakelock.c static long has_wake_lock_locked(int type)
static long has_wake_lock_locked(int type)
{
{
static int power_suspend_late(
static int power_suspend_late( struct wake_lock *lock, *n;
struct wake_lock *lock, *n;
struct platform_device *pdev,
struct platform_device *pdev, long max_timeout = 0;
long max_timeout = 0;
pm_message_t state)
pm_message_t state) BUG_ON(type >= WAKE_LOCK_TYPE_COUNT);
BUG_ON(type >= WAKE_LOCK_TYPE_COUNT);
{
{ list_for_each_entry_safe(lock, n,
list_for_each_entry_safe(lock, n,
int ret =
int ret = &active_wake_locks[type], link) {
&active_wake_locks[type], link) {
has_wake_lock(WAKE_LOCK_SUSPEND) ?
has_wake_lock(WAKE_LOCK_SUSPEND) ? if (lock->flags & WAKE_LOCK_AUTO_EXPIRE) {
if (lock->flags & WAKE_LOCK_AUTO_EXPIRE) {
-EAGAIN : 0;
-EAGAIN : 0; long timeout = lock->expires - jiffies;
long timeout = lock->expires - jiffies;
return ret;
return ret; if (timeout <= 0)
if (timeout <= 0)
}
} expire_wake_lock(lock);
expire_wake_lock(lock);
else if (timeout > max_timeout)
else if (timeout > max_timeout)
static struct platform_driver power_driver = {
static struct platform_driver power_driver = { max_timeout = timeout;
max_timeout = timeout;
.driver.name = "power",
.driver.name = "power", } else
} else
.suspend_late = power_suspend_late,
.suspend_late = power_suspend_late, return -1;
return -1;
};
}; }
}
static struct platform_device power_device = {
static struct platform_device power_device = { return max_timeout;
return max_timeout;
.name = "power",
.name = "power", }
}
};
};
long has_wake_lock(int type)
long has_wake_lock(int type)
{
{
long ret;
long ret;
unsigned long irqflags;
unsigned long irqflags;
spin_lock_irqsave(&list_lock, irqflags);
spin_lock_irqsave(&list_lock, irqflags);
ret = has_wake_lock_locked(type);
ret = has_wake_lock_locked(type);
spin_unlock_irqrestore(&list_lock, irqflags);
spin_unlock_irqrestore(&list_lock, irqflags);
return ret;
return ret;
}
} 32

Android PM Kernel APIs
kernel/kernel/power/wakelock.c

static int __init wakelocks_init(void)
static int __init wakelocks_init(void)
{
{
int ret;
int ret;
int i;
int i;

for (i = 0; i < ARRAY_SIZE(active_wake_locks); i++)
for (i = 0; i < ARRAY_SIZE(active_wake_locks); i++)
INIT_LIST_HEAD(&active_wake_locks[i]);
INIT_LIST_HEAD(&active_wake_locks[i]);

wake_lock_init(&main_wake_lock, WAKE_LOCK_SUSPEND, "main");
wake_lock_init(&main_wake_lock, WAKE_LOCK_SUSPEND, "main");
wake_lock(&main_wake_lock);
wake_lock(&main_wake_lock);
wake_lock_init(&unknown_wakeup, WAKE_LOCK_SUSPEND, "unknown_wakeups");
wake_lock_init(&unknown_wakeup, WAKE_LOCK_SUSPEND, "unknown_wakeups");

ret = platform_device_register(&power_device);
ret = platform_device_register(&power_device);
if (ret) {
if (ret) {
pr_err("wakelocks_init: platform_device_register
pr_err("wakelocks_init: platform_device_register failedn");
failedn");
goto err_platform_device_register;
goto err_platform_device_register;
}
}
ret = platform_driver_register(&power_driver);
ret = platform_driver_register(&power_driver);
if (ret) {
if (ret) {
pr_err("wakelocks_init: platform_driver_register
pr_err("wakelocks_init: platform_driver_register failedn");
failedn");
goto err_platform_driver_register;
goto err_platform_driver_register;
}
}

suspend_work_queue = create_singlethread_workqueue("suspend");
suspend_work_queue = create_singlethread_workqueue("suspend");
if (suspend_work_queue == NULL) {
if (suspend_work_queue == NULL) {
ret = -ENOMEM;
ret = -ENOMEM;
goto err_suspend_work_queue;
goto err_suspend_work_queue;
}
} 33

Back to userspace

Use “grep -r acquire_wake_lock” to discover.
frameworks/base/libs/ui/EventHub.cpp
EventHub::EventHub(), EventHub::~EventHub(),
EventHub::getEvent()
hardware/ril/libril/ril.cpp
RIL_onUnsolicitedResponse(),
system/wlan/ti/sta_dk_4_0_4_32/CUDK/tiwlan_loader/tiwlan_loader.c
start_cli()

34

Back to userspace

Use “grep -ri power” to discover.
base/core/jni/android_net_wifi_Wifi.cpp
android.net.wifi.WifiNative.setPowerModeCommand
(android_net_wifi_setPowerModeCommand)
base/core/java/com/android/internal/os/BatteryStats.java
base/core/java/com/android/internal/os/BatteryStatsImpl.java
base/core/java/com/android/os/PowerManager.java
base/core/java/com/android/os/LocalPowerManager.java
base/core/java/com/android/os/Power.java
base/core/java/com/android/app/ApplicationContext.java
base/core/java/android/bluetooth/ScoSocket.java
base/core/java/android/bluetooth/HeadsetBase.java
base/core/media/java/android/media/MediaPlayer.java
base/core/media/java/android/media/AsyncPlayer.java
35

Back to userspace

base/telephony/java/com/android/internal/telephony/gsm/GSMConnection.
java
base/telephony/java/com/android/internal/telephony/gsm/RIL.java
base/telephony/java/com/android/internal/telephony/gsm/GSMPhone.java
base/services/jni/com_android_server_BatteryService.cpp
base/services/java/com/android/server/am/ActivityManagerService.java
base/services/java/com/android/server/SystemServer.java
base/services/java/com/android/server/BatteryService.java
base/services/java/com/android/server/AlarmManagerService.java
base/services/java/com/android/server/LocationManagerService.java
base/services/java/com/android/server/HardwareService.java
base/services/java/com/android/server/PowerManagerService.java

36

Back to userspace

system/core/toolbox/powerd.c
system/core/toolbox/alarm.c
system/core/vold/mmc.c
system/core/vold/uevent.c

37

Basically, it is the slim wrapper for Linux Power Management

38

So, Android PM is simple

Key concept is “Wake Locks”, which is simple and portable.
The implementation should be introspected.

If there are no active wake locks, CPU will be turned
off.
If there is are partial wake locks, screen and
keyboard will be turned off.

Nowadays, CPU can enter into more states for power saving and
usability purpose! → applied in modern SoC
39

Interface to Physical World

Analog
The Real Signal Signal
Conditioning Conversion
World
to Digital
Temperature

Pressure

Position

Speed
Power Digital Signal Processor
Flow
Management
Humidity

Sound

Light
Digital
Signal
Signal
Conditioning Interface
Conversion
to Analog
Clocks & Timers
40

Advanced Power Management
Power management aims to improve battery life of equipment by
minimizing power consumption while guaranteeing expected system
performance

Active power consumption occurs while some processing is on-going
Dynamic power consumption (transistor switching) + Leakage
consumption

Static (also Standby or Idle) power consumption occurs when
limited or no processing is on-going and the system is waiting for a
wakeup event
Very limited dynamic power consumption + Leakage consumption
Managed by
Dynamic Voltage & Frequency Scaling (DVFS)
Adaptive Voltage Scaling (AVS)
Dynamic Power Switching (DPS)

On OMAP35xx, power management is handled by the
On OMAP35xx, power management is handled by the
Power, Reset and Clock Management (PRCM) module
Power, Reset and Clock Management (PRCM) module
41

Adaptive Voltage Scaling

Silicon manufacturing process yields a distribution of performance
capability
For a given frequency requirement:
Devices on hot/strong/fast end of distribution can meet this at a lower voltage
Devices on cold/weak/slow end of distribution need higher voltage

Simple system will set the higher voltage for operating all devices
Smarter system will adapt operating voltage per device.

hotter device
hotter device

42

Power Block Diagram

VDD1 DCDC MPU IVA
VDD2 DCDC
OMAP3530
CORE
VRTC
VIO DCDC
IOs Memory Display
Wake-Up domains
USB CP
VDAC Video DAC

USB PHY
VPLL PLL

VPP or
VAUX
Camera IF
VMMC

MMC1
TPS659xx
43

Reference Software Architecture

Applications

GUI

App Framework

2D/3D APIs Multimedia Optional
Framework DRM

GFX Driver
Codec Engine and Link
Codec Engine FC BIOS
Linux Kernel / WinCE
audio video image
& Power Management video image audio video image audio

GFX ARM C64x+ DSP and Video Acceleration

44

Linux PM Mechanisms

cpuidle
Generic framework for supporting software-controlled
idle processor power management
Hardware specific drivers
Various governing for the state transition decisions

Latency and power management
Framework for expressing latency constraints, and make sure
that they are taken into account for power management
decisions

Needs to be integrated into Android Partial Wake Locks
45

Summary

Integration is the key to the most
power-saving system

46

Reference

PDK :: Android – Power Management
http://www.netmite.com/android/mydroid/development/pdk/docs/power_ma
nagement.html
Android Cupcake source code
http://www.netmite.com/android/mydroid/cupcake/
BeagleBoard and its Linux support
http://elinux.org/BeagleBoard
class PowerManager
http://developer.android.com/reference/android/os/PowerManager.html
Free-Electrons
http://free-electrons.com/training
CELF's power management specification
http://elinux.org/Power_Management
47

Power Management from Linux Kernel to Android

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