Power Management

Zephyr RTOS power management subsystem provides several means for a system integrator to implement power management support that can take full advantage of the power saving features of SOCs.

Terminology

SOC interface
This is a general term for the components that have knowledge of the SOC and provide interfaces to the hardware features. It will abstract the SOC specific implementations to the applications and the OS.
CPU LPS (Low Power State)
Refers to any one of the low power states supported by the CPU. The CPU is usually powered on while the clocks are power gated.
Active State
The CPU and clocks are powered on. This is the normal operating state when the system is running.
Deep Sleep State
The CPU is power gated and loses context. Most peripherals would also be power gated. RAM is selectively retained.
SOC Power State
SOC Power State describes processor and device power states implemented at the SOC level. Deep Sleep State is an example of SOC Power State.
Idle Thread
A system thread that runs when there are no other threads ready to run.
Power gating
Power gating reduces power consumption by shutting off current to blocks of the integrated circuit that are not in use.

Overview

The interfaces and APIs provided by the power management subsystem are designed to be architecture and SOC independent. This enables power management implementations to be easily adapted to different SOCs and architectures. The kernel does not implement any power schemes of its own, giving the system integrator the flexibility of implementing custom power schemes.

The architecture and SOC independence is achieved by separating the core infrastructure and the SOC specific implementations. The SOC specific implementations are abstracted to the application and the OS using hardware abstraction layers.

The power management features are classified into the following categories.

  • Tickless Idle
  • System Power Management
  • Device Power Management

Tickless Idle

This is the name used to identify the event-based idling mechanism of the Zephyr RTOS kernel scheduler. The kernel scheduler can run in two modes. During normal operation, when at least one thread is active, it sets up the system timer in periodic mode and runs in an interval-based scheduling mode. The interval-based mode allows it to time slice between threads. Many times, the threads would be waiting on semaphores, timeouts or for events. When there are no threads running, it is inefficient for the kernel scheduler to run in interval-based mode. This is because, in this mode the timer would trigger an interrupt at fixed intervals causing the scheduler to be invoked at each interval. The scheduler checks if any thread is ready to run. If no thread is ready to run then it is a waste of power because of the unnecessary CPU processing. This is avoided by the kernel switching to event-based idling mode whenever there is no thread ready to run.

The kernel holds an ordered list of thread timeouts in the system. These are the amount of time each thread has requested to wait. When the last active thread goes to wait, the idle thread is scheduled. The idle thread programs the timer to one-shot mode and programs the count to the earliest timeout from the ordered thread timeout list. When the timer expires, a timer event is generated. The ISR of this event will invoke the scheduler, which would schedule the thread associated with the timeout. Before scheduling the thread, the scheduler would switch the timer again to periodic mode. This method saves power because the CPU is removed from the wait only when there is a thread ready to run or if an external event occurred.

System Power Management

This consists of the hook functions that the power management subsystem calls when the kernel enters and exits the idle state, in other words, when the kernel has nothing to schedule. Enabling system power management compels Zephyr kernel scheduler to work in tickless idle mode (see CONFIG_TICKLESS_IDLE).

This section provides a general overview of the hook functions. Refer to Power Management APIs for the detailed description of the APIs.

Suspend Hook function

int _sys_soc_suspend(s32_t ticks);

When the kernel is about to go idle, the power management subsystem calls the _sys_soc_suspend() function, notifying the SOC interface that the kernel is ready to enter the idle state.

At this point, the kernel has disabled interrupts and computed the maximum time the system can remain idle. The function passes the time that the system can remain idle. The SOC interface performs power operations that can be done in the available time. The power management operation must halt execution on a CPU or SOC low power state. Before entering the low power state, the SOC interface must setup a wake event.

The power management subsystem expects the _sys_soc_suspend() to return one of the following values based on the power management operations the SOC interface executed:

SYS_PM_NOT_HANDLED

Indicates that no power management operations were performed.

SYS_PM_LOW_POWER_STATE

Indicates that the CPU was put in a low power state.

SYS_PM_DEEP_SLEEP

Indicates that the SOC was put in a deep sleep state.

Resume Hook function

void _sys_soc_resume(void);

The power management subsystem optionally calls this hook function when exiting kernel idling if power management operations were performed in _sys_soc_suspend(). Any necessary recovery operations can be performed in this function before the kernel scheduler schedules another thread. Some power states may not need this notification. It can be disabled by calling _sys_soc_pm_idle_exit_notification_disable() from _sys_soc_suspend().

Resume From Deep Sleep Hook function

void _sys_soc_resume_from_deep_sleep(void);

This function is optionally called when exiting from deep sleep if the SOC interface does not have bootloader support to handle resume from deep sleep. This function should restore context to the point where system entered the deep sleep state.

Note

Since the hook functions are called with the interrupts disabled, the SOC interface should ensure that its operations are completed quickly. Thus, the SOC interface ensures that the kernel’s scheduling performance is not disrupted.

Power Schemes

When the power management subsystem notifies the SOC interface that the kernel is about to enter a system idle state, it specifies the period of time the system intends to stay idle. The SOC interface can perform various power management operations during this time. For example, put the processor or the SOC in a low power state, turn off some or all of the peripherals or power gate device clocks.

Different levels of power savings and different wake latencies characterize these power schemes. In general, operations that save more power have a higher wake latency. When making decisions, the SOC interface chooses the scheme that saves the most power. At the same time, the scheme’s total execution time must fit within the idle time allotted by the power management subsystem.

The power management subsystem classifies power management schemes into two categories based on whether the CPU loses execution context during the power state transition.

  • SYS_PM_LOW_POWER_STATE
  • SYS_PM_DEEP_SLEEP

SYS_PM_LOW_POWER_STATE

CPU does not lose execution context. Devices also do not lose power while entering power states in this category. The wake latencies of power states in this category are relatively low.

SYS_PM_DEEP_SLEEP

CPU is power gated and loses execution context. Execution will resume at OS startup code or at a resume point determined by a bootloader that supports deep sleep resume. Depending on the SOC’s implementation of the power saving feature, it may turn off power to most devices. RAM may be retained by some implementations, while others may remove power from RAM saving considerable power. Power states in this category save more power than SYS_PM_LOW_POWER_STATE and would have higher wake latencies.

Device Power Management Infrastructure

The device power management infrastructure consists of interfaces to the Zephyr RTOS device model. These APIs send control commands to the device driver to update its power state or to get its current power state. Refer to Power Management APIs for detailed descriptions of the APIs.

Zephyr RTOS supports two methods of doing device power management.

  • Distributed method
  • Central method

Distributed method

In this method, the application or any component that deals with devices directly and has the best knowledge of their use does the device power management. This saves power if some devices that are not in use can be turned off or put in power saving mode. This method allows saving power even when the CPU is active. The components that use the devices need to be power aware and should be able to make decisions related to managing device power. In this method, the SOC interface can enter CPU or SOC low power states quickly when _sys_soc_suspend() gets called. This is because it does not need to spend time doing device power management if the devices are already put in the appropriate low power state by the application or component managing the devices.

Central method

In this method device power management is mostly done inside _sys_soc_suspend() along with entering a CPU or SOC low power state.

If a decision to enter deep sleep is made, the implementation would enter it only after checking if the devices are not in the middle of a hardware transaction that cannot be interrupted. This method can be used in implementations where the applications and components using devices are not expected to be power aware and do not implement device power management.

This method can also be used to emulate a hardware feature supported by some SOCs which cause automatic entry to deep sleep when all devices are idle. Refer to Busy Status Indication to see how to indicate whether a device is busy or idle.

Device Power Management States

The Zephyr RTOS power management subsystem defines four device states. These states are classified based on the degree of device context that gets lost in those states, kind of operations done to save power, and the impact on the device behavior due to the state transition. Device context includes device registers, clocks, memory etc.

The four device power states:

DEVICE_PM_ACTIVE_STATE

Normal operation of the device. All device context is retained.

DEVICE_PM_LOW_POWER_STATE

Device context is preserved by the HW and need not be restored by the driver.

DEVICE_PM_SUSPEND_STATE

Most device context is lost by the hardware. Device drivers must save and restore or reinitialize any context lost by the hardware.

DEVICE_PM_OFF_STATE

Power has been fully removed from the device. The device context is lost when this state is entered. Need to reinitialize the device when powering it back on.

Device Power Management Operations

Zephyr RTOS power management subsystem provides a control function interface to device drivers to indicate power management operations to perform. The supported PM control commands are:

  • DEVICE_PM_SET_POWER_STATE
  • DEVICE_PM_GET_POWER_STATE

Each device driver defines:

  • The device’s supported power states.
  • The device’s supported transitions between power states.
  • The device’s necessary operations to handle the transition between power states.

The following are some examples of operations that the device driver may perform in transition between power states:

  • Save/Restore device states.
  • Gate/Un-gate clocks.
  • Gate/Un-gate power.
  • Mask/Un-mask interrupts.

Device Model with Power Management Support

Drivers initialize the devices using macros. See Device Drivers and Device Model for details on how these macros are used. Use the DEVICE_DEFINE macro to initialize drivers providing power management support via the PM control function. One of the macro parameters is the pointer to the device_pm_control handler function.

Default Initializer Function

int device_pm_control_nop(struct device *unused_device, u32_t unused_ctrl_command, void *unused_context);

If the driver doesn’t implement any power control operations, the driver can initialize the corresponding pointer with this default nop function. This default nop function does nothing and should be used instead of implementing a dummy function to avoid wasting code memory in the driver.

Device Power Management API

The SOC interface and application use these APIs to perform power management operations on the devices.

Get Device List

void device_list_get(struct device **device_list, int *device_count);

The Zephyr RTOS kernel internally maintains a list of all devices in the system. The SOC interface uses this API to get the device list. The SOC interface can use the list to identify the devices on which to execute power management operations.

Note

Ensure that the SOC interface does not alter the original list. Since the kernel uses the original list, it must remain unchanged.

Device Set Power State

int device_set_power_state(struct device *device, u32_t device_power_state);

Calls the device_pm_control() handler function implemented by the device driver with DEVICE_PM_SET_POWER_STATE command.

Device Get Power State

int device_get_power_state(struct device *device, u32_t * device_power_state);

Calls the device_pm_control() handler function implemented by the device driver with DEVICE_PM_GET_POWER_STATE command.

Busy Status Indication

The SOC interface executes some power policies that can turn off power to devices, causing them to lose their state. If the devices are in the middle of some hardware transaction, like writing to flash memory when the power is turned off, then such transactions would be left in an inconsistent state. This infrastructure guards such transactions by indicating to the SOC interface that the device is in the middle of a hardware transaction.

When the _sys_soc_suspend() is called, the SOC interface checks if any device is busy. The SOC interface can then decide to execute a power management scheme other than deep sleep or to defer power management operations until the next call of _sys_soc_suspend().

An alternative to using the busy status mechanism is to use the distributed method of device power management. In such a method where the device power management is handled in a distributed manner rather than centrally in _sys_soc_suspend(), the decision to enter deep sleep can be made based on whether all devices are already turned off.

This feature can be also used to emulate a hardware feature found in some SOCs that causes the system to automatically enter deep sleep when all devices are idle. In such an usage, the busy status can be set by default and cleared as each device becomes idle. When _sys_soc_suspend() is called, deep sleep can be entered if no device is found to be busy.

Here are the APIs used to set, clear, and check the busy status of devices.

Indicate Busy Status API

void device_busy_set(struct device *busy_dev);

Sets a bit corresponding to the device, in a data structure maintained by the kernel, to indicate whether or not it is in the middle of a transaction.

Clear Busy Status API

void device_busy_clear(struct device *busy_dev);

Clears the bit corresponding to the device in a data structure maintained by the kernel to indicate that the device is not in the middle of a transaction.

Check Busy Status of Single Device API

int device_busy_check(struct device *chk_dev);

Checks whether a device is busy. The API returns 0 if the device is not busy.

Check Busy Status of All Devices API

int device_any_busy_check(void);

Checks if any device is busy. The API returns 0 if no device in the system is busy.

Power Management Configuration Flags

The Power Management features can be individually enabled and disabled using the following configuration flags.

CONFIG_SYS_POWER_MANAGEMENT

This flag enables the power management subsystem.

CONFIG_TICKLESS_IDLE

This flag enables the tickless idle power saving feature.

CONFIG_SYS_POWER_LOW_POWER_STATE

The SOC interface enables this flag to use the SYS_PM_LOW_POWER_STATE policy.

CONFIG_SYS_POWER_DEEP_SLEEP

This flag enables support for the SYS_PM_DEEP_SLEEP policy.

CONFIG_DEVICE_POWER_MANAGEMENT

This flag is enabled if the SOC interface and the devices support device power management.