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DeviceWake

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The DeviceWake member of DEVICE_CAPABILITIES contains the lowest (least-powered) device power state from which the device can signal a wake event, or PowerDeviceUnspecified if the device cannot wake in response to an external signal.

The bus driver sets this value. A higher-level driver can change the value to a higher-powered state. For example, if the bus driver sets DeviceWake to D3 but a driver further up the device stack supports wake-up only from D2, the higher-level driver can change the value to D2.

Note that if a driver changes DeviceWake, it might also have to change SystemWake to avoid conflicts with the system-to-device mappings in the DeviceState array. For example, assume that the bus driver sets the following:

  • DeviceState[PowerSystemSleeping1] = PowerDeviceD1

  • DeviceState[PowerSystemSleeping2] = PowerDeviceD3

  • DeviceWake = PowerDeviceD3

  • SystemWake = PowerSystemSleeping2

If a higher-level driver determines that its device cannot wake the system from D3, but only from D2 or higher, it can change DeviceWake to D2. However, this change causes the mapping from S2 to D3 to be impossible. Remember that the DeviceState array lists the highest device power state a device can support for a given system power state. If the system power state in the example is PowerSystemSleeping2, the device power state cannot be PowerDeviceD2. To eliminate this problem, the driver must also change SystemWake to PowerSystemSleeping1. The same is true for the WakeFromDx and DeviceDx settings. A driver must ensure that any changes it makes to SystemWake or DeviceWake do not conflict with the WakeFromDx and DeviceDx values. The values of WakeFromDx and DeviceDx reflect hardware characteristics that a driver cannot change.

If both the SystemWake and DeviceWake members are nonzero (that is, not PowerSystemUnspecified), then the device and its drivers support wake-up on this system.

On non-ACPI hardware, the DeviceWake member contains zero (PowerSystemUnspecified).