Note that some devices are capable of several different low power modes within a single device power state. Such a device can use these modes if its driver can automatically switch the device from one mode to another without changing the device power state. As a general rule, however, if there is no user-perceptible difference between the modes, the device should use only the lowest power mode. If a low power mode, such as a low-speed mode, adversely affects performance or is not transparent to software other than the device driver, the hardware should not automatically use it. See the Device Class Power Management Reference Specification for details.
A driver or the power manager can request a device power state transition, and all drivers must be prepared to handle IRPs that request such transitions. For more information, see the following topics:
Device Ntpnp Pci0012 Driver
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In the preceding graph, the transition from D3hot to D3cold is the only direct transition between device low-power states. All other transitions between low-power states require an intermediate transition to D0, which allows the device driver to configure the device hardware, as required, either to enter the next low-power state or to stay in D0. However, a device exits D3hot and enters D3cold when power to the device is shut off, which requires no intervention from the device driver. This driver does any necessary configuration of the device hardware before the device enters D3hot; no additional configuration is required to prepare the device for the transition from D3hot to D3cold. For more information, see Supporting D3cold in a Driver. 2ff7e9595c
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