Power Domain

Introduction

The Zephyr power domain abstraction is designed to support groupings of devices powered by a common source to be notified of power source state changes in a generic fashion. Application code that is using device A does not need to know that device B is on the same power domain and should also be configured into a low power state.

Power domains are optional on Zephyr, to enable this feature the option CONFIG_PM_DEVICE_POWER_DOMAIN has to be set.

When a power domain turns itself on or off, it is the responsibilty of the power domain to notify all devices using it through their power management callback called with PM_DEVICE_ACTION_TURN_ON or PM_DEVICE_ACTION_TURN_OFF respectively. This work flow is illustrated in the diagram bellow.

Fig. 31 Power domain work flow

Internal Power Domains

Most of the devices in an SoC have independent power control that can be turned on or off to reduce power consumption. But there is a significant amount of static current leakage that can’t be controlled only using device power management. To solve this problem, SoCs normally are divided into several regions grouping devices that are generally used together, so that an unused region can be completely powered off to eliminate this leakage. These regions are called “power domains”, can be present in a hierarchy and can be nested.

External Power Domains

Devices external to a SoC can be powered from sources other than the main power source of the SoC. These external sources are typically a switch, a regulator, or a dedicated power IC. Multiple devices can be powered from the same source, and this grouping of devices is typically called a “power domain”.

Placing devices on power domains can be done for a variety of reasons, including to enable devices with high power consumption in low power mode to be completely turned off when not in use.

Implementation guidelines

In a first place, a device that acts as a power domain needs to declare compatible with power-domain. Taking Power domain work flow as example, the following code defines a domain called gpio_domain.

gpio_domain: gpio_domain@4 {
        compatible = "power-domain";
        ...
};

A power domain needs to implement the PM action callback used by the PM subsystem to turn devices on and off.

static int mydomain_pm_action(const struct device *dev,
                           enum pm_device_action *action)
{
    switch (action) {
    case PM_DEVICE_ACTION_RESUME:
        /* resume the domain */
        ...
        /* notify children domain is now powered */
        pm_device_children_action_run(dev, PM_DEVICE_ACTION_TURN_ON, NULL);
        break;
    case PM_DEVICE_ACTION_SUSPEND:
        /* notify children domain is going down */
        pm_device_children_action_run(dev, PM_DEVICE_ACTION_TURN_OFF, NULL);
        /* suspend the domain */
        ...
        break;
    case PM_DEVICE_ACTION_TURN_ON:
        /* turn on the domain (e.g. setup control pins to disabled) */
        ...
        break;
    case PM_DEVICE_ACTION_TURN_OFF:
        /* turn off the domain (e.g. reset control pins to default state) */
        ...
        break;
    default:
        return -ENOTSUP;
    }

    return 0;
}

Devices belonging to this device can be declared referring it in the power-domain node’s property. The example below declares devices gpio0 and gpio1 belonging to domain gpio_domain`.

&gpio0 {
        compatible = "zephyr,gpio-emul";
        gpio-controller;
        power-domain = <&gpio_domain>;
};

&gpio1 {
        compatible = "zephyr,gpio-emul";
        gpio-controller;
        power-domain = <&gpio_domain>;
};

All devices under a domain will be notified when the domain changes state. These notifications are sent as actions in the device PM action callback and can be used by them to do any additional work required. They can safely be ignored though.

static int mydev_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:
        /* configure the device into low power mode */
        ...
        break;
    case PM_DEVICE_ACTION_TURN_OFF:
        /* prepare the device for power down */
        ...
        break;
    default:
        return -ENOTSUP;
    }

    return 0;
}

Note

It is responsibility of driver or the application to set the domain as “wakeup” source if a device depending on it is used as “wakeup” source.