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Device Power Management Infrastructure

The device power management infrastructure consists of interfaces to the Device Driver Model. 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 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 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.

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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 device states in pm_device_state. 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.

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 Device Driver 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 PM action callback. If the driver doesn’t implement any power control operations, it can initialize the corresponding pointer with NULL.

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 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.

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 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:

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 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.