Device Power Management ####################### Introduction ************ Device power management (PM) on Zephyr is a feature that enables devices to save energy when they are not being used. This feature can be enabled by setting :kconfig:option:`CONFIG_PM_DEVICE` to ``y``. When this option is selected, device drivers implementing power management will be able to take advantage of the device power management subsystem. Zephyr supports two methods of device power management: - :ref:`Device Runtime Power Management ` - :ref:`System-Managed Device Power Management ` .. _pm-device-runtime-pm: Device Runtime Power Management ******************************* Device runtime power management involves coordinated interaction between device drivers, subsystems, and applications. While device drivers play a crucial role in directly controlling the power state of devices, the decision to suspend or resume a device can also be influenced by higher layers of the software stack. Each layer—device drivers, subsystems, and applications—can operate independently without needing to know about the specifics of the other layers because the subsystem uses reference count to check when it needs to suspend or resume a device. - **Device drivers** are responsible for managing the power state of devices. They interact directly with the hardware to put devices into low-power states (suspend) when they are not in use, and bring them back (resume) when needed. Drivers should use the :ref:`device runtime power management APIs ` provided by Zephyr to control the power state of devices. - **Subsystems**, such as sensors, file systems, and network, can also influence device power management. Subsystems may have better knowledge about the overall system state and workload, allowing them to make informed decisions about when to suspend or resume devices. For example, a networking subsystem may decide to keep a network interface powered on if it expects network activity in the near future. - **Applications** running on Zephyr can impact device power management as well. An application may have specific requirements regarding device usage and power consumption. For example, an application that streams data over a network may need to keep the network interface powered on continuously. Coordination between device drivers, subsystems, and applications is key to efficient device power management. For example, a device driver may not know that a subsystem will perform a series of sequential operations that require a device to remain powered on. In such cases, the subsystem can use device runtime power management to ensure that the device remains in an active state until the operations are complete. When using this Device Runtime Power Management, the System Power Management subsystem is able to change power states quickly because it does not need to spend time suspending and resuming devices that are runtime enabled. For more information, see :ref:`pm-device-runtime`. .. _pm-device-system-pm: System-Managed Device Power Management ************************************** When using this method, device power management is mostly done inside :c:func:`pm_system_suspend()` along with entering a CPU or SOC power state. If a decision to enter a CPU lower power state is made, the power management subsystem will suspend devices before changing state. The subsystem takes care of suspending devices following their initialization order, ensuring that possible dependencies between them are satisfied. As soon as the CPU wakes up from a sleep state, devices are resumed in the opposite order that they were suspended. .. note:: When using :ref:`pm-system`, device transitions can be run from the idle thread. As functions in this context cannot block, transitions that intend to use blocking APIs **must** check whether they can do so with :c:func:`k_can_yield`. This method of device power management can be useful in the following scenarios: - Systems with no device requiring any blocking operations when suspending and resuming. This implementation is reasonably simpler than device runtime power management. - For devices that can not make any power management decision and have to be always active. For example a firmware using Zephyr that is controlled by an external entity (e.g Host CPU). In this scenario, some devices have to be always active and should be suspended together with the SoC when requested by this external entity. It is important to emphasize that this method has drawbacks (see above) and :ref:`Device Runtime Power Management ` is the **preferred** method for implementing device power management. .. note:: When using this method of device power management, the CPU will not enter a low-power state if a device cannot be suspended. For example, if a device returns an error such as ``-EBUSY`` in response to the ``PM_DEVICE_ACTION_SUSPEND`` action, indicating it is in the middle of a transaction that cannot be interrupted. Another condition that prevents the CPU from entering a low-power state is if the option :kconfig:option:`CONFIG_PM_NEED_ALL_DEVICES_IDLE` is set and a device is marked as busy. .. note:: This method of device power management is disabled when :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_EXCLUSIVE` is set to ``y`` (that is the default value when :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME` is enabled) .. note:: Devices are suspended only when the last active core is entering a low power state and devices are resumed by the first core that becomes active. Device Power Management States ****************************** The power management subsystem defines device states in :c:enum:`pm_device_state`. This method is used to track power states of a particular device. It is important to emphasize that, although the state is tracked by the subsystem, it is the responsibility of each device driver to handle device actions(:c:enum:`pm_device_action`) which change device state. Each :c:enum:`pm_device_action` have a direct an unambiguous relationship with a :c:enum:`pm_device_state`. .. graphviz:: :caption: Device actions x states digraph { node [shape=circle]; rankdir=LR; subgraph { SUSPENDED [label=PM_DEVICE_STATE_SUSPENDED]; SUSPENDING [label=PM_DEVICE_STATE_SUSPENDING]; ACTIVE [label=PM_DEVICE_STATE_ACTIVE]; OFF [label=PM_DEVICE_STATE_OFF]; ACTIVE -> SUSPENDING -> SUSPENDED; ACTIVE -> SUSPENDED ["label"="PM_DEVICE_ACTION_SUSPEND"]; SUSPENDED -> ACTIVE ["label"="PM_DEVICE_ACTION_RESUME"]; {rank = same; SUSPENDED; SUSPENDING;} OFF -> SUSPENDED ["label"="PM_DEVICE_ACTION_TURN_ON"]; SUSPENDED -> OFF ["label"="PM_DEVICE_ACTION_TURN_OFF"]; ACTIVE -> OFF ["label"="PM_DEVICE_ACTION_TURN_OFF"]; } } As mentioned above, device drivers do not directly change between these states. This is entirely done by the power management subsystem. Instead, drivers are responsible for implementing any hardware-specific tasks needed to handle state changes. Device Model with Power Management Support ****************************************** Drivers initialize devices using macros. See :ref:`device_model_api` for details on how these macros are used. A driver which implements device power management support must provide these macros with arguments that describe its power management implementation. Use :c:macro:`PM_DEVICE_DEFINE` or :c:macro:`PM_DEVICE_DT_DEFINE` to define the power management resources required by a driver. These macros allocate the driver-specific state which is required by the power management subsystem. Drivers can use :c:macro:`PM_DEVICE_GET` or :c:macro:`PM_DEVICE_DT_GET` to get a pointer to this state. These pointers should be passed to ``DEVICE_DEFINE`` or ``DEVICE_DT_DEFINE`` to initialize the power management field in each :c:struct:`device`. Here is some example code showing how to implement device power management support in a device driver. .. code-block:: c #define DT_DRV_COMPAT dummy_device static int dummy_driver_pm_action(const struct device *dev, enum pm_device_action action) { switch (action) { case PM_DEVICE_ACTION_SUSPEND: /* suspend the device */ ... break; case PM_DEVICE_ACTION_RESUME: /* resume the device */ ... break; case PM_DEVICE_ACTION_TURN_ON: /* * powered on the device, used when the power * domain this device belongs is resumed. */ ... break; case PM_DEVICE_ACTION_TURN_OFF: /* * power off the device, used when the power * domain this device belongs is suspended. */ ... break; default: return -ENOTSUP; } return 0; } PM_DEVICE_DT_INST_DEFINE(0, dummy_driver_pm_action); DEVICE_DT_INST_DEFINE(0, &dummy_init, PM_DEVICE_DT_INST_GET(0), NULL, NULL, POST_KERNEL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, NULL); .. _pm-device-busy: Busy Status Indication ********************** When the system is idle and the SoC is going to sleep, the power management subsystem can suspend devices, as described in :ref:`pm-device-system-pm`. This can cause device hardware to lose some states. Suspending a device which is in the middle of a hardware transaction, such as writing to a flash memory, may lead to undefined behavior or inconsistent states. This API guards such transactions by indicating to the kernel that the device is in the middle of an operation and should not be suspended. When :c:func:`pm_device_busy_set` is called, the device is marked as busy and the system will not do power management on it. After the device is no longer doing an operation and can be suspended, it should call :c:func:`pm_device_busy_clear`. 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 responsibility of driver or the application to do any additional configuration required by the device to support it. Power Domain ************ Power domain on Zephyr is represented as a regular device. The power management subsystem ensures that a domain is resumed before and suspended after devices using it. For more details, see :ref:`pm-power-domain`.