Device Power Management Specification

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Background
Various devices on embedded platforms upport low-power states that can be employed by CE products at times when full operation of the device is not required. The ability to manage device power usage may be crucial to many CE products, especially those powered by batteries.

Generic Linux contains some support for these topics. However, device suspend/resume support is not a priority at present, and neither is support for embedded platforms. This specification addresses the potential lack of device power management features on a platform used for CE products by requiring that a CELF-conforming Linux for that platform support its basic device power management capabilities. This specification also requires a minimal set of functionality closely associated with device power management. The basic capabilities outlined here may be extended by future CELF specifications that cover additional features useful for CE products.

Terminology

 * CE platform device : : A device that is closely associated with the platform, that is supported by a Linux driver under an open-source license, and that may reasonably be expected to appear in an actual consumer electronics product based on the platform. See further discussion below.


 * Device resume : : The process of restoring the state of a device that was previously suspended to normal operation.


 * Device suspend : : The process of placing a device into a device suspend state, and/or of preparing the device for a system suspend. This may occur for such reasons as explicit instructions to power down an unneeded device by an application, or as part of a system suspend.  Depending on the platform and device, device suspend may include saving state to allow later restoration of state at device resume time.


 * Device suspend state : : A reduced-power state supported by a device. Many, but not all, devices support at least one device suspend state, which may prevent operation of the device until a device resume is performed.  The various device suspend states may be activated by device-specific interfaces and/or automatic criteria, or may follow standards such as ACPI.  This specification primarily targets reduced-power states that are activated by the device driver when needed (rather than automatic hardware mechanisms, such as inactivity timers).

Device Suspend/Resume Discussion
Device suspend may occur for reasons that include:


 * an application explicitly manipulates device state, such as to power down a device no longer required by the application
 * a system suspend occurs, which suspends all devices prior to suspending the system
 * a power policy management subsystem, such as DPM, places the system in a state that is incompatible with operation of the device
 * a hardware or software mechanism triggers a low-power state after a period of inactivity
 * the driver powers down the device because applications no longer hold an open reference to the device

In many cases, CELF specifies support for evaluation or reference boards, based on which CE products may be derived using a custom hardware design that incorporates the processor and various devices. This specification targets only devices and drivers termed "CE platform devices" here, which meet these criteria:


 * The device is closely associated with the platform. A CE platform device may be physically located on a single-board computer or in some other way be tightly coupled to the platform supported by CELF-conforming kernel source, such that its presence is likely in many products that may be based on the platform.  Devices not included in this definition include arbitrary cards that plug into buses provided on the platform, such as PCI or PCMCIA, or that may be attached in custom hardware designs.
 * The device is supported by a Linux driver under an open-source license.
 * The device may reasonably be expected to appear in an actual consumer electronics product based on the platform. A CE platform device may be distinguished from devices that are present on evaluation or reference boards for development or debugging purposes, such as an ethernet interface that is unlikely to appear in an actual CE product.

This specification targets CE platform devices exclusively, in order to give product designers the necessary tools to save power in actual product configurations. This distinction is made in order to avoid mandating power management capabilities for:
 * devices not present in the CELF-supported evaluation/reference boards
 * devices for which no open source driver has been made available
 * devices that serve only a development or diagnostic function

Device state may need to be be saved during the device suspend operation, such that device operation can later be restored to approximately the same condition at device resume. If so, device state is typically saved in SDRAM since SDRAM is usually powered (perhaps in self-refresh state) during the suspend interval -- a platform that does not preserve SDRAM during suspend generally must reboot at resume time, whereupon device state can be restored from stable storage if needed.

System suspend may remove power from some or all devices, depending on the platform and the particular system suspend state entered, leaving the devices unpowered during the suspend interval and restoring power at system resume. This may affect the manner in which device resume occurs during system resume, since recovering from a power cycle may require different procedures than are needed for individual device suspend/resume. This may also affect the actions to be taken to accomplish a device suspend during system suspend; for example, entering a low-power suspend state may not be useful if the platform is about to remove power from the device.

Specification

 * 1) If a CE platform device that supports device suspend/resume actions, then both kernel programmatic interfaces and userspace interfaces MUST be provided to individually perform device suspend and device resume for only that device. These interfaces SHOULD suspend other devices that depend upon the selected device for correct operation. Other devices that do not depend upon the selected device for correct operation MUST NOT be suspended or resumed by these interfaces.


 * 1) If a CE platform device supports a device suspend state, or if any actions are needed in order to correctly resume device operation after a system resume from at least one system suspend state supported by the Linux kernel, then the driver for the device SHOULD implement the support necessary for the device suspend and resume interfaces.


 * 1) If a CE platform device supports one device suspend state, then the device suspend processing performed by the driver for the device MUST be capable of causing the device to enter the supported device suspend state. Where multiple device suspend states are available, the driver SHOULD be capable of entering each of these states.


 * 1) The device resume processing performed by the driver for a CE platform device SHOULD restore device operation to approximate pre- suspend conditions.


 * 1) A mechanism for requesting device suspend for all active CE platform devices at system suspend time MUST be provided. A mechanism for system resume to restore to an active state all CE platform devices that were in an active state prior to system suspend MUST be provided.


 * 1) It is RECOMMENDED that drivers place devices into low-power states when not in use or after a period of inactivity.


 * 1) It is RECOMMENDED that drivers and platform support code make use of hardware features to automatically place devices into lower-power states, such as to stop clocks (sometimes referred to as "automatic clock gating"), after a period of inactivity or when the hardware is in some manner able to detect that the device is not in use.

Non-normative notes
Among the choices for device suspend/resume interfaces are: A version of the Linux 2.5/2.6 technology described above has been backported to Linux 2.4 for use in CELF-conforming systems based on the 2.4 Linux kernel.
 * The Linux 2.5/2.6 Linux Driver Model (LDM) implements a kernel API for driver suspend and resume functions. LDM also provides a kernel API for calling drivers to suspend all devices at system suspend, and to restore normal operation of the devices at system resume.
 * The Linux 2.5/2.6 sysfs filesystem exports interfaces that applications may use to suspend and resume devices (individually); these interfaces call the LDM driver suspend and resume kernel APIs.
 * APM function calls (such as pm_register) for device suspend/resume. Note that these interfaces are generally being replaced with Linux Driver Model interfaces in future Linux versions.

Comments on the 4.5.2 specification section are the following:
 * 4.5.2.4 and 4.5.2.6 does not make specific requirements upon devices that may change state in significant ways between device suspend and device resume, for example, a controller for a hotpluggable bus such as PCMCIA or MMC, where the consumer may insert or eject cards during the suspend interval.


 * 4.5.2.5: In 2.6 kernel,Linux Driver Model (LDM) has this mechanism. CE platform devices that are already suspended via a previous individual device suspend should be skipped, if it is not necessary to perform any further device suspend processing prior to a system suspend (such as to save state in RAM in preparation for power-off). Devices that were already individually suspended prior to system suspend should be left in a suspended state after system resume.


 * 4.5.2.6: For example, when the last open file descriptor for a device is closed, the driver may suspend the device.