.. _power_management_api: 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 *********** :dfn:`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. :dfn:`Idle Thread` A system thread that runs when there are no other threads ready to run. :dfn:`Power gating` Power gating reduces power consumption by shutting off current to blocks of the integrated circuit that are not in use. :dfn:`Power State` SOC Power State describes processor and device power states implemented at the SOC level. Power states are represented by :c:enum:`pm_state` and each one has a different meaning. :dfn:`Device Runtime Power Management` Device Runtime Power Management (PM) refers the capability of devices be able of saving energy independently of the the system. Devices will keep reference of their usage and will automatically be suspended or resumed. This feature is enabled via the ::kconfig:`CONFIG_PM_DEVICE_RUNTIME` Kconfig option. 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 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. * System Power Management * Device Power Management System Power Management *********************** The kernel enters the idle state when it has nothing to schedule. If enabled via the :kconfig:`CONFIG_PM` Kconfig option, the Power Management Subsystem can put an idle system in one of the supported power states, based on the selected power management policy and the duration of the idle time allotted by the kernel. It is an application responsibility to set up a wake up event. A wake up event will typically be an interrupt triggered by one of the SoC peripheral modules such as a SysTick, RTC, counter, or GPIO. Depending on the power mode entered, only some SoC peripheral modules may be active and can be used as a wake up source. The following diagram describes system power management: .. image:: ../../../../../zephyr/doc/reference/power_management/system-pm.svg :align: center :alt: System power management Some handful examples using different power management features: * :zephyr_file:`samples/boards/stm32/power_mgmt/blinky/` * :zephyr_file:`samples/boards/nrf/system_off/` * :zephyr_file:`samples/subsys/pm/device_pm/` * :zephyr_file:`tests/subsys/pm/power_mgmt/` * :zephyr_file:`tests/subsys/pm/power_mgmt_soc/` * :zephyr_file:`tests/subsys/pm/power_state_api/` Power States ============ The power management subsystem contains a set of states based on power consumption and context retention. The list of available power states is defined by :c:enum:`pm_state`. In general power states with higher indexes will offer greater power savings and have higher wake latencies. Following is a thorough list of available states: .. doxygenenumvalue:: PM_STATE_ACTIVE .. doxygenenumvalue:: PM_STATE_RUNTIME_IDLE .. doxygenenumvalue:: PM_STATE_SUSPEND_TO_IDLE .. doxygenenumvalue:: PM_STATE_STANDBY .. doxygenenumvalue:: PM_STATE_SUSPEND_TO_RAM .. doxygenenumvalue:: PM_STATE_SUSPEND_TO_DISK .. doxygenenumvalue:: PM_STATE_SOFT_OFF .. _pm_constraints: Power States Constraint ======================= The power management subsystem allows different Zephyr components and applications to set constraints on various power states preventing the system from transitiioning into these states. This can be used by devices when executing tasks in background to avoid the system to go to a specific state where it would lose context. Constraints can be set, released and checked using the follow APIs: .. doxygenfunction:: pm_constraint_set .. doxygenfunction:: pm_constraint_release .. doxygenfunction:: pm_constraint_get Power Management Policies ========================= The power management subsystem supports the following power management policies: * Residency based * Application defined The policy manager is responsible for informing the power subsystem which power state the system should transition to based on states defined by the platform and possible runtime :ref:`constraints` Information about states can be found in the device tree, see :zephyr_file:`dts/bindings/power/state.yaml`. Residency --------- The power management system enters the power state which offers the highest power savings, and with a minimum residency value (in device tree, see :zephyr_file:`dts/bindings/power/state.yaml`) less than or equal to the scheduled system idle time duration. This policy also accounts for the time necessary to become active again. The core logic used by this policy to select the best power state is: .. code-block:: c if (time_to_next_scheduled_event >= (state.min_residency_us + state.exit_latency))) { return state } Application ----------- The power management policy is defined by the application which has to implement the following function. .. code-block:: c struct pm_state_info pm_policy_next_state(int32_t ticks); In this policy the application is free to decide which power state the system should transition to based on the remaining time for the next scheduled timeout. An example of an application that defines its own policy can be found in :zephyr_file:`tests/subsys/pm/power_mgmt/`. Device Power Management Infrastructure ************************************** The device power management infrastructure consists of interfaces to the :ref:`device_model_api`. These APIs send control commands to the device driver to update its power state or to get its current power state. Zephyr RTOS supports two methods of doing device power management. * Runtime Device Power Management * System Power Management Runtime Device Power Management =============================== In this method, the application or any component that deals with devices directly and has the best knowledge of their use, performs 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 power states quickly when :code:`pm_system_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 power state by the application or component managing the devices. System Power Management ======================= In this method device power management is mostly done inside :code:`pm_system_suspend()` along with entering a CPU or SOC power state. If a decision to enter a lower power state 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 runtime device power management. .. image:: ../../../../../zephyr/doc/reference/power_management/central_method.svg :align: center This method can also be used to emulate a hardware feature supported by some SOCs which triggers automatic entry to a lower power state 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 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: :code:`PM_DEVICE_STATE_ACTIVE` Normal operation of the device. All device context is retained. :code:`PM_DEVICE_STATE_LOW_POWER` Device context is preserved by the HW and need not be restored by the driver. :code:`PM_DEVICE_STATE_SUSPENDED` Most device context is lost by the hardware. Device drivers must save and restore or reinitialize any context lost by the hardware. :code:`PM_DEVICE_STATE_OFF` 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. 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 :ref:`device_model_api` 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 pm_control handler function. If the driver doesn't implement any power control operations, it can initialize the corresponding pointer with ``NULL``. Device Power Management API =========================== The SOC interface and application use these APIs to perform power management operations on the devices. Get Device List --------------- .. code-block:: c size_t z_device_get_all_static(struct device const **device_list); 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 ---------------------- .. code-block:: c int pm_device_state_set(const struct device *dev, enum pm_device_state state); Calls the :c:func:`pm_control()` handler function implemented by the device driver with the provided state. Device Get Power State ---------------------- .. code-block:: c int pm_device_state_get(const struct device *dev, enum pm_device_state *state); 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 :c:func:`pm_system_suspend()` is called, depending on the power state returned by the policy manager, the system may suspend or put devices in low power if they are not marked as busy. Here are the APIs used to set, clear, and check the busy status of devices. Indicate Busy Status API ------------------------ .. code-block:: c void device_busy_set(const 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 --------------------- .. code-block:: c void device_busy_clear(const 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 -------------------------------------- .. code-block:: c int device_busy_check(const struct device *chk_dev); Checks whether a device is busy. The API returns 0 if the device is not busy. This API is used by the system power management. Check Busy Status of All Devices API ------------------------------------ .. code-block:: c int device_any_busy_check(void); Checks if any device is busy. The API returns 0 if no device in the system is busy. Wakeup capability ----------------- Some devices are capable of waking the system up from a sleep state. When a device has such capability, applications can enable or disable this feature on a device dynamically using :c:func:`pm_device_wakeup_enable`. This property can be set on device declaring the property ``wakeup-source`` in the device node in devicetree. For example, this devicetree fragment sets the ``gpio0`` device as a "wakeup" source: .. code-block:: devicetree gpio0: gpio@40022000 { compatible = "ti,cc13xx-cc26xx-gpio"; reg = <0x40022000 0x400>; interrupts = <0 0>; status = "disabled"; label = "GPIO_0"; gpio-controller; wakeup-source; #gpio-cells = <2>; }; By default, "wakeup" capable devices do not have this functionality enabled during the device initialization. Applications can enable this functionality later calling :c:func:`pm_device_wakeup_enable`. .. note:: This property is **only** used by the system power management to identify devices that should not be suspended. It is responsability of driver or the application to do any additional configuration required by the device to support it. Device Runtime Power Management ******************************* The Device Runtime Power Management framework is an Active Power Management mechanism which reduces the overall system Power consumtion by suspending the devices which are idle or not being used while the System is active or running. The framework uses :c:func:`pm_device_state_set()` API set the device power state accordingly based on the usage count. The interfaces and APIs provided by the Device Runtime PM are designed to be generic and architecture independent. Device Runtime Power Management API =================================== The Device Drivers use these APIs to perform device runtime power management operations on the devices. Enable Device Runtime Power Management of a Device API ------------------------------------------------------ .. code-block:: c void pm_device_enable(const struct device *dev); Enables Runtime Power Management of the device. Disable Device Runtime Power Management of a Device API ------------------------------------------------------- .. code-block:: c void pm_device_disable(const struct device *dev); Disables Runtime Power Management of the device. Resume Device asynchronously API -------------------------------- .. code-block:: c int pm_device_get_async(const struct device *dev); Marks the device as being used. This API will asynchronously bring the device to resume state if it was suspended. If the device was already active, it just increments the device usage count. The API returns 0 on success. Device drivers can monitor this operation to finish calling :c:func:`pm_device_wait`. Resume Device synchronously API ------------------------------- .. code-block:: c int pm_device_get(const struct device *dev); Marks the device as being used. It will bring up or resume the device if it is in suspended state based on the device usage count. This call is blocked until the device PM state is changed to active. The API returns 0 on success. Suspend Device asynchronously API --------------------------------- .. code-block:: c int pm_device_put_async(const struct device *dev); Releases a device. This API asynchronously puts the device to suspend state if not already in suspend state if the usage count of this device reaches 0. Device drivers can monitor this operation to finish calling :c:func:`pm_device_wait`. Suspend Device synchronously API -------------------------------- .. code-block:: c int pm_device_put(const struct device *dev); Marks the device as being released. It will put the device to suspended state if is is in active state based on the device usage count. This call is blocked until the device PM state is changed to resume. The API returns 0 on success. This call is blocked until the device is suspended. Power Management Configuration Flags ************************************ The Power Management features can be individually enabled and disabled using the following configuration flags. :kconfig:`CONFIG_PM` This flag enables the power management subsystem. :kconfig:`CONFIG_PM_DEVICE` This flag is enabled if the SOC interface and the devices support device power management. :kconfig:`CONFIG_PM_DEVICE_RUNTIME` This flag enables the Runtime Power Management. API Reference ************* Power Management Hook Interface =============================== .. doxygengroup:: power_management_hook_interface System Power Management APIs ============================ .. doxygengroup:: system_power_management_api Device Power Management APIs ============================ .. doxygengroup:: device_power_management_api